What is linux equivalent of "host.docker.internal" [duplicate]

What is linux equivalent of "host.docker.internal" [duplicate] - docker

This question already has answers here:
From inside of a Docker container, how do I connect to the localhost of the machine?
(41 answers)
Closed 10 months ago.
On Mac and Windows it is possible to use host.docker.internal (Docker 18.03+) inside container.
Is there one for Linux that will work out of the box without passing env variables or extracting it using various CLI commands?

Depends what you're trying to do. If you're running with --net=host, localhost should work fine. If you're using default networking, use the static IP 172.17.0.1. I suspect neither will behave quite the same as those domains.

For linux systems, you can – starting from major version 20.04 of the docker engine – now also communicate with the host via host.docker.internal. This won't work automatically, but you need to provide the following run flag:
--add-host=host.docker.internal:host-gateway
See the answer here: https://stackoverflow.com/a/61424570/3757139
See also this answer below to add to a docker-compose file - https://stackoverflow.com/a/67158212/243392

If you are using Docker Compose + Linux, you have to add it manually (at least for now). Use extra_hosts on your docker-compose.yaml file:
version: '3.7'
services:
fpm:
build:
context: .
extra_hosts:
- "host.docker.internal:host-gateway"
Do not forget to update Docker since this only works with Docker v20.10+.
Source : https://github.com/docker/for-linux/issues/264#issuecomment-784985736

One solution is to use a special container which redirects traffic to the host. You can find such a container here: https://github.com/qoomon/docker-host. The idea is to grab the default route from within the container and install that as a NAT gateway for incoming connections.
An imaginary example usage:
docker-host:
image: qoomon/docker-host
cap_add: [ 'NET_ADMIN', 'NET_RAW' ]
restart: on-failure
environment:
- PORTS=999
some-service:
image: ...
environment:
SERVER_URL: "http://docker-host:999"
command: ...
depends_on:
- docker-host

This is my solution:
IP_ADDRESS=$(ip addr show | grep "\binet\b.*\bdocker0\b" | awk '{print $2}' | cut -d '/' -f 1)
then in docker-compose:
extra_hosts:
docker.host: ${IP_ADDRESS}

For linux there isn't a default DNS name for the host machine. This can be verified by running the command:
docker run -it alpine cat /etc/hosts
This feature has been requested, however wasn't implemented. You can check this issue. As discussed you can use the following command to find the IP of the host from the container.
netstat -nr | grep '^0\.0\.0\.0' | awk '{print $2}'
Alternatively, you can provide the host ip to the run command via docker run --add-host dockerHost:<ip-address> ...

https://github.com/docker/for-linux/issues/264
IP=$(ip -4 route list match 0/0 | awk '{print $3}')
echo "Host ip is $IP"
echo "$IP host.docker.internal" | sudo tee -a /etc/hosts
It will add host.docker.internal to your hosts. Then you can use it in xdebug config.
Here is example of env variable in docker-compose.yml
XDEBUG_CONFIG: remote_host=host.docker.internal remote_autostart=On remote_enable=On idekey=XDEBUG remote_log=/tmp/xdebug.log remote_port=9999

host.docker.internal exists only in Windows WSL because Docker Desktop for Windows runs Docker daemon inside the special WSL VM Docker-Desktop. It has its own localhost and its own WSL2 interface to communicate with Windows. This VM has no static IP. The IP is generated every time when VM is created and passed via host.docker.internal in the generated /etc/hosts to every distro. Although there is no bridge or real v-switch all ports opened on eth0 of VM internal network are mapped on the host Local network, BUT NOT ON THE ETH0 OF THE HOST.
There is no real bridge and port mapping - nothing to configure.
Inside WSL VM its Localhost is the same as the localhost of the Linux machine. 2 processes inside WSL VM can communicate via localhost. Cross-distro IPC must use host.docker.internal. It is possible to create bridge inside WSL VM -Docker does it.

tldr; Access the host via the static IP 172.17.0.1
Doing HTTP request towards the host:
Run the following command to get the static IP: ip addr show | grep "\binet\b.*\bdocker0\b" | awk '{print $2}' | cut -d '/' -f 1
Add the new IP to allowed hosts
Use the IP address just found in your requests: req = requests.get('http://172.17.0.1:8000/api/YOUR_ENDPOINT')

Using the docker0 interface ip, say 172.17.0.1, could be a good workaround.
Just be sure that the service you need to reach listens to external connections. A typical example is Mysql who binds to 127.0.0.1 by default, resulting unreachable until you allow external connections (es. binding to 0.0.0.0)

For linux, I was able to use the service name I was trying to connect to, e.g. one of my containers (php-fpm) was trying to connect to mysql, so I used mysql as the host name, since that's the service name in my docker-compose

Related

Docker internal DNS not resolving [duplicate]

I plan to split my monolthic server up into many small docker containers but haven't found a good solution for "inter-container communication" yet. This is my target scenario:
I know how to link containers together and how to expose ports, but none of these solutions are satisfying to me.
Is there any solution to communicate via hostnames (container names) between the containers like in a traditional server network?

The new networking feature allows you to connect to containers by
their name, so if you create a new network, any container connected to
that network can reach other containers by their name. Example:
1) Create new network
$ docker network create <network-name>
2) Connect containers to network
$ docker run --net=<network-name> ...
or
$ docker network connect <network-name> <container-name>
3) Ping container by name
docker exec -ti <container-name-A> ping <container-name-B>
64 bytes from c1 (172.18.0.4): icmp_seq=1 ttl=64 time=0.137 ms
64 bytes from c1 (172.18.0.4): icmp_seq=2 ttl=64 time=0.073 ms
64 bytes from c1 (172.18.0.4): icmp_seq=3 ttl=64 time=0.074 ms
64 bytes from c1 (172.18.0.4): icmp_seq=4 ttl=64 time=0.074 ms
See this section of the documentation;
Note: Unlike legacy links the new networking will not create environment variables, nor share environment variables with other containers.
This feature currently doesn't support aliases

Edit: After Docker 1.9, the docker network command (see below https://stackoverflow.com/a/35184695/977939) is the recommended way to achieve this.
My solution is to set up a dnsmasq on the host to have DNS record automatically updated: "A" records have the names of containers and point to the IP addresses of the containers automatically (every 10 sec). The automatic updating script is pasted here:
#!/bin/bash
# 10 seconds interval time by default
INTERVAL=${INTERVAL:-10}
# dnsmasq config directory
DNSMASQ_CONFIG=${DNSMASQ_CONFIG:-.}
# commands used in this script
DOCKER=${DOCKER:-docker}
SLEEP=${SLEEP:-sleep}
TAIL=${TAIL:-tail}
declare -A service_map
while true
do
changed=false
while read line
do
name=${line##* }
ip=$(${DOCKER} inspect --format '{{.NetworkSettings.IPAddress}}' $name)
if [ -z ${service_map[$name]} ] || [ ${service_map[$name]} != $ip ] # IP addr changed
then
service_map[$name]=$ip
# write to file
echo $name has a new IP Address $ip >&2
echo "host-record=$name,$ip" > "${DNSMASQ_CONFIG}/docker-$name"
changed=true
fi
done < <(${DOCKER} ps | ${TAIL} -n +2)
# a change of IP address occured, restart dnsmasq
if [ $changed = true ]
then
systemctl restart dnsmasq
fi
${SLEEP} $INTERVAL
done
Make sure your dnsmasq service is available on docker0. Then, start your container with --dns HOST_ADDRESS to use this mini dns service.
Reference: http://docs.blowb.org/setup-host/dnsmasq.html

That should be what --link is for, at least for the hostname part.
With docker 1.10, and PR 19242, that would be:
docker network create --net-alias=[]: Add network-scoped alias for the container
(see last section below)
That is what Updating the /etc/hosts file details
In addition to the environment variables, Docker adds a host entry for the source container to the /etc/hosts file.
For instance, launch an LDAP server:
docker run -t --name openldap -d -p 389:389 larrycai/openldap
And define an image to test that LDAP server:
FROM ubuntu
RUN apt-get -y install ldap-utils
RUN touch /root/.bash_aliases
RUN echo "alias lds='ldapsearch -H ldap://internalopenldap -LL -b
ou=Users,dc=openstack,dc=org -D cn=admin,dc=openstack,dc=org -w
password'" > /root/.bash_aliases
ENTRYPOINT bash
You can expose the 'openldap' container as 'internalopenldap' within the test image with --link:
docker run -it --rm --name ldp --link openldap:internalopenldap ldaptest
Then, if you type 'lds', that alias will work:
ldapsearch -H ldap://internalopenldap ...
That would return people. Meaning internalopenldap is correctly reached from the ldaptest image.
Of course, docker 1.7 will add libnetwork, which provides a native Go implementation for connecting containers. See the blog post.
It introduced a more complete architecture, with the Container Network Model (CNM)
That will Update the Docker CLI with new “network” commands, and document how the “-net” flag is used to assign containers to networks.
docker 1.10 has a new section Network-scoped alias, now officially documented in network connect:
While links provide private name resolution that is localized within a container, the network-scoped alias provides a way for a container to be discovered by an alternate name by any other container within the scope of a particular network.
Unlike the link alias, which is defined by the consumer of a service, the network-scoped alias is defined by the container that is offering the service to the network.
Continuing with the above example, create another container in isolated_nw with a network alias.
$ docker run --net=isolated_nw -itd --name=container6 -alias app busybox
8ebe6767c1e0361f27433090060b33200aac054a68476c3be87ef4005eb1df17
--alias=[]
Add network-scoped alias for the container
You can use --link option to link another container with a preferred alias
You can pause, restart, and stop containers that are connected to a network. Paused containers remain connected and can be revealed by a network inspect. When the container is stopped, it does not appear on the network until you restart it.
If specified, the container's IP address(es) is reapplied when a stopped container is restarted. If the IP address is no longer available, the container fails to start.
One way to guarantee that the IP address is available is to specify an --ip-range when creating the network, and choose the static IP address(es) from outside that range. This ensures that the IP address is not given to another container while this container is not on the network.
$ docker network create --subnet 172.20.0.0/16 --ip-range 172.20.240.0/20 multi-host-network
$ docker network connect --ip 172.20.128.2 multi-host-network container2
$ docker network connect --link container1:c1 multi-host-network container2

