Iptables rules are notoriously difficult to set up when Docker is running on the host, and I thought I had a definitive solution in this fantastic blog post: https://unrouted.io/2017/08/15/docker-firewall/
The configuration described in this blog post has served me well for a long time, but I'm now facing a problem I never had before.
I am running a docker container that exposes a service on port 465 on the host. Port 465 maps to port 25 in the container. Here's how to simulate such a service:
$ docker run --rm -it -p 465:25 python:3.6 python3 -m http.server 25
My problem is that I cannot access port 465 on my server from the outside:
$ curl mydomain.com:465
curl: (7) Failed to connect to mydomain.com port 465: No route to host
However, and here comes the interesting part, I do manage to access the service if the port on the host maps to the same port in the container. In other words, when I run on the host:
$ docker run --rm -it -p 465:465 python:3.6 python3 -m http.server 465
then I can access the service from the outside:
$ curl mydomain.com:465
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org...
This whole problem is due to my iptables definition: I know that because when I flush the iptables rules, I do manage to access the service from outside, whatever the port mapping.
Here are my iptable rules:
*filter
# Source: https://unrouted.io/2017/08/15/docker-firewall/
:INPUT ACCEPT [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:FILTERS - [0:0]
:DOCKER-USER - [0:0]
-F INPUT
-F DOCKER-USER
-F FILTERS
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -s 127.0.0.0/8 -j REJECT
-A INPUT -p icmp --icmp-type any -j ACCEPT
-A INPUT -j FILTERS
-A DOCKER-USER -i eth0 -j FILTERS
-A FILTERS -m state --state ESTABLISHED,RELATED -j ACCEPT
-A FILTERS -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A FILTERS -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A FILTERS -m state --state NEW -m tcp -p tcp --dport 443 -j ACCEPT
-A FILTERS -m state --state NEW -m tcp -p tcp --dport 465 -j ACCEPT
-A FILTERS -j REJECT --reject-with icmp-host-prohibited
COMMIT
How should I modify my iptables so that I can access my container from the outside, whatever the port mapping?
EDIT:
Here are the complete iptables rules in the failing scenario (465:25 mapping):
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere
REJECT all -- loopback/8 anywhere reject-with icmp-port-unreachable
ACCEPT icmp -- anywhere anywhere icmp any
FILTERS all -- anywhere anywhere
Chain FORWARD (policy DROP)
target prot opt source destination
DOCKER-USER all -- anywhere anywhere
DOCKER-ISOLATION-STAGE-1 all -- anywhere anywhere
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
DOCKER all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
DOCKER all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
DOCKER all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
Chain DOCKER (3 references)
target prot opt source destination
ACCEPT tcp -- anywhere 172.19.0.4 tcp dpt:3000
ACCEPT tcp -- anywhere 172.17.0.3 tcp dpt:smtp
Chain DOCKER-ISOLATION-STAGE-1 (1 references)
target prot opt source destination
DOCKER-ISOLATION-STAGE-2 all -- anywhere anywhere
DOCKER-ISOLATION-STAGE-2 all -- anywhere anywhere
DOCKER-ISOLATION-STAGE-2 all -- anywhere anywhere
RETURN all -- anywhere anywhere
Chain DOCKER-ISOLATION-STAGE-2 (3 references)
target prot opt source destination
DROP all -- anywhere anywhere
DROP all -- anywhere anywhere
DROP all -- anywhere anywhere
RETURN all -- anywhere anywhere
Chain DOCKER-USER (1 references)
target prot opt source destination
FILTERS all -- anywhere anywhere
Chain FILTERS (2 references)
target prot opt source destination
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:ssh
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:http
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:https
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:urd
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited
Thanks for reaching out to me on Twitter. I've actually looked into this issue before without someone else who noticed it and I think I know whats happening. In your example:
docker run --rm -it -p 465:25 python:3.6 python3 -m http.server 25
If you look at your full firewall config with iptables-save you'll see a bunch of NAT rules. You'll probably see something that looks like this in the *nat section:
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
... snip ...
-A DOCKER ! -i br-abbaabbaabba -p tcp -m tcp --dport 465 -j DNAT --to-destination 172.18.0.10:25
So this rule is executed in the PREROUTING phase and rewrites the incoming packet to look like it was always for port 25 and not port 465. And this happens before the filter tables INPUT chain runs.
So I reckon if you allowed traffic to port 25, then actually you would be able to access port 465 too. Obviously you don't want to allow access to all port 25 because that includes your host's port 25.
All the usual tricks you'd do at this point are made that much more difficult because of Docker.
