Docker: Connection from inside the container to localhost:port Refused - docker

Question

I'm trying to insure the connection between the different containers and the localhost address (127.0.0.1) used with port 8040.( My web application container run using this port.)
root#a70b20fbda00:~# curl -v http://127.0.0.1
* Rebuilt URL to: http://127.0.0.1/
* Hostname was NOT found in DNS cache
* Trying 127.0.0.1...
* connect to 127.0.0.1 port 80 failed: Connection refused
* Failed to connect to 127.0.0.1 port 80: Connection refused
* Closing connection 0
curl: (7) Failed to connect to 127.0.0.1 port 80: Connection refused
This is what I get when I want to connect to localhost from inside the container
root#a70b20fbda00:~# curl -v http://127.0.0.1:8040
* Rebuilt URL to: http://127.0.0.1:8040/
* Hostname was NOT found in DNS cache
* Trying 127.0.0.1...
* connect to 127.0.0.1 port 8040 failed: Connection refused
* Failed to connect to 127.0.0.1 port 8040: Connection refused
* Closing connection 0
curl: (7) Failed to connect to 127.0.0.1 port 8040: Connection refused
About iptables in each container:
root#a70b20fbda00:~# iptables
bash: iptables: command not found
Connection between the container is good
root#635114ca18b7:~# ping 172.17.0.1
PING 172.17.0.1 (172.17.0.1) 56(84) bytes of data.
64 bytes from 172.17.0.1: icmp_seq=1 ttl=64 time=0.061 ms
64 bytes from 172.17.0.1: icmp_seq=2 ttl=64 time=0.253 ms
--- 172.17.0.1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1002ms
root#635114ca18b7:~# ping 127.0.0.1
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.080 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.100 ms
--- 127.0.0.1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
root#635114ca18b7:~# ping 172.17.0.3
PING 172.17.0.3 (172.17.0.3) 56(84) bytes of data.
64 bytes from 172.17.0.3: icmp_seq=1 ttl=64 time=0.149 ms
64 bytes from 172.17.0.3: icmp_seq=2 ttl=64 time=0.180 ms
--- 172.17.0.3 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 0.149/0.164/0.180/0.020 ms
Ping the 127.0.0.1:8040
root#635114ca18b7:~# ping 127.0.01:8040
ping: unknown host 127.0.0.1:8040
What I need to do in this case?
So the Global image that there is two containers ,
The first container contains a tomcat server that deploy my web application and it turnes perfectly.
The second is a container that need to connect to the web application
URL. http://127.0.0.1:8040/my_app

Answer 1

you will have to use docker run --network host IMAGE:TAG for achieving the desired connection
further read here
example:-
docker run --network host --name CONTAINER1 IMAGE:tag
docker run --network host --name CONTAINER2 IMAGE:tag
inside container - CONTAINER2 you will be able to access other container as host CONTAINER1
And for accessing the service you will have to do CONTAINER:

Answer 2

Based on the information provided, looks like there are two containers. If these two containers are started by docker without --net=host then each of them get two different IP addresses. Say your first container got 172.17.0.2 and the second one 172.17.0.3.
In this scenario each container gets it's own networking stack. So 127.0.0.1 refers to it's own networking stack not the same.
As pointed out by #kakabali, it's possible to run the containers with host network, sharing the networking stack of the host.
One of the other options is to use the actual IP address of the first container in the second one.
second-container# curl http://172.17.0.2
Or another option is to run the second container as the sidekick/sidecar container sharing the networking stack of the first one.
docker run --net=container:${ID_OF_FIRST_CONTAINER} ${IMAGE_SECOND}:${IMAGE_TAG_SECOND}
Or if you use links correctly:
docker run --name web -itd ${IMAGE_FIRST}:${TAG_FIRST}
docker run --link web -itd ${IMAGE_SECOND}:${TAG_SECOND}
Note: docker --link feature is deprecated.
Another option is to use container management platforms which take care of service discovery for you automatically.
PS: You cannot ping an IP address on a different port. For more info, click here.