EDIT : It is not bleeding edge anymore : http://blog.docker.com/2016/02/docker-1-10/
Original Answer
I battled with it the whole night.
If you're not afraid of bleeding edge, the latest version of Docker engine and Docker compose both implement libnetwork.
With the right config file (that need to be put in version 2), you will create services that will all see each other. And, bonus, you can scale them with docker-compose as well (you can scale any service you want that doesn't bind port on the host)
Here is an example file
version: "2"
services:
router:
build: services/router/
ports:
- "8080:8080"
auth:
build: services/auth/
todo:
build: services/todo/
data:
build: services/data/
And the reference for this new version of compose file:
https://github.com/docker/compose/blob/1.6.0-rc1/docs/networking.md

As far as I know, by using only Docker this is not possible. You need some DNS to map container ip:s to hostnames.
If you want out of the box solution. One solution is to use for example Kontena. It comes with network overlay technology from Weave and this technology is used to create virtual private LAN networks for each service and every service can be reached by service_name.kontena.local-address.
Here is simple example of Wordpress application's YAML file where Wordpress service connects to MySQL server with wordpress-mysql.kontena.local address:
wordpress:
image: wordpress:4.1
stateful: true
ports:
- 80:80
links:
- mysql:wordpress-mysql
environment:
- WORDPRESS_DB_HOST=wordpress-mysql.kontena.local
- WORDPRESS_DB_PASSWORD=secret
mysql:
image: mariadb:5.5
stateful: true
environment:
- MYSQL_ROOT_PASSWORD=secret

Docker Compose networking - Export networks.ipam.config.gateway to service env variables [duplicate]

I have a Nginx running inside a docker container. I have a MySql running on the host system. I want to connect to the MySql from within my container. MySql is only binding to the localhost device.
Is there any way to connect to this MySql or any other program on localhost from within this docker container?
This question is different from "How to get the IP address of the docker host from inside a docker container" due to the fact that the IP address of the docker host could be the public IP or the private IP in the network which may or may not be reachable from within the docker container (I mean public IP if hosted at AWS or something). Even if you have the IP address of the docker host it does not mean you can connect to docker host from within the container given that IP address as your Docker network may be overlay, host, bridge, macvlan, none etc which restricts the reachability of that IP address.

Edit:
If you are using Docker-for-mac or Docker-for-Windows 18.03+, connect to your mysql service using the host host.docker.internal (instead of the 127.0.0.1 in your connection string).
If you are using Docker-for-Linux 20.10.0+, you can also use the host host.docker.internal if you started your Docker container with the --add-host host.docker.internal:host-gateway option.
Otherwise, read below
TLDR
Use --network="host" in your docker run command, then 127.0.0.1 in your docker container will point to your docker host.
Note: This mode only works on Docker for Linux, per the documentation.
Note on docker container networking modes
Docker offers different networking modes when running containers. Depending on the mode you choose you would connect to your MySQL database running on the docker host differently.
docker run --network="bridge" (default)
Docker creates a bridge named docker0 by default. Both the docker host and the docker containers have an IP address on that bridge.
on the Docker host, type sudo ip addr show docker0 you will have an output looking like:
[vagrant#docker:~] $ sudo ip addr show docker0
4: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
link/ether 56:84:7a:fe:97:99 brd ff:ff:ff:ff:ff:ff
inet 172.17.42.1/16 scope global docker0
valid_lft forever preferred_lft forever
inet6 fe80::5484:7aff:fefe:9799/64 scope link
valid_lft forever preferred_lft forever
So here my docker host has the IP address 172.17.42.1 on the docker0 network interface.
Now start a new container and get a shell on it: docker run --rm -it ubuntu:trusty bash and within the container type ip addr show eth0 to discover how its main network interface is set up:
root#e77f6a1b3740:/# ip addr show eth0
863: eth0: <BROADCAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 66:32:13:f0:f1:e3 brd ff:ff:ff:ff:ff:ff
inet 172.17.1.192/16 scope global eth0
valid_lft forever preferred_lft forever
inet6 fe80::6432:13ff:fef0:f1e3/64 scope link
valid_lft forever preferred_lft forever
Here my container has the IP address 172.17.1.192. Now look at the routing table:
root#e77f6a1b3740:/# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default 172.17.42.1 0.0.0.0 UG 0 0 0 eth0
172.17.0.0 * 255.255.0.0 U 0 0 0 eth0
So the IP Address of the docker host 172.17.42.1 is set as the default route and is accessible from your container.
root#e77f6a1b3740:/# ping 172.17.42.1
PING 172.17.42.1 (172.17.42.1) 56(84) bytes of data.
64 bytes from 172.17.42.1: icmp_seq=1 ttl=64 time=0.070 ms
64 bytes from 172.17.42.1: icmp_seq=2 ttl=64 time=0.201 ms
64 bytes from 172.17.42.1: icmp_seq=3 ttl=64 time=0.116 ms
docker run --network="host"
Alternatively you can run a docker container with network settings set to host. Such a container will share the network stack with the docker host and from the container point of view, localhost (or 127.0.0.1) will refer to the docker host.
Be aware that any port opened in your docker container would be opened on the docker host. And this without requiring the -p or -P docker run option.
IP config on my docker host:
[vagrant#docker:~] $ ip addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 08:00:27:98:dc:aa brd ff:ff:ff:ff:ff:ff
inet 10.0.2.15/24 brd 10.0.2.255 scope global eth0
valid_lft forever preferred_lft forever
inet6 fe80::a00:27ff:fe98:dcaa/64 scope link
valid_lft forever preferred_lft forever
and from a docker container in host mode:
[vagrant#docker:~] $ docker run --rm -it --network=host ubuntu:trusty ip addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 08:00:27:98:dc:aa brd ff:ff:ff:ff:ff:ff
inet 10.0.2.15/24 brd 10.0.2.255 scope global eth0
valid_lft forever preferred_lft forever
inet6 fe80::a00:27ff:fe98:dcaa/64 scope link
valid_lft forever preferred_lft forever
As you can see both the docker host and docker container share the exact same network interface and as such have the same IP address.
Connecting to MySQL from containers
bridge mode
To access MySQL running on the docker host from containers in bridge mode, you need to make sure the MySQL service is listening for connections on the 172.17.42.1 IP address.
To do so, make sure you have either bind-address = 172.17.42.1 or bind-address = 0.0.0.0 in your MySQL config file (my.cnf).
If you need to set an environment variable with the IP address of the gateway, you can run the following code in a container :
export DOCKER_HOST_IP=$(route -n | awk '/UG[ \t]/{print $2}')
then in your application, use the DOCKER_HOST_IP environment variable to open the connection to MySQL.
Note: if you use bind-address = 0.0.0.0 your MySQL server will listen for connections on all network interfaces. That means your MySQL server could be reached from the Internet ; make sure to set up firewall rules accordingly.
Note 2: if you use bind-address = 172.17.42.1 your MySQL server won't listen for connections made to 127.0.0.1. Processes running on the docker host that would want to connect to MySQL would have to use the 172.17.42.1 IP address.
host mode
To access MySQL running on the docker host from containers in host mode, you can keep bind-address = 127.0.0.1 in your MySQL configuration and connect to 127.0.0.1 from your containers:
[vagrant#docker:~] $ docker run --rm -it --network=host mysql mysql -h 127.0.0.1 -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 36
Server version: 5.5.41-0ubuntu0.14.04.1 (Ubuntu)
Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>
note: Do use mysql -h 127.0.0.1 and not mysql -h localhost; otherwise the MySQL client would try to connect using a unix socket.

For all platforms
Docker v 20.10 and above (since December 14th 2020)
Use your internal IP address or connect to the special DNS name host.docker.internal which will resolve to the internal IP address used by the host.
On Linux, using the Docker command, add --add-host=host.docker.internal:host-gateway to your Docker command to enable this feature.
To enable this in Docker Compose on Linux, add the following lines to the container definition:
extra_hosts:
- "host.docker.internal:host-gateway"
For older macOS and Windows versions of Docker
Docker v 18.03 and above (since March 21st 2018)
Use your internal IP address or connect to the special DNS name host.docker.internal which will resolve to the internal IP address used by the host.
Linux support pending https://github.com/docker/for-linux/issues/264
For older macOS versions of Docker
Docker for Mac v 17.12 to v 18.02
Same as above but use docker.for.mac.host.internal instead.
Docker for Mac v 17.06 to v 17.11
Same as above but use docker.for.mac.localhost instead.
Docker for Mac 17.05 and below
To access host machine from the docker container you must attach an IP alias to your network interface. You can bind whichever IP you want, just make sure you're not using it to anything else.
sudo ifconfig lo0 alias 123.123.123.123/24
Then make sure that you server is listening to the IP mentioned above or 0.0.0.0. If it's listening on localhost 127.0.0.1 it will not accept the connection.
Then just point your docker container to this IP and you can access the host machine!
To test you can run something like curl -X GET 123.123.123.123:3000 inside the container.
The alias will reset on every reboot so create a start-up script if necessary.
Solution and more documentation here: https://docs.docker.com/docker-for-mac/networking/#use-cases-and-workarounds

Use
host.docker.internal
instead of
localhost

I doing a hack similar to above posts of get the local IP to map to a alias name (DNS) in the container. The major problem is to get dynamically with a simple script that works both in Linux and OSX the host IP address. I did this script that works in both environments (even in Linux distribution with "$LANG" != "en_*" configured):
ifconfig | grep -E "([0-9]{1,3}\.){3}[0-9]{1,3}" | grep -v 127.0.0.1 | awk '{ print $2 }' | cut -f2 -d: | head -n1
So, using Docker Compose, the full configuration will be:
Startup script (docker-run.sh):
export DOCKERHOST=$(ifconfig | grep -E "([0-9]{1,3}\.){3}[0-9]{1,3}" | grep -v 127.0.0.1 | awk '{ print $2 }' | cut -f2 -d: | head -n1)
docker-compose -f docker-compose.yml up
docker-compose.yml:
myapp:
build: .
ports:
- "80:80"
extra_hosts:
- "dockerhost:$DOCKERHOST"
Then change http://localhost to http://dockerhost in your code.
For a more advance guide of how to customize the DOCKERHOST script, take a look at this post with a explanation of how it works.

Solution for Linux (kernel >=3.6).
Ok, your localhost server has a default docker interface docker0 with IP address 172.17.0.1. Your container started with default network settings --net="bridge".
Enable route_localnet for docker0 interface:
$ sysctl -w net.ipv4.conf.docker0.route_localnet=1
Add these rules to iptables:
$ iptables -t nat -I PREROUTING -i docker0 -d 172.17.0.1 -p tcp --dport 3306 -j DNAT --to 127.0.0.1:3306
$ iptables -t filter -I INPUT -i docker0 -d 127.0.0.1 -p tcp --dport 3306 -j ACCEPT
Create MySQL user with access from '%' that means - from anyone, excluding localhost:
CREATE USER 'user'#'%' IDENTIFIED BY 'password';
Change in your script the mysql-server address to 172.17.0.1.
From the kernel documentation:
route_localnet - BOOLEAN: Do not consider loopback addresses as martian source or destination while routing. This enables the use of 127/8 for local routing purposes (default FALSE).