Option 1
You could go down the explicit is better than implicit route and split the host vs docker rules:
*filter
:INPUT ACCEPT [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:FILTERS - [0:0]
:DOCKER-USER - [0:0]
-F INPUT
-F DOCKER-USER
-F FILTERS
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -s 127.0.0.0/8 -j REJECT
-A INPUT -p icmp --icmp-type any -j ACCEPT
# Rules for services running on the host:
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 443 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
# Rules for services running in containers:
-A DOCKER-USER -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT
# This says dport 25, but is actually 465. Yay for prerouting + NAT.
# Service on real host port 25 should still be inaccessible because DOCKER-USER
# is only accessible via `FORWARD` and not `INPUT`...
-A DOCKER-USER -i eth0 -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT
-A DOCKER-USER -j REJECT --reject-with icmp-host-prohibited
COMMIT
It's still unsatisfying that you are allowing traffic to port 25..
Option 2
I believe right now Docker doesn't put anything in *raw or *mangle so its safe to add your own rules there. Obviously there are limitations with these tables (raw is before connection tracking, mangle is only for marking connections) so thats not great either.
Option 3
Finally, the only other thing I can think the conntrack iptables module might have the answer with --ctorigdstport, but i've never tried it myself. Looking at this you could try:
iptables -A FILTERS -p tcp --dport 25 -m conntrack --ctstate NEW --ctorigdstport 465 -j ACCEPT
A bit ugly to look at, but explicit about what is happening. If you try this one and it works let me know and i'll see about writing it up / updating that blog post.
Related
I am trying to add a rule which looks like:
Chain DOCKER (2 references)
target prot opt source destination
ACCEPT tcp -- anywhere 172.18.0.8 tcp dpt:21102
I am using the following command:
sudo iptables -A DOCKER -t tcp -s anywhere -d 172.18.0.8 --dport 21102 -j ACCEPT
However, I am getting the following error:
table 'tcp' does not exist
Perhaps iptables or your kernel needs to be upgraded.
Can someone please guide me where am I going wrong ?
-t : Specifies the packet matching table such as nat, filter, raw.etc
-p : Sets the IP protocol for the rule, which can be either icmp, tcp, udp, or all.
So the command should be:
sudo iptables -A DOCKER -p tcp -s {source_ip} -d 172.18.0.8 --dport 21102 -j ACCEPT
I'm running a dockerized app on an ubuntu machine. It's a test environment so I want to limit acces to a few IP addresses. I use the following iptables rules:
iptables -I DOCKER-USER -p tcp --dport 80 -j REJECT
iptables -I DOCKER-USER -p tcp --dport 443 -j REJECT
iptables -I DOCKER-USER -p tcp --dport 3306 -j REJECT
iptables -I DOCKER-USER -s <my ip> -p tcp --dport 443 -j RETURN
iptables -I DOCKER-USER -s 172.18.0.0/16 -p tcp --dport 3306 -j RETURN
Works ok to block all traffic, except it also blocks all outgoing traffic, rendering e.g. api-calls to other systems useless. How can I block all incoming traffic on 443, and also allow outgoing traffic on 443?
You can specify rules for the DOCKER-USER chain targeting the docker interface with -i as input and -o as output.
iptables -I DOCKER-USER -i ${docker_interface} -p tcp --dport 443 -j REJECT
iptables -I DOCKER-USER -o ${docker_interface} -p tcp --dport 443 -j ACCEPT
instead of :
iptables -I DOCKER-USER -p tcp --dport 443 -j REJECT
If you're running your docker container with default bridged configuration, the ${docker_interface} should be set with bridge. If it is not the case, use the docker network ls command to retrieve it.
My application need access webservice requires white list IP.
Since I have many app servers, i end up a solution like this
iptables -t nat -A OUTPUT -p tcp --dport 80 -d a.webservices.dimension.abc.com -j DNAT --to 52.1.2.3:9999
that means outgoing connection to a.webservices.dimension.abc.com will be routed to 52.1.2.3:9999 ( a proxy server )
It works.
And now I move my application server ( ec2 vm) to Docker container. But doing this, the iptables rule doesn't work.
here is the final rules
Chain PREROUTING (policy ACCEPT)
target prot opt source destination
DOCKER all -- anywhere anywhere ADDRTYPE match dst-type LOCALChain INPUT (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
DOCKER all -- anywhere !127.0.0.0/8 ADDRTYPE match dst-type LOCAL
DNAT tcp -- anywhere 1.2.3.4 tcp dpt:http to:52.1.2.3:9999
Chain POSTROUTING (policy ACCEPT)
target prot opt source destination
MASQUERADE all -- ip-172-17-0-0.us-west-2.compute.internal/16 anywhere
MASQUERADE tcp -- ip-172-17-0-2.us-west-2.compute.internal ip-172-17-0-2.us-west-2.compute.internal tcp dpt:http
Chain DOCKER (2 references)
target prot opt source destination
LOG all -- anywhere anywhere limit: avg 2/min burst 5 LOG level warning prefix "DOCKER CHAIN "
RETURN all -- anywhere anywhere
DNAT tcp -- anywhere anywhere tcp dpt:9090 to:172.17.0.2:80
As a result, the following command d
docker exec -it some-nginx curl -iL 1.2.3.4
doesn't work
BUT
curl -iL 1.2.3.4
works
I don't know why.
Proxying outgoing connection from Docker container should be common but I haven't found out any existing solution.
Please help.
UPDATE
The following link
https://serverfault.com/questions/734526/whitelisting-outgoing-traffic-from-docker-containers
shed me some lights.
use this rule to make it work
iptables -t nat -A PREROUTING -i docker0 -p tcp a.webservices.dimension.abc.com -j DNAT --to 52.1.2.3:9999
I'm running a docker container that has the port 9000:9000 binding to the host, but I also have ufw enabled. The only ports I've allowed are 22, 80, 443.
So why is it that I'm able to connect to this container using the host's IP address? Shouldn't port 9000 be blocked by ufw?
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
14417c4f71fb dockerui/dockerui "/dockerui" 2 seconds ago Up 2 seconds 0.0.0.0:9000->9000/tcp docker_ui
root#docker:~# ufw status
Status: active
To Action From
-- ------ ----
22 ALLOW Anywhere
80 ALLOW Anywhere
443 ALLOW Anywhere
22 (v6) ALLOW Anywhere (v6)
80 (v6) ALLOW Anywhere (v6)
443 (v6) ALLOW Anywhere (v6)
Aren't all ports blocked by default when you enable ufw?
Docker silently modifies iptables. You can start the Docker daemon with the --iptables=false option by editing DOCKER_OPTS in /etc/default/docker
Docker modifies iptables and UFW is not aware of this.
This answer describes what is happening and how to fix:
https://stackoverflow.com/a/46266757
Please follow it.
adding "--iptables=false" is not a good solution
as I said here https://stackoverflow.com/a/54486957/1712906
a better solution is to add these lines to /etc/ufw/after.rules
# BEGIN UFW AND DOCKER
*filter
:ufw-user-forward - [0:0]
:DOCKER-USER - [0:0]
-A DOCKER-USER -j RETURN -s 10.0.0.0/8
-A DOCKER-USER -j RETURN -s 172.16.0.0/12
-A DOCKER-USER -j RETURN -s 192.168.0.0/16
-A DOCKER-USER -p udp -m udp --sport 53 --dport 1024:65535 -j RETURN
-A DOCKER-USER -j ufw-user-forward
-A DOCKER-USER -j DROP -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 192.168.0.0/16
-A DOCKER-USER -j DROP -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 10.0.0.0/8
-A DOCKER-USER -j DROP -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 172.16.0.0/12
-A DOCKER-USER -j DROP -p udp -m udp --dport 0:32767 -d 192.168.0.0/16
-A DOCKER-USER -j DROP -p udp -m udp --dport 0:32767 -d 10.0.0.0/8
-A DOCKER-USER -j DROP -p udp -m udp --dport 0:32767 -d 172.16.0.0/12
-A DOCKER-USER -j RETURN
COMMIT
# END UFW AND DOCKER
As mentioned by #kliew, Docker will add its own iptables chain, granting access to all exposed ports and pre-empting any rules that you define using ufw.
If you're not comfortable completely disabling Docker's control over iptables, though, there is another documented way to have your own rules respected. From the same docs:
If you need to add rules which load before Docker’s rules, add them to the DOCKER-USER chain. These rules are applied before any rules Docker creates automatically.
Source: https://docs.docker.com/network/iptables/#add-iptables-policies-before-dockers-rules
This would preclude the use of ufw, though, because ufw only operates within its own iptables chains and doesn't give you control over the DOCKER-USER chain. You'll have to do this using iptables directly.
Just realized a few days ago that Docker seems to bypass my iptable rules. I am not incredible experienced with Docker nor iptables. Tried a lot of different things the last days. Also saw that there was big change in recent docker versions with a special DOCKER-chain that should allow me to do that. However not sure what I am doing wrong but it never does what I expect it to do.
So what I want is quite simple. I want that it behaves like expected. That if I have an ACCEPT-Rule to go through and if not it gets blocked.
My iptable looked originally like that (so before my many unsuccessful attempts):
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [779:162776]
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
-A INPUT -s 1.2.3.4 -p tcp -m tcp --dport 123 -j ACCEPT
-A INPUT -j DROP
COMMIT
Hoped that it does exactly what I want. Just allow access to ports 22 and 80 and also allow port 123 from the ip 1.2.3.4. However If I create a container with "-p 123:123" everybody can access it. Can anybody help me and tell me how I have to change the above file?