This worked for me on an NGINX/PHP-FPM stack without touching any code or networking where the app's just expecting to be able to connect to localhost
Mount mysqld.sock from the host to inside the container.
Find the location of the mysql.sock file on the host running mysql:
netstat -ln | awk '/mysql(.*)?\.sock/ { print $9 }'
Mount that file to where it's expected in the docker:
docker run -v /hostpath/to/mysqld.sock:/containerpath/to/mysqld.sock
Possible locations of mysqld.sock:
/tmp/mysqld.sock
/var/run/mysqld/mysqld.sock
/var/lib/mysql/mysql.sock
/Applications/MAMP/tmp/mysql/mysql.sock # if running via MAMP

Until host.docker.internal is working for every platform, you can use my container acting as a NAT gateway without any manual setup:
https://github.com/qoomon/docker-host

Simplest solution for Mac OSX
Just use the IP address of your Mac. On the Mac run this to get the IP address and use it from within the container:
ifconfig | grep 'inet 192'| awk '{ print $2}'
As long as the server running locally on your Mac or in another docker container is listening to 0.0.0.0, the docker container will be able to reach out at that address.
If you just want to access another docker container that is listening on 0.0.0.0 you can use 172.17.0.1

Very simple and quick, check your host IP with ifconfig (linux) or ipconfig (windows) and then create a docker-compose.yml:
version: '3' # specify docker-compose version
services:
nginx:
build: ./ # specify the directory of the Dockerfile
ports:
- "8080:80" # specify port mapping
extra_hosts:
- "dockerhost:<yourIP>"
This way, your container will be able to access your host. When accessing your DB, remember to use the name you specified before, in this case dockerhost and the port of your host in which the DB is running.

Several solutions come to mind:
Move your dependencies into containers first
Make your other services externally accessible and connect to them with that external IP
Run your containers without network isolation
Avoid connecting over the network, use a socket that is mounted as a volume instead
The reason this doesn't work out of the box is that containers run with their own network namespace by default. That means localhost (or 127.0.0.1 pointing to the loopback interface) is unique per container. Connecting to this will connect to the container itself, and not services running outside of docker or inside of a different docker container.
Option 1: If your dependency can be moved into a container, I would do this first. It makes your application stack portable as others try to run your container on their own environment. And you can still publish the port on your host where other services that have not been migrated can still reach it. You can even publish the port to the localhost interface on your docker host to avoid it being externally accessible with a syntax like: -p 127.0.0.1:3306:3306 for the published port.
Option 2: There are a variety of ways to detect the host IP address from inside of the container, but each have a limited number of scenarios where they work (e.g. requiring Docker for Mac). The most portable option is to inject your host IP into the container with something like an environment variable or configuration file, e.g.:
docker run --rm -e "HOST_IP=$(ip route get 1 | sed -n 's/^.*src $[0-9.]*$ .*$/\1/p')" ...
This does require that your service is listening on that external interface, which could be a security concern. For other methods to get the host IP address from inside of the container, see this post.
Slightly less portable is to use host.docker.internal. This works in current versions of Docker for Windows and Docker for Mac. And in 20.10, the capability has been added to Docker for Linux when you pass a special host entry with:
docker run --add-host host.docker.internal:host-gateway ...
The host-gateway is a special value added in Docker 20.10 that automatically expands to a host IP. For more details see this PR.
Option 3: Running without network isolation, i.e. running with --net host, means your application is running on the host network namespace. This is less isolation for the container, and it means you cannot access other containers over a shared docker network with DNS (instead, you need to use published ports to access other containerized applications). But for applications that need to access other services on the host that are only listening on 127.0.0.1 on the host, this can be the easiest option.
Option 4: Various services also allow access over a filesystem based socket. This socket can be mounted into the container as a bind mounted volume, allowing you to access the host service without going over the network. For access to the docker engine, you often see examples of mounting /var/run/docker.sock into the container (giving that container root access to the host). With mysql, you can try something like -v /var/run/mysqld/mysqld.sock:/var/run/mysqld/mysql.sock and then connect to localhost which mysql converts to using the socket.

Solution for Windows 10
Docker Community Edition 17.06.0-ce-win18 2017-06-28 (stable)
You can use DNS name of the host docker.for.win.localhost, to resolve to the internal IP. (Warning some sources mentioned windows but it should be win)
Overview
I needed to do something similar, that is connect from my Docker container to my localhost, which was running the Azure Storage Emulator and CosmosDB Emulator.
The Azure Storage Emulator by default listens on 127.0.0.1, while you can change the IP its bound too, I was looking for a solution that would work with default settings.
This also works for connecting from my Docker container to SQL Server and IIS, both running locally on my host with default port settings.

For windows,
I have changed the database url in spring configuration: spring.datasource.url=jdbc:postgresql://host.docker.internal:5432/apidb
Then build the image and run. It worked for me.

None of the answers worked for me when using Docker Toolbox on Windows 10 Home, but 10.0.2.2 did, since it uses VirtualBox which exposes the host to the VM on this address.

This is not an answer to the actual question. This is how I solved a similar problem. The solution comes totally from: Define Docker Container Networking so Containers can Communicate. Thanks to Nic Raboy
Leaving this here for others who might want to do REST calls between one container and another. Answers the question: what to use in place of localhost in a docker environment?
Get how your network looks like docker network ls
Create a new network docker network create -d my-net
Start the first container docker run -d -p 5000:5000 --network="my-net" --name "first_container" <MyImage1:v0.1>
Check out network settings for first container docker inspect first_container. "Networks": should have 'my-net'
Start the second container docker run -d -p 6000:6000 --network="my-net" --name "second_container" <MyImage2:v0.1>
Check out network settings for second container docker inspect second_container. "Networks": should have 'my-net'
ssh into your second container docker exec -it second_container sh or docker exec -it second_container bash.
Inside of the second container, you can ping the first container by ping first_container. Also, your code calls such as http://localhost:5000 can be replaced by http://first_container:5000

If you're running with --net=host, localhost should work fine. If you're using default networking, use the static IP 172.17.0.1.
See this - https://stackoverflow.com/a/48547074/14120621

For those on Windows, assuming you're using the bridge network driver, you'll want to specifically bind MySQL to the IP address of the hyper-v network interface.
This is done via the configuration file under the normally hidden C:\ProgramData\MySQL folder.
Binding to 0.0.0.0 will not work. The address needed is shown in the docker configuration as well, and in my case was 10.0.75.1.

Edit: I ended up prototyping out the concept on GitHub. Check out: https://github.com/sivabudh/system-in-a-box
First, my answer is geared towards 2 groups of people: those who use a Mac, and those who use Linux.
The host network mode doesn't work on a Mac. You have to use an IP alias, see: https://stackoverflow.com/a/43541681/2713729
What is a host network mode? See: https://docs.docker.com/engine/reference/run/#/network-settings
Secondly, for those of you who are using Linux (my direct experience was with Ubuntu 14.04 LTS and I'm upgrading to 16.04 LTS in production soon), yes, you can make the service running inside a Docker container connect to localhost services running on the Docker host (eg. your laptop).
How?
The key is when you run the Docker container, you have to run it with the host mode. The command looks like this:
docker run --network="host" -id <Docker image ID>
When you do an ifconfig (you will need to apt-get install net-tools your container for ifconfig to be callable) inside your container, you will see that the network interfaces are the same as the one on Docker host (eg. your laptop).
It's important to note that I'm a Mac user, but I run Ubuntu under Parallels, so using a Mac is not a disadvantage. ;-)
And this is how you connect NGINX container to the MySQL running on a localhost.

For Linux, where you cannot change the interface the localhost service binds to
There are two problems we need to solve
Getting the IP of the host
Making our localhost service available to Docker
The first problem can be solved using qoomon's docker-host image, as given by other answers.
You will need to add this container to the same bridge network as your other container so that you can access it. Open a terminal inside your container and ensure that you can ping dockerhost.
bash-5.0# ping dockerhost
PING dockerhost (172.20.0.2): 56 data bytes
64 bytes from 172.20.0.2: seq=0 ttl=64 time=0.523 ms
Now, the harder problem, making the service accessible to docker.
We can use telnet to check if we can access a port on the host (you may need to install this).
The problem is that our container will only be able to access services that bind to all interfaces, such as SSH:
bash-5.0# telnet dockerhost 22
SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.3
But services bound only to localhost will be inaccessible:
bash-5.0# telnet dockerhost 1025
telnet: can't connect to remote host (172.20.0.2): Connection refused
The proper solution here would be to bind the service to dockers bridge network. However, this answer assumes that it is not possible for you to change this. So we will instead use iptables.
First, we need to find the name of the bridge network that docker is using with ifconfig. If you are using an unnamed bridge, this will just be docker0. However, if you are using a named network you will have a bridge starting with br- that docker will be using instead. Mine is br-5cd80298d6f4.
Once we have the name of this bridge, we need to allow routing from this bridge to localhost. This is disabled by default for security reasons:
sysctl -w net.ipv4.conf.<bridge_name>.route_localnet=1
Now to set up our iptables rule. Since our container can only access ports on the docker bridge network, we are going to pretend that our service is actually bound to a port on this network.
To do this, we will forward all requests to <docker_bridge>:port to localhost:port
iptables -t nat -A PREROUTING -p tcp -i <docker_bridge_name> --dport <service_port> -j DNAT --to-destination 127.0.0.1:<service_port>
For example, for my service on port 1025
iptables -t nat -A PREROUTING -p tcp -i br-5cd80298d6f4 --dport 1025 -j DNAT --to-destination 127.0.0.1:1025
You should now be able to access your service from the container:
bash-5.0# telnet dockerhost 1025
220 127.0.0.1 ESMTP Service Ready

First see this answer for the options that you have to fix this problem. But if you use docker-compose you can add network_mode: host to your service and then use 127.0.0.1 to connect to the local host. This is just one of the options described in the answer above. Below you can find how I modified docker-compose.yml from https://github.com/geerlingguy/php-apache-container.git:
---
version: "3"
services:
php-apache:
+ network_mode: host
image: geerlingguy/php-apache:latest
container_name: php-apache
...
+ indicates the line I added.
[Additional info] This has also worked in version 2.2. and "host" or just 'host' are both worked in docker-compose.
---
version: "2.2"
services:
php-apache:
+ network_mode: "host"
or
+ network_mode: host
...

I disagree with the answer from Thomasleveil.
Making mysql bind to 172.17.42.1 will prevent other programs using the database on the host to reach it. This will only work if all your database users are dockerized.
Making mysql bind to 0.0.0.0 will open the db to outside world, which is not only a very bad thing to do, but also contrary to what the original question author wants to do. He explicitly says "The MySql is running on localhost and not exposing a port to the outside world, so its bound on localhost"
To answer the comment from ivant
"Why not bind mysql to docker0 as well?"
This is not possible. The mysql/mariadb documentation explicitly says it is not possible to bind to several interfaces. You can only bind to 0, 1, or all interfaces.
As a conclusion, I have NOT found any way to reach the (localhost only) database on the host from a docker container. That definitely seems like a very very common pattern, but I don't know how to do it.

Try this:
version: '3.5'
services:
yourservice-here:
container_name: container_name
ports:
- "4000:4000"
extra_hosts: # <---- here
- localhost:192.168.1.202
- or-vitualhost.local:192.168.1.202
To get 192.168.1.202, uses ifconfig
This worked for me. Hope this help!

In 7 years the question was asked, it is either docker has changed, or no one tried this way. So I will include my own answer.
I have found all answers use complex methods. Today, I have needed this, and found 2 very simple ways:
use ipconfig or ifconfig on your host and make note of all IP addresses. At least two of them can be used by the container.
I have a fixed local network address on WiFi LAN Adapter: 192.168.1.101. This could be 10.0.1.101. the result will change depending on your router
I use WSL on windows, and it has its own vEthernet address: 172.19.192.1
use host.docker.internal. Most answers have this or another form of it depending on OS. The name suggests it is now globally used by docker.
A third option is to use WAN address of the machine, or in other words IP given by the service provider. However, this may not work if IP is not static, and requires routing and firewall settings.

You need to know the gateway! My solution with local server was to expose it under 0.0.0.0:8000, then run docker with subnet and run container like:
docker network create --subnet=172.35.0.0/16 --gateway 172.35.0.1 SUBNET35
docker run -d -p 4444:4444 --net SUBNET35 <container-you-want-run-place-here>
So, now you can access your loopback through http://172.35.0.1:8000

Connect to the gateway address.
❯ docker network inspect bridge | grep Gateway
"Gateway": "172.17.0.1"
Make sure the process on the host is listening on this interface or on all interfaces and is started after docker. If using systemd, you can add the below to make sure it is started after docker.
[Unit]
After=docker.service
Example
❯ python -m http.server &> /dev/null &
[1] 149976
❯ docker run --rm python python -c "from urllib.request import urlopen;print(b'Directory listing for' in urlopen('http://172.17.0.1:8000').read())"
True

Here is my solution : it works for my case
set local mysql server to public access by comment
#bind-address = 127.0.0.1
in /etc/mysql/mysql.conf.d
restart mysql server
sudo /etc/init.d/mysql restart
run the following command to open user root access any host
mysql -uroot -proot
GRANT ALL PRIVILEGES ON *.* TO 'root'#'%' IDENTIFIED BY 'root' WITH
GRANT OPTION;
FLUSH PRIVILEGES;
create sh script : run_docker.sh
#!bin/bash
HOSTIP=`ip -4 addr show scope global dev eth0 | grep inet | awk '{print \$2}' | cut -d / -f 1`
docker run -it -d --name web-app \
--add-host=local:${HOSTIP} \
-p 8080:8080 \
-e DATABASE_HOST=${HOSTIP} \
-e DATABASE_PORT=3306 \
-e DATABASE_NAME=demo \
-e DATABASE_USER=root \
-e DATABASE_PASSWORD=root \
sopheamak/springboot_docker_mysql
run with docker-composer
version: '2.1'
services:
tomcatwar:
extra_hosts:
- "local:10.1.2.232"
image: sopheamak/springboot_docker_mysql
ports:
- 8080:8080
environment:
- DATABASE_HOST=local
- DATABASE_USER=root
- DATABASE_PASSWORD=root
- DATABASE_NAME=demo
- DATABASE_PORT=3306

You can get the host ip using alpine image
docker run --rm alpine ip route | awk 'NR==1 {print $3}'
This would be more consistent as you're always using alpine to run the command.
Similar to Mariano's answer you can use same command to set an environment variable
DOCKER_HOST=$(docker run --rm alpine ip route | awk 'NR==1 {print $3}') docker-compose up

you can use net alias for your machine
OSX
sudo ifconfig lo0 alias 123.123.123.123/24 up
LINUX
sudo ifconfig lo:0 123.123.123.123 up
then from the container you can see the machine by 123.123.123.123

The CGroups and Namespaces are playing major role in the Container Ecosystem.
Namespace provide a layer of isolation. Each container runs in a separate namespace and its access is limited to that namespace. The Cgroups controls the resource utilization of each container, whereas Namespace controls what a process can see and access the respective resource.
Here is the basic understanding of the solution approach you could follow,
Use Network Namespace
When a container spawns out of image, a network interface is defined and create. This gives the container unique IP address and interface.
$ docker run -it alpine ifconfig
By changing the namespace to host, cotainers networks does not remain isolated to its interface, the process will have access to host machines network interface.
$ docker run -it --net=host alpine ifconfig
If the process listens on ports, they'll be listened on the host interface and mapped to the container.
Use PID Namespace
By changing the Pid namespace allows a container to interact with other process beyond its normal scope.
This container will run in its own namespace.
$ docker run -it alpine ps aux
By changing the namespace to the host, the container can also see all the other processes running on the system.
$ docker run -it --pid=host alpine ps aux
Sharing Namespace
This is a bad practice to do this in production because you are breaking out of the container security model which might open up for vulnerabilities, and easy access to eavesdropper. This is only for debugging tools and understating the loopholes in container security.
The first container is nginx server. This will create a new network and process namespace. This container will bind itself to port 80 of newly created network interface.
$ docker run -d --name http nginx:alpine
Another container can now reuse this namespace,
$ docker run --net=container:http mohan08p/curl curl -s localhost
Also, this container can see the interface with the processes in a shared container.
$ docker run --pid=container:http alpine ps aux
This will allow you give more privileges to containers without changing or restarting the application. In the similar way you can connect to mysql on host, run and debug your application. But, its not recommend to go by this way. Hope it helps.

Until fix is not merged into master branch, to get host IP just run from inside of the container:
ip -4 route list match 0/0 | cut -d' ' -f3
(as suggested by #Mahoney here).

I solved it by creating a user in MySQL for the container's ip:
$ sudo mysql<br>
mysql> create user 'username'#'172.17.0.2' identified by 'password';<br>
Query OK, 0 rows affected (0.00 sec)
mysql> grant all privileges on database_name.* to 'username'#'172.17.0.2' with grant option;<br>
Query OK, 0 rows affected (0.00 sec)
$ sudo vim /etc/mysql/mysql.conf.d/mysqld.cnf
<br>bind-address = 172.17.0.1
$ sudo systemctl restart mysql.service
Then on container: jdbc:mysql://<b>172.17.0.1</b>:3306/database_name

How to connect my local mysql database in containerised spring mvc web application [duplicate]

I have a Nginx running inside a docker container. I have a MySql running on the host system. I want to connect to the MySql from within my container. MySql is only binding to the localhost device.
Is there any way to connect to this MySql or any other program on localhost from within this docker container?
This question is different from "How to get the IP address of the docker host from inside a docker container" due to the fact that the IP address of the docker host could be the public IP or the private IP in the network which may or may not be reachable from within the docker container (I mean public IP if hosted at AWS or something). Even if you have the IP address of the docker host it does not mean you can connect to docker host from within the container given that IP address as your Docker network may be overlay, host, bridge, macvlan, none etc which restricts the reachability of that IP address.

Use
host.docker.internal
instead of
localhost

I doing a hack similar to above posts of get the local IP to map to a alias name (DNS) in the container. The major problem is to get dynamically with a simple script that works both in Linux and OSX the host IP address. I did this script that works in both environments (even in Linux distribution with "$LANG" != "en_*" configured):
ifconfig | grep -E "([0-9]{1,3}\.){3}[0-9]{1,3}" | grep -v 127.0.0.1 | awk '{ print $2 }' | cut -f2 -d: | head -n1
So, using Docker Compose, the full configuration will be:
Startup script (docker-run.sh):
export DOCKERHOST=$(ifconfig | grep -E "([0-9]{1,3}\.){3}[0-9]{1,3}" | grep -v 127.0.0.1 | awk '{ print $2 }' | cut -f2 -d: | head -n1)
docker-compose -f docker-compose.yml up
docker-compose.yml:
myapp:
build: .
ports:
- "80:80"
extra_hosts:
- "dockerhost:$DOCKERHOST"
Then change http://localhost to http://dockerhost in your code.
For a more advance guide of how to customize the DOCKERHOST script, take a look at this post with a explanation of how it works.

Solution for Linux (kernel >=3.6).
Ok, your localhost server has a default docker interface docker0 with IP address 172.17.0.1. Your container started with default network settings --net="bridge".
Enable route_localnet for docker0 interface:
$ sysctl -w net.ipv4.conf.docker0.route_localnet=1
Add these rules to iptables:
$ iptables -t nat -I PREROUTING -i docker0 -d 172.17.0.1 -p tcp --dport 3306 -j DNAT --to 127.0.0.1:3306
$ iptables -t filter -I INPUT -i docker0 -d 127.0.0.1 -p tcp --dport 3306 -j ACCEPT
Create MySQL user with access from '%' that means - from anyone, excluding localhost:
CREATE USER 'user'#'%' IDENTIFIED BY 'password';
Change in your script the mysql-server address to 172.17.0.1.
From the kernel documentation:
route_localnet - BOOLEAN: Do not consider loopback addresses as martian source or destination while routing. This enables the use of 127/8 for local routing purposes (default FALSE).

This worked for me on an NGINX/PHP-FPM stack without touching any code or networking where the app's just expecting to be able to connect to localhost
Mount mysqld.sock from the host to inside the container.
Find the location of the mysql.sock file on the host running mysql:
netstat -ln | awk '/mysql(.*)?\.sock/ { print $9 }'
Mount that file to where it's expected in the docker:
docker run -v /hostpath/to/mysqld.sock:/containerpath/to/mysqld.sock
Possible locations of mysqld.sock:
/tmp/mysqld.sock
/var/run/mysqld/mysqld.sock
/var/lib/mysql/mysql.sock
/Applications/MAMP/tmp/mysql/mysql.sock # if running via MAMP

Until host.docker.internal is working for every platform, you can use my container acting as a NAT gateway without any manual setup:
https://github.com/qoomon/docker-host

Simplest solution for Mac OSX
Just use the IP address of your Mac. On the Mac run this to get the IP address and use it from within the container:
ifconfig | grep 'inet 192'| awk '{ print $2}'
As long as the server running locally on your Mac or in another docker container is listening to 0.0.0.0, the docker container will be able to reach out at that address.
If you just want to access another docker container that is listening on 0.0.0.0 you can use 172.17.0.1

Very simple and quick, check your host IP with ifconfig (linux) or ipconfig (windows) and then create a docker-compose.yml:
version: '3' # specify docker-compose version
services:
nginx:
build: ./ # specify the directory of the Dockerfile
ports:
- "8080:80" # specify port mapping
extra_hosts:
- "dockerhost:<yourIP>"
This way, your container will be able to access your host. When accessing your DB, remember to use the name you specified before, in this case dockerhost and the port of your host in which the DB is running.

Solution for Windows 10
Docker Community Edition 17.06.0-ce-win18 2017-06-28 (stable)
You can use DNS name of the host docker.for.win.localhost, to resolve to the internal IP. (Warning some sources mentioned windows but it should be win)
Overview
I needed to do something similar, that is connect from my Docker container to my localhost, which was running the Azure Storage Emulator and CosmosDB Emulator.
The Azure Storage Emulator by default listens on 127.0.0.1, while you can change the IP its bound too, I was looking for a solution that would work with default settings.
This also works for connecting from my Docker container to SQL Server and IIS, both running locally on my host with default port settings.

For windows,
I have changed the database url in spring configuration: spring.datasource.url=jdbc:postgresql://host.docker.internal:5432/apidb
Then build the image and run. It worked for me.

None of the answers worked for me when using Docker Toolbox on Windows 10 Home, but 10.0.2.2 did, since it uses VirtualBox which exposes the host to the VM on this address.

This is not an answer to the actual question. This is how I solved a similar problem. The solution comes totally from: Define Docker Container Networking so Containers can Communicate. Thanks to Nic Raboy
Leaving this here for others who might want to do REST calls between one container and another. Answers the question: what to use in place of localhost in a docker environment?
Get how your network looks like docker network ls
Create a new network docker network create -d my-net
Start the first container docker run -d -p 5000:5000 --network="my-net" --name "first_container" <MyImage1:v0.1>
Check out network settings for first container docker inspect first_container. "Networks": should have 'my-net'
Start the second container docker run -d -p 6000:6000 --network="my-net" --name "second_container" <MyImage2:v0.1>
Check out network settings for second container docker inspect second_container. "Networks": should have 'my-net'
ssh into your second container docker exec -it second_container sh or docker exec -it second_container bash.
Inside of the second container, you can ping the first container by ping first_container. Also, your code calls such as http://localhost:5000 can be replaced by http://first_container:5000

If you're running with --net=host, localhost should work fine. If you're using default networking, use the static IP 172.17.0.1.
See this - https://stackoverflow.com/a/48547074/14120621

For those on Windows, assuming you're using the bridge network driver, you'll want to specifically bind MySQL to the IP address of the hyper-v network interface.
This is done via the configuration file under the normally hidden C:\ProgramData\MySQL folder.
Binding to 0.0.0.0 will not work. The address needed is shown in the docker configuration as well, and in my case was 10.0.75.1.

Edit: I ended up prototyping out the concept on GitHub. Check out: https://github.com/sivabudh/system-in-a-box
First, my answer is geared towards 2 groups of people: those who use a Mac, and those who use Linux.
The host network mode doesn't work on a Mac. You have to use an IP alias, see: https://stackoverflow.com/a/43541681/2713729
What is a host network mode? See: https://docs.docker.com/engine/reference/run/#/network-settings
Secondly, for those of you who are using Linux (my direct experience was with Ubuntu 14.04 LTS and I'm upgrading to 16.04 LTS in production soon), yes, you can make the service running inside a Docker container connect to localhost services running on the Docker host (eg. your laptop).
How?
The key is when you run the Docker container, you have to run it with the host mode. The command looks like this:
docker run --network="host" -id <Docker image ID>
When you do an ifconfig (you will need to apt-get install net-tools your container for ifconfig to be callable) inside your container, you will see that the network interfaces are the same as the one on Docker host (eg. your laptop).
It's important to note that I'm a Mac user, but I run Ubuntu under Parallels, so using a Mac is not a disadvantage. ;-)
And this is how you connect NGINX container to the MySQL running on a localhost.

First see this answer for the options that you have to fix this problem. But if you use docker-compose you can add network_mode: host to your service and then use 127.0.0.1 to connect to the local host. This is just one of the options described in the answer above. Below you can find how I modified docker-compose.yml from https://github.com/geerlingguy/php-apache-container.git:
---
version: "3"
services:
php-apache:
+ network_mode: host
image: geerlingguy/php-apache:latest
container_name: php-apache
...
+ indicates the line I added.
[Additional info] This has also worked in version 2.2. and "host" or just 'host' are both worked in docker-compose.
---
version: "2.2"
services:
php-apache:
+ network_mode: "host"
or
+ network_mode: host
...

I disagree with the answer from Thomasleveil.
Making mysql bind to 172.17.42.1 will prevent other programs using the database on the host to reach it. This will only work if all your database users are dockerized.
Making mysql bind to 0.0.0.0 will open the db to outside world, which is not only a very bad thing to do, but also contrary to what the original question author wants to do. He explicitly says "The MySql is running on localhost and not exposing a port to the outside world, so its bound on localhost"
To answer the comment from ivant
"Why not bind mysql to docker0 as well?"
This is not possible. The mysql/mariadb documentation explicitly says it is not possible to bind to several interfaces. You can only bind to 0, 1, or all interfaces.
As a conclusion, I have NOT found any way to reach the (localhost only) database on the host from a docker container. That definitely seems like a very very common pattern, but I don't know how to do it.

Try this:
version: '3.5'
services:
yourservice-here:
container_name: container_name
ports:
- "4000:4000"
extra_hosts: # <---- here
- localhost:192.168.1.202
- or-vitualhost.local:192.168.1.202
To get 192.168.1.202, uses ifconfig
This worked for me. Hope this help!

In 7 years the question was asked, it is either docker has changed, or no one tried this way. So I will include my own answer.
I have found all answers use complex methods. Today, I have needed this, and found 2 very simple ways:
use ipconfig or ifconfig on your host and make note of all IP addresses. At least two of them can be used by the container.
I have a fixed local network address on WiFi LAN Adapter: 192.168.1.101. This could be 10.0.1.101. the result will change depending on your router
I use WSL on windows, and it has its own vEthernet address: 172.19.192.1
use host.docker.internal. Most answers have this or another form of it depending on OS. The name suggests it is now globally used by docker.
A third option is to use WAN address of the machine, or in other words IP given by the service provider. However, this may not work if IP is not static, and requires routing and firewall settings.

You need to know the gateway! My solution with local server was to expose it under 0.0.0.0:8000, then run docker with subnet and run container like:
docker network create --subnet=172.35.0.0/16 --gateway 172.35.0.1 SUBNET35
docker run -d -p 4444:4444 --net SUBNET35 <container-you-want-run-place-here>
So, now you can access your loopback through http://172.35.0.1:8000

Connect to the gateway address.
❯ docker network inspect bridge | grep Gateway
"Gateway": "172.17.0.1"
Make sure the process on the host is listening on this interface or on all interfaces and is started after docker. If using systemd, you can add the below to make sure it is started after docker.
[Unit]
After=docker.service
Example
❯ python -m http.server &> /dev/null &
[1] 149976
❯ docker run --rm python python -c "from urllib.request import urlopen;print(b'Directory listing for' in urlopen('http://172.17.0.1:8000').read())"
True

Here is my solution : it works for my case
set local mysql server to public access by comment
#bind-address = 127.0.0.1
in /etc/mysql/mysql.conf.d
restart mysql server
sudo /etc/init.d/mysql restart
run the following command to open user root access any host
mysql -uroot -proot
GRANT ALL PRIVILEGES ON *.* TO 'root'#'%' IDENTIFIED BY 'root' WITH
GRANT OPTION;
FLUSH PRIVILEGES;
create sh script : run_docker.sh
#!bin/bash
HOSTIP=`ip -4 addr show scope global dev eth0 | grep inet | awk '{print \$2}' | cut -d / -f 1`
docker run -it -d --name web-app \
--add-host=local:${HOSTIP} \
-p 8080:8080 \
-e DATABASE_HOST=${HOSTIP} \
-e DATABASE_PORT=3306 \
-e DATABASE_NAME=demo \
-e DATABASE_USER=root \
-e DATABASE_PASSWORD=root \
sopheamak/springboot_docker_mysql
run with docker-composer
version: '2.1'
services:
tomcatwar:
extra_hosts:
- "local:10.1.2.232"
image: sopheamak/springboot_docker_mysql
ports:
- 8080:8080
environment:
- DATABASE_HOST=local
- DATABASE_USER=root
- DATABASE_PASSWORD=root
- DATABASE_NAME=demo
- DATABASE_PORT=3306

You can get the host ip using alpine image
docker run --rm alpine ip route | awk 'NR==1 {print $3}'
This would be more consistent as you're always using alpine to run the command.
Similar to Mariano's answer you can use same command to set an environment variable
DOCKER_HOST=$(docker run --rm alpine ip route | awk 'NR==1 {print $3}') docker-compose up

you can use net alias for your machine
OSX
sudo ifconfig lo0 alias 123.123.123.123/24 up
LINUX
sudo ifconfig lo:0 123.123.123.123 up
then from the container you can see the machine by 123.123.123.123

The CGroups and Namespaces are playing major role in the Container Ecosystem.
Namespace provide a layer of isolation. Each container runs in a separate namespace and its access is limited to that namespace. The Cgroups controls the resource utilization of each container, whereas Namespace controls what a process can see and access the respective resource.
Here is the basic understanding of the solution approach you could follow,
Use Network Namespace
When a container spawns out of image, a network interface is defined and create. This gives the container unique IP address and interface.
$ docker run -it alpine ifconfig
By changing the namespace to host, cotainers networks does not remain isolated to its interface, the process will have access to host machines network interface.
$ docker run -it --net=host alpine ifconfig
If the process listens on ports, they'll be listened on the host interface and mapped to the container.
Use PID Namespace
By changing the Pid namespace allows a container to interact with other process beyond its normal scope.
This container will run in its own namespace.
$ docker run -it alpine ps aux
By changing the namespace to the host, the container can also see all the other processes running on the system.
$ docker run -it --pid=host alpine ps aux
Sharing Namespace
This is a bad practice to do this in production because you are breaking out of the container security model which might open up for vulnerabilities, and easy access to eavesdropper. This is only for debugging tools and understating the loopholes in container security.
The first container is nginx server. This will create a new network and process namespace. This container will bind itself to port 80 of newly created network interface.
$ docker run -d --name http nginx:alpine
Another container can now reuse this namespace,
$ docker run --net=container:http mohan08p/curl curl -s localhost
Also, this container can see the interface with the processes in a shared container.
$ docker run --pid=container:http alpine ps aux
This will allow you give more privileges to containers without changing or restarting the application. In the similar way you can connect to mysql on host, run and debug your application. But, its not recommend to go by this way. Hope it helps.

Until fix is not merged into master branch, to get host IP just run from inside of the container:
ip -4 route list match 0/0 | cut -d' ' -f3
(as suggested by #Mahoney here).

I solved it by creating a user in MySQL for the container's ip:
$ sudo mysql<br>
mysql> create user 'username'#'172.17.0.2' identified by 'password';<br>
Query OK, 0 rows affected (0.00 sec)
mysql> grant all privileges on database_name.* to 'username'#'172.17.0.2' with grant option;<br>
Query OK, 0 rows affected (0.00 sec)
$ sudo vim /etc/mysql/mysql.conf.d/mysqld.cnf
<br>bind-address = 172.17.0.1
$ sudo systemctl restart mysql.service
Then on container: jdbc:mysql://<b>172.17.0.1</b>:3306/database_name

how to enable containers from docker-compose "bridge" network to reach out to service running on Docker host? [duplicate]

I have a Nginx running inside a docker container. I have a MySql running on the host system. I want to connect to the MySql from within my container. MySql is only binding to the localhost device.
Is there any way to connect to this MySql or any other program on localhost from within this docker container?
This question is different from "How to get the IP address of the docker host from inside a docker container" due to the fact that the IP address of the docker host could be the public IP or the private IP in the network which may or may not be reachable from within the docker container (I mean public IP if hosted at AWS or something). Even if you have the IP address of the docker host it does not mean you can connect to docker host from within the container given that IP address as your Docker network may be overlay, host, bridge, macvlan, none etc which restricts the reachability of that IP address.

Use
host.docker.internal
instead of
localhost

I doing a hack similar to above posts of get the local IP to map to a alias name (DNS) in the container. The major problem is to get dynamically with a simple script that works both in Linux and OSX the host IP address. I did this script that works in both environments (even in Linux distribution with "$LANG" != "en_*" configured):
ifconfig | grep -E "([0-9]{1,3}\.){3}[0-9]{1,3}" | grep -v 127.0.0.1 | awk '{ print $2 }' | cut -f2 -d: | head -n1
So, using Docker Compose, the full configuration will be:
Startup script (docker-run.sh):
export DOCKERHOST=$(ifconfig | grep -E "([0-9]{1,3}\.){3}[0-9]{1,3}" | grep -v 127.0.0.1 | awk '{ print $2 }' | cut -f2 -d: | head -n1)
docker-compose -f docker-compose.yml up
docker-compose.yml:
myapp:
build: .
ports:
- "80:80"
extra_hosts:
- "dockerhost:$DOCKERHOST"
Then change http://localhost to http://dockerhost in your code.
For a more advance guide of how to customize the DOCKERHOST script, take a look at this post with a explanation of how it works.

Solution for Linux (kernel >=3.6).
Ok, your localhost server has a default docker interface docker0 with IP address 172.17.0.1. Your container started with default network settings --net="bridge".
Enable route_localnet for docker0 interface:
$ sysctl -w net.ipv4.conf.docker0.route_localnet=1
Add these rules to iptables:
$ iptables -t nat -I PREROUTING -i docker0 -d 172.17.0.1 -p tcp --dport 3306 -j DNAT --to 127.0.0.1:3306
$ iptables -t filter -I INPUT -i docker0 -d 127.0.0.1 -p tcp --dport 3306 -j ACCEPT
Create MySQL user with access from '%' that means - from anyone, excluding localhost:
CREATE USER 'user'#'%' IDENTIFIED BY 'password';
Change in your script the mysql-server address to 172.17.0.1.
From the kernel documentation:
route_localnet - BOOLEAN: Do not consider loopback addresses as martian source or destination while routing. This enables the use of 127/8 for local routing purposes (default FALSE).

This worked for me on an NGINX/PHP-FPM stack without touching any code or networking where the app's just expecting to be able to connect to localhost
Mount mysqld.sock from the host to inside the container.
Find the location of the mysql.sock file on the host running mysql:
netstat -ln | awk '/mysql(.*)?\.sock/ { print $9 }'
Mount that file to where it's expected in the docker:
docker run -v /hostpath/to/mysqld.sock:/containerpath/to/mysqld.sock
Possible locations of mysqld.sock:
/tmp/mysqld.sock
/var/run/mysqld/mysqld.sock
/var/lib/mysql/mysql.sock
/Applications/MAMP/tmp/mysql/mysql.sock # if running via MAMP

Until host.docker.internal is working for every platform, you can use my container acting as a NAT gateway without any manual setup:
https://github.com/qoomon/docker-host

Simplest solution for Mac OSX
Just use the IP address of your Mac. On the Mac run this to get the IP address and use it from within the container:
ifconfig | grep 'inet 192'| awk '{ print $2}'
As long as the server running locally on your Mac or in another docker container is listening to 0.0.0.0, the docker container will be able to reach out at that address.
If you just want to access another docker container that is listening on 0.0.0.0 you can use 172.17.0.1

Very simple and quick, check your host IP with ifconfig (linux) or ipconfig (windows) and then create a docker-compose.yml:
version: '3' # specify docker-compose version
services:
nginx:
build: ./ # specify the directory of the Dockerfile
ports:
- "8080:80" # specify port mapping
extra_hosts:
- "dockerhost:<yourIP>"
This way, your container will be able to access your host. When accessing your DB, remember to use the name you specified before, in this case dockerhost and the port of your host in which the DB is running.

Solution for Windows 10
Docker Community Edition 17.06.0-ce-win18 2017-06-28 (stable)
You can use DNS name of the host docker.for.win.localhost, to resolve to the internal IP. (Warning some sources mentioned windows but it should be win)
Overview
I needed to do something similar, that is connect from my Docker container to my localhost, which was running the Azure Storage Emulator and CosmosDB Emulator.
The Azure Storage Emulator by default listens on 127.0.0.1, while you can change the IP its bound too, I was looking for a solution that would work with default settings.
This also works for connecting from my Docker container to SQL Server and IIS, both running locally on my host with default port settings.

For windows,
I have changed the database url in spring configuration: spring.datasource.url=jdbc:postgresql://host.docker.internal:5432/apidb
Then build the image and run. It worked for me.

None of the answers worked for me when using Docker Toolbox on Windows 10 Home, but 10.0.2.2 did, since it uses VirtualBox which exposes the host to the VM on this address.

This is not an answer to the actual question. This is how I solved a similar problem. The solution comes totally from: Define Docker Container Networking so Containers can Communicate. Thanks to Nic Raboy
Leaving this here for others who might want to do REST calls between one container and another. Answers the question: what to use in place of localhost in a docker environment?
Get how your network looks like docker network ls
Create a new network docker network create -d my-net
Start the first container docker run -d -p 5000:5000 --network="my-net" --name "first_container" <MyImage1:v0.1>
Check out network settings for first container docker inspect first_container. "Networks": should have 'my-net'
Start the second container docker run -d -p 6000:6000 --network="my-net" --name "second_container" <MyImage2:v0.1>
Check out network settings for second container docker inspect second_container. "Networks": should have 'my-net'
ssh into your second container docker exec -it second_container sh or docker exec -it second_container bash.
Inside of the second container, you can ping the first container by ping first_container. Also, your code calls such as http://localhost:5000 can be replaced by http://first_container:5000

If you're running with --net=host, localhost should work fine. If you're using default networking, use the static IP 172.17.0.1.
See this - https://stackoverflow.com/a/48547074/14120621

For those on Windows, assuming you're using the bridge network driver, you'll want to specifically bind MySQL to the IP address of the hyper-v network interface.
This is done via the configuration file under the normally hidden C:\ProgramData\MySQL folder.
Binding to 0.0.0.0 will not work. The address needed is shown in the docker configuration as well, and in my case was 10.0.75.1.

Edit: I ended up prototyping out the concept on GitHub. Check out: https://github.com/sivabudh/system-in-a-box
First, my answer is geared towards 2 groups of people: those who use a Mac, and those who use Linux.
The host network mode doesn't work on a Mac. You have to use an IP alias, see: https://stackoverflow.com/a/43541681/2713729
What is a host network mode? See: https://docs.docker.com/engine/reference/run/#/network-settings
Secondly, for those of you who are using Linux (my direct experience was with Ubuntu 14.04 LTS and I'm upgrading to 16.04 LTS in production soon), yes, you can make the service running inside a Docker container connect to localhost services running on the Docker host (eg. your laptop).
How?
The key is when you run the Docker container, you have to run it with the host mode. The command looks like this:
docker run --network="host" -id <Docker image ID>
When you do an ifconfig (you will need to apt-get install net-tools your container for ifconfig to be callable) inside your container, you will see that the network interfaces are the same as the one on Docker host (eg. your laptop).
It's important to note that I'm a Mac user, but I run Ubuntu under Parallels, so using a Mac is not a disadvantage. ;-)
And this is how you connect NGINX container to the MySQL running on a localhost.

First see this answer for the options that you have to fix this problem. But if you use docker-compose you can add network_mode: host to your service and then use 127.0.0.1 to connect to the local host. This is just one of the options described in the answer above. Below you can find how I modified docker-compose.yml from https://github.com/geerlingguy/php-apache-container.git:
---
version: "3"
services:
php-apache:
+ network_mode: host
image: geerlingguy/php-apache:latest
container_name: php-apache
...
+ indicates the line I added.
[Additional info] This has also worked in version 2.2. and "host" or just 'host' are both worked in docker-compose.
---
version: "2.2"
services:
php-apache:
+ network_mode: "host"
or
+ network_mode: host
...

I disagree with the answer from Thomasleveil.
Making mysql bind to 172.17.42.1 will prevent other programs using the database on the host to reach it. This will only work if all your database users are dockerized.
Making mysql bind to 0.0.0.0 will open the db to outside world, which is not only a very bad thing to do, but also contrary to what the original question author wants to do. He explicitly says "The MySql is running on localhost and not exposing a port to the outside world, so its bound on localhost"
To answer the comment from ivant
"Why not bind mysql to docker0 as well?"
This is not possible. The mysql/mariadb documentation explicitly says it is not possible to bind to several interfaces. You can only bind to 0, 1, or all interfaces.
As a conclusion, I have NOT found any way to reach the (localhost only) database on the host from a docker container. That definitely seems like a very very common pattern, but I don't know how to do it.

Try this:
version: '3.5'
services:
yourservice-here:
container_name: container_name
ports:
- "4000:4000"
extra_hosts: # <---- here
- localhost:192.168.1.202
- or-vitualhost.local:192.168.1.202
To get 192.168.1.202, uses ifconfig
This worked for me. Hope this help!

In 7 years the question was asked, it is either docker has changed, or no one tried this way. So I will include my own answer.
I have found all answers use complex methods. Today, I have needed this, and found 2 very simple ways:
use ipconfig or ifconfig on your host and make note of all IP addresses. At least two of them can be used by the container.
I have a fixed local network address on WiFi LAN Adapter: 192.168.1.101. This could be 10.0.1.101. the result will change depending on your router
I use WSL on windows, and it has its own vEthernet address: 172.19.192.1
use host.docker.internal. Most answers have this or another form of it depending on OS. The name suggests it is now globally used by docker.
A third option is to use WAN address of the machine, or in other words IP given by the service provider. However, this may not work if IP is not static, and requires routing and firewall settings.

You need to know the gateway! My solution with local server was to expose it under 0.0.0.0:8000, then run docker with subnet and run container like:
docker network create --subnet=172.35.0.0/16 --gateway 172.35.0.1 SUBNET35
docker run -d -p 4444:4444 --net SUBNET35 <container-you-want-run-place-here>
So, now you can access your loopback through http://172.35.0.1:8000

Connect to the gateway address.
❯ docker network inspect bridge | grep Gateway
"Gateway": "172.17.0.1"
Make sure the process on the host is listening on this interface or on all interfaces and is started after docker. If using systemd, you can add the below to make sure it is started after docker.
[Unit]
After=docker.service
Example
❯ python -m http.server &> /dev/null &
[1] 149976
❯ docker run --rm python python -c "from urllib.request import urlopen;print(b'Directory listing for' in urlopen('http://172.17.0.1:8000').read())"
True

Here is my solution : it works for my case
set local mysql server to public access by comment
#bind-address = 127.0.0.1
in /etc/mysql/mysql.conf.d
restart mysql server
sudo /etc/init.d/mysql restart
run the following command to open user root access any host
mysql -uroot -proot
GRANT ALL PRIVILEGES ON *.* TO 'root'#'%' IDENTIFIED BY 'root' WITH
GRANT OPTION;
FLUSH PRIVILEGES;
create sh script : run_docker.sh
#!bin/bash
HOSTIP=`ip -4 addr show scope global dev eth0 | grep inet | awk '{print \$2}' | cut -d / -f 1`
docker run -it -d --name web-app \
--add-host=local:${HOSTIP} \
-p 8080:8080 \
-e DATABASE_HOST=${HOSTIP} \
-e DATABASE_PORT=3306 \
-e DATABASE_NAME=demo \
-e DATABASE_USER=root \
-e DATABASE_PASSWORD=root \
sopheamak/springboot_docker_mysql
run with docker-composer
version: '2.1'
services:
tomcatwar:
extra_hosts:
- "local:10.1.2.232"
image: sopheamak/springboot_docker_mysql
ports:
- 8080:8080
environment:
- DATABASE_HOST=local
- DATABASE_USER=root
- DATABASE_PASSWORD=root
- DATABASE_NAME=demo
- DATABASE_PORT=3306

You can get the host ip using alpine image
docker run --rm alpine ip route | awk 'NR==1 {print $3}'
This would be more consistent as you're always using alpine to run the command.
Similar to Mariano's answer you can use same command to set an environment variable
DOCKER_HOST=$(docker run --rm alpine ip route | awk 'NR==1 {print $3}') docker-compose up

you can use net alias for your machine
OSX
sudo ifconfig lo0 alias 123.123.123.123/24 up
LINUX
sudo ifconfig lo:0 123.123.123.123 up
then from the container you can see the machine by 123.123.123.123

The CGroups and Namespaces are playing major role in the Container Ecosystem.
Namespace provide a layer of isolation. Each container runs in a separate namespace and its access is limited to that namespace. The Cgroups controls the resource utilization of each container, whereas Namespace controls what a process can see and access the respective resource.
Here is the basic understanding of the solution approach you could follow,
Use Network Namespace
When a container spawns out of image, a network interface is defined and create. This gives the container unique IP address and interface.
$ docker run -it alpine ifconfig
By changing the namespace to host, cotainers networks does not remain isolated to its interface, the process will have access to host machines network interface.
$ docker run -it --net=host alpine ifconfig
If the process listens on ports, they'll be listened on the host interface and mapped to the container.
Use PID Namespace
By changing the Pid namespace allows a container to interact with other process beyond its normal scope.
This container will run in its own namespace.
$ docker run -it alpine ps aux
By changing the namespace to the host, the container can also see all the other processes running on the system.
$ docker run -it --pid=host alpine ps aux
Sharing Namespace
This is a bad practice to do this in production because you are breaking out of the container security model which might open up for vulnerabilities, and easy access to eavesdropper. This is only for debugging tools and understating the loopholes in container security.
The first container is nginx server. This will create a new network and process namespace. This container will bind itself to port 80 of newly created network interface.
$ docker run -d --name http nginx:alpine
Another container can now reuse this namespace,
$ docker run --net=container:http mohan08p/curl curl -s localhost
Also, this container can see the interface with the processes in a shared container.
$ docker run --pid=container:http alpine ps aux
This will allow you give more privileges to containers without changing or restarting the application. In the similar way you can connect to mysql on host, run and debug your application. But, its not recommend to go by this way. Hope it helps.

Until fix is not merged into master branch, to get host IP just run from inside of the container:
ip -4 route list match 0/0 | cut -d' ' -f3
(as suggested by #Mahoney here).

I solved it by creating a user in MySQL for the container's ip:
$ sudo mysql<br>
mysql> create user 'username'#'172.17.0.2' identified by 'password';<br>
Query OK, 0 rows affected (0.00 sec)
mysql> grant all privileges on database_name.* to 'username'#'172.17.0.2' with grant option;<br>
Query OK, 0 rows affected (0.00 sec)
$ sudo vim /etc/mysql/mysql.conf.d/mysqld.cnf
<br>bind-address = 172.17.0.1
$ sudo systemctl restart mysql.service
Then on container: jdbc:mysql://<b>172.17.0.1</b>:3306/database_name

How to get the IP address of the docker host from inside a docker container [duplicate]

This question already has answers here:
From inside of a Docker container, how do I connect to the localhost of the machine?
(41 answers)
Closed 10 months ago.
As the title says, I need to be able to retrieve the IP address the docker hosts and the portmaps from the host to the container, and doing that inside of the container.

/sbin/ip route|awk '/default/ { print $3 }'
As #MichaelNeale noticed, there is no sense to use this method in Dockerfile (except when we need this IP during build time only), because this IP will be hardcoded during build time.

As of version 18.03, you can use host.docker.internal as the host's IP.
Works in Docker for Mac, Docker for Windows, and perhaps other platforms as well.
This is an update from the Mac-specific docker.for.mac.localhost, available since version 17.06, and docker.for.mac.host.internal, available since version 17.12, which may also still work on that platform.
Note, as in the Mac and Windows documentation, this is for development purposes only.
For example, I have environment variables set on my host:
MONGO_SERVER=host.docker.internal
In my docker-compose.yml file, I have this:
version: '3'
services:
api:
build: ./api
volumes:
- ./api:/usr/src/app:ro
ports:
- "8000"
environment:
- MONGO_SERVER
command: /usr/local/bin/gunicorn -c /usr/src/app/gunicorn_config.py -w 1 -b :8000 wsgi

Update: On Docker for Mac, as of version 18.03, you can use host.docker.internal as the host's IP. See aljabear's answer. For prior versions of Docker for Mac the following answer may still be useful:
On Docker for Mac the docker0 bridge does not exist, so other answers here may not work. All outgoing traffic however, is routed through your parent host, so as long as you try to connect to an IP it recognizes as itself (and the docker container doesn't think is itself) you should be able to connect. For example if you run this from the parent machine run:
ipconfig getifaddr en0
This should show you the IP of your Mac on its current network and your docker container should be able to connect to this address as well. This is of course a pain if this IP address ever changes, but you can add a custom loopback IP to your Mac that the container doesn't think is itself by doing something like this on the parent machine:
sudo ifconfig lo0 alias 192.168.46.49
You can then test the connection from within the docker container with telnet. In my case I wanted to connect to a remote xdebug server:
telnet 192.168.46.49 9000
Now when traffic comes into your Mac addressed for 192.168.46.49 (and all the traffic leaving your container does go through your Mac) your Mac will assume that IP is itself. When you are finish using this IP, you can remove the loopback alias like this:
sudo ifconfig lo0 -alias 192.168.46.49
One thing to be careful about is that the docker container won't send traffic to the parent host if it thinks the traffic's destination is itself. So check the loopback interface inside the container if you have trouble:
sudo ip addr show lo
In my case, this showed inet 127.0.0.1/8 which means I couldn't use any IPs in the 127.* range. That's why I used 192.168.* in the example above. Make sure the IP you use doesn't conflict with something on your own network.

AFAIK, in the case of Docker for Linux (standard distribution), the IP address of the host will always be 172.17.0.1 (on the main network of docker, see comments to learn more).
The easiest way to get it is via ifconfig (interface docker0) from the host:
ifconfig
From inside a docker, the following command from a docker: ip -4 route show default | cut -d" " -f3
You can run it quickly in a docker with the following command line:
# 1. Run an ubuntu docker
# 2. Updates dependencies (quietly)
# 3. Install ip package (quietly)
# 4. Shows (nicely) the ip of the host
# 5. Removes the docker (thanks to `--rm` arg)
docker run -it --rm ubuntu:22.04 bash -c "apt-get update > /dev/null && apt-get install iproute2 -y > /dev/null && ip -4 route show default | cut -d' ' -f3"

For those running Docker in AWS, the instance meta-data for the host is still available from inside the container.
curl http://169.254.169.254/latest/meta-data/local-ipv4
For example:
$ docker run alpine /bin/sh -c "apk update ; apk add curl ; curl -s http://169.254.169.254/latest/meta-data/local-ipv4 ; echo"
fetch http://dl-cdn.alpinelinux.org/alpine/v3.3/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.3/community/x86_64/APKINDEX.tar.gz
v3.3.1-119-gb247c0a [http://dl-cdn.alpinelinux.org/alpine/v3.3/main]
v3.3.1-59-g48b0368 [http://dl-cdn.alpinelinux.org/alpine/v3.3/community]
OK: 5855 distinct packages available
(1/4) Installing openssl (1.0.2g-r0)
(2/4) Installing ca-certificates (20160104-r2)
(3/4) Installing libssh2 (1.6.0-r1)
(4/4) Installing curl (7.47.0-r0)
Executing busybox-1.24.1-r7.trigger
Executing ca-certificates-20160104-r2.trigger
OK: 7 MiB in 15 packages
172.31.27.238
$ ifconfig eth0 | grep -oP 'inet addr:\K\S+'
172.31.27.238

The only way is passing the host information as environment when you create a container
run --env <key>=<value>

The --add-host could be a more cleaner solution (but without the port part, only the host can be handled with this solution). So, in your docker run command, do something like:
docker run --add-host dockerhost:`/sbin/ip route|awk '/default/ { print $3}'` [my container]
(From https://stackoverflow.com/a/26864854/127400 )

docker network inspect bridge -f '{{range .IPAM.Config}}{{.Gateway}}{{end}}'
It's possible to retrieve it using docker network inspect

The standard best practice for most apps looking to do this automatically is: you don't. Instead you have the person running the container inject an external hostname/ip address as configuration, e.g. as an environment variable or config file. Allowing the user to inject this gives you the most portable design.
Why would this be so difficult? Because containers will, by design, isolate the application from the host environment. The network is namespaced to just that container by default, and details of the host are protected from the process running inside the container which may not be fully trusted.
There are different options depending on your specific situation:
If your container is running with host networking, then you can look at the routing table on the host directly to see the default route out. From this question the following works for me e.g.:
ip route get 1 | sed -n 's/^.*src $[0-9.]*$ .*$/\1/p'
An example showing this with host networking in a container looks like:
docker run --rm --net host busybox /bin/sh -c \
"ip route get 1 | sed -n 's/^.*src $[0-9.]*$ .*$/\1/p'"
For current versions of Docker Desktop, they injected a DNS entry into the embedded VM:
getent hosts host.docker.internal | awk '{print $1}'
With the 20.10 release, the host.docker.internal alias can also work on Linux if you run your containers with an extra option:
docker run --add-host host.docker.internal:host-gateway ...
If you are running in a cloud environment, you can check the metadata service from the cloud provider, e.g. the AWS one:
curl http://169.254.169.254/latest/meta-data/local-ipv4
If you want your external/internet address, you can query a remote service like:
curl ifconfig.co
Each of these have limitations and only work in specific scenarios. The most portable option is still to run your container with the IP address injected as a configuration, e.g. here's an option running the earlier ip command on the host and injecting it as an environment variable:
export HOST_IP=$(ip route get 1 | sed -n 's/^.*src $[0-9.]*$ .*$/\1/p')
docker run --rm -e HOST_IP busybox printenv HOST_IP

TLDR for Mac and Windows
docker run -it --rm alpine nslookup host.docker.internal
... prints the host's IP address ...
nslookup: can't resolve '(null)': Name does not resolve
Name: host.docker.internal
Address 1: 192.168.65.2
Details
On Mac and Windows, you can use the special DNS name host.docker.internal.
The host has a changing IP address (or none if you have no network access). From 18.03 onwards our recommendation is to connect to the special DNS name host.docker.internal, which resolves to the internal IP address used by the host. This is for development purpose and will not work in a production environment outside of Docker Desktop for Mac.

If you want real IP address (not a bridge IP) on Windows and you have docker 18.03 (or more recent) do the following:
Run bash on container from host where image name is nginx (works on Alpine Linux distribution):
docker run -it nginx /bin/ash
Then run inside container
/ # nslookup host.docker.internal
Name: host.docker.internal
Address 1: 192.168.65.2
192.168.65.2 is the host's IP - not the bridge IP like in spinus accepted answer.
I am using here host.docker.internal:
The host has a changing IP address (or none if you have no network access). From 18.03 onwards our recommendation is to connect to the special DNS name host.docker.internal, which resolves to the internal IP address used by the host. This is for development purpose and will not work in a production environment outside of Docker for Windows.

In linux you can run
HOST_IP=`hostname -I | awk '{print $1}'`
In macOS your host machine is not the Docker host. Docker will install it's host OS in VirtualBox.
HOST_IP=`docker run busybox ping -c 1 docker.for.mac.localhost | awk 'FNR==2 {print $4}' | sed s'/.$//'`

I have Ubuntu 16.03. For me
docker run --add-host dockerhost:`/sbin/ip route|awk '/default/ { print $3}'` [image]
does NOT work (wrong ip was generating)
My working solution was that:
docker run --add-host dockerhost:`docker network inspect --format='{{range .IPAM.Config}}{{.Gateway}}{{end}}' bridge` [image]

Docker for Mac
I want to connect from a container to a service on the host
The host has a changing IP address (or none if you have no network access). From 18.03 onwards our recommendation is to connect to the special DNS name host.docker.internal, which resolves to the internal IP address used by the host.
The gateway is also reachable as gateway.docker.internal.
https://docs.docker.com/docker-for-mac/networking/#use-cases-and-workarounds

If you enabled the docker remote API (via -Htcp://0.0.0.0:4243 for instance) and know the host machine's hostname or IP address this can be done with a lot of bash.
Within my container's user's bashrc:
export hostIP=$(ip r | awk '/default/{print $3}')
export containerID=$(awk -F/ '/docker/{print $NF;exit;}' /proc/self/cgroup)
export proxyPort=$(
curl -s http://$hostIP:4243/containers/$containerID/json |
node -pe 'JSON.parse(require("fs").readFileSync("/dev/stdin").toString()).NetworkSettings.Ports["DESIRED_PORT/tcp"][0].HostPort'
)
The second line grabs the container ID from your local /proc/self/cgroup file.
Third line curls out to the host machine (assuming you're using 4243 as docker's port) then uses node to parse the returned JSON for the DESIRED_PORT.

My solution:
docker run --net=host
then in docker container:
hostname -I | awk '{print $1}'

Here is another option for those running Docker in AWS. This option avoids having using apk to add the curl package and saves the precious 7mb of space. Use the built-in wget (part of the monolithic BusyBox binary):
wget -q -O - http://169.254.169.254/latest/meta-data/local-ipv4

use hostname -I command on the terminal

Try this:
docker run --rm -i --net=host alpine ifconfig

So... if you are running your containers using a Rancher server, Rancher v1.6 (not sure if 2.0 has this) containers have access to http://rancher-metadata/ which has a lot of useful information.
From inside the container the IP address can be found here:
curl http://rancher-metadata/latest/self/host/agent_ip
For more details see:
https://rancher.com/docs/rancher/v1.6/en/rancher-services/metadata-service/

This is a minimalistic implementation in Node.js for who is running the host on AWS EC2 instances, using the afore mentioned EC2 Metadata instance
const cp = require('child_process');
const ec2 = function (callback) {
const URL = 'http://169.254.169.254/latest/meta-data/local-ipv4';
// we make it silent and timeout to 1 sec
const args = [URL, '-s', '--max-time', '1'];
const opts = {};
cp.execFile('curl', args, opts, (error, stdout) => {
if (error) return callback(new Error('ec2 ip error'));
else return callback(null, stdout);
})
.on('error', (error) => callback(new Error('ec2 ip error')));
}//ec2
and used as
ec2(function(err, ip) {
if(err) console.log(err)
else console.log(ip);
})

If you are running a Windows container on a Service Fabric cluster, the host's IP address is available via the environment variable Fabric_NodeIPOrFQDN. Service Fabric environment variables

Here is how I do it. In this case, it adds a hosts entry into /etc/hosts within the docker image pointing taurus-host to my local machine IP: :
TAURUS_HOST=`ipconfig getifaddr en0`
docker run -it --rm -e MY_ENVIRONMENT='local' --add-host "taurus-host:${TAURUS_HOST}" ...
Then, from within Docker container, script can use host name taurus-host to get out to my local machine which hosts the docker container.

Maybe the container I've created is useful as well https://github.com/qoomon/docker-host
You can simply use container name dns to access host system e.g. curl http://dockerhost:9200, so no need to hassle with any IP address.

The solution I use is based on a "server" that returns the external address of the Docker host when it receives a http request.
On the "server":
1) Start jwilder/nginx-proxy
# docker run -d -p <external server port>:80 -v /var/run/docker.sock:/tmp/docker.sock:ro jwilder/nginx-proxy
2) Start ipify container
# docker run -e VIRTUAL_HOST=<external server name/address> --detach --name ipify osixia/ipify-api:0.1.0
Now when a container sends a http request to the server, e.g.
# curl http://<external server name/address>:<external server port>
the IP address of the Docker host is returned by ipify via http header "X-Forwarded-For"
Example (ipify server has name "ipify.example.com" and runs on port 80, docker host has IP 10.20.30.40):
# docker run -d -p 80:80 -v /var/run/docker.sock:/tmp/docker.sock:ro jwilder/nginx-proxy
# docker run -e VIRTUAL_HOST=ipify.example.com --detach --name ipify osixia/ipify-api:0.1.0
Inside the container you can now call:
# curl http://ipify.example.com
10.20.30.40

On Ubuntu, hostname command can be used with the following options:
-i, --ip-address addresses for the host name
-I, --all-ip-addresses all addresses for the host
For example:
$ hostname -i
172.17.0.2
To assign to the variable, the following one-liner can be used:
IP=$(hostname -i)

Another approach is based on traceroute and it's working on a Linux host for me, e.g. in a container based on Alpine:
traceroute -n 8.8.8.8 -m 4 -w 1 | awk '$1~/\d/&&$2!~/^172\./{print$2}' | head -1
It takes a moment, but lists the first hop's IP that does not start with 172. If there is no successful response, try increasing the limit on the tested hops using -m 4 argument.

With https://docs.docker.com/machine/install-machine/
a) $ docker-machine ip
b) Get the IP address of one or more machines.
$ docker-machine ip host_name
$ docker-machine ip host_name1 host_name2

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