Thanks!
Docker-Version: 1.6.2
Edit:
Left initially my different tries out to not overcomplicate the question. However adding at least one of them could maybe be helpful.
*nat
:PREROUTING ACCEPT [319:17164]
:INPUT ACCEPT [8:436]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [16:960]
:DOCKER - [0:0]
COMMIT
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [779:162776]
:DOCKER - [0:0]
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
-A DOCKER -s 1.2.3.4 -p tcp -m tcp --dport 123 -j ACCEPT
-A DOCKER -j DROP
-A INPUT -j DROP
COMMIT
The above kind of works. However get then a lot of other problems. For example do I get problems with container linking, DNS does not work anymore, and so on. So then end up adding a lot of additional rules to fix that issues but I get never to a state where it runs properly. So I guess there most be better and easier solution out there.
Solution:
Ended up doing more or less exactly what larsks said. Just did not add it to the FORWARD chain, I added it to the DOCKER chain instead. The problem with the FORWARD chain is that Docker adds its stuff in there when it restarts in first position. Which results in having my rules getting pushed down and not having any effect. However for the DOCKER chain it seems Docker appends only additional rules so mine stay in effect. So when I save my rules and then restart the server everything still works fine.
So now it looks more or less like that:
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [779:162776]
:DOCKER - [0:0]
# That I can access from IP 1.2.3.4
-A DOCKER -s 1.2.3.4/32 -p tcp -m tcp --dport 123 -j ACCEPT
# That I can access from other Docker containers
-A DOCKER -o docker0 -p tcp -m tcp --dport 123 -j ACCEPT
# Does not allow it for anything else
-A DOCKER -p tcp --dport 123 -j DROP
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
-A INPUT -j DROP
COMMIT
I'm not an expert on iptables but I know that if you run the container with -p 127.0.0.1:123:123 then the port won't be exposed on all interfaces, just on the loopback.
Your iptables configuration looks a little broken right now, as if you cleared it out at some point without restarting Docker. For example, you have a DOCKER chain available in both the filter and nat tables, but no rules that reference it, so rules placed in that chain will have no affect.
In general, if you want to implement iptables rules that affect your Docker containers they need to go in the FORWARD chain of the filter table. Each container has it's own ip address, which means that your host is simply accepting packets and then FORWARDing them to the container address.
Rules in the INPUT chain are only for packets with a final destination of an address on an interface in the host's global network namespace.
However, I'm not sure that iptables is actually your problem.
If you are trying to expose services in containers such that they are available to other systems, you need to publish those ports using the -p flag to docker run. You can read more about that
in this section of the documentation.
If you want to update your question with a specific example of what you are trying to accomplish I can provide a more targeted answer.
Update
It's true that when you publish a container port using -p it will generally be available to any source ip address. In order to restrict access to a published port you would need to add a new rule to your FORWARD chain. For example, if I start a web server:
docker run --name web -p 80:8080 larsks/mini-httpd
The web server in the container is now available on port 8080 on my host. If I want to block access to this port, I need to insert a rule into the FORWARD chain that blocks access to port 80 on the container ip. So first I need the container ip address:
$ web_ip=$(docker inspect --format '{{ .NetworkSettings.IPAddress }}' web)
$ echo $web_ip
172.17.0.5
The rule I create in the FORWARD chain needs to come before the rules that docker creates, so I will need to specify an explicit position:
iptables -I FORWARD 1 -d $web_ip -p tcp --dport 80 \! -s 192.168.1.10 -j DROP
This would block all traffic from hosts other than 192.168.1.10.
If you want a rule to apply to all containers, rather than a specific container, you can bind it to the docker0 interface rather than a specific ip address:
-A FORWARD -o docker0 -p tcp --dport 80 \! -s 192.168.1.10 -j DROP
This would prohibit access to port 80 on any container.
Ended up doing more or less exactly what larsks said. Just did not add
it to the FORWARD chain, I added it to the DOCKER chain instead.
I've found the same in the docs: https://docs.docker.com/v1.5/articles/networking/#the-world
Docker will not delete or modify any pre-existing rules from the
DOCKER filter chain. This allows the user to create in advance any
rules required to further restrict access to the containers.
Docker's forward rules permit all external source IPs by default. To
allow only a specific IP or network to access the containers, insert a
negated rule at the top of the DOCKER filter chain. For example, to
restrict external access such that only source IP 8.8.8.8 can access
the containers, the following rule could be added:
$ iptables -I DOCKER -i ext_if ! -s 8.8.8.8 -j DROP
To use iptables on published ports from docker containers, you need a combination of things:
DOCKER-USER table: docker uses this table for iptables rules that affect containers and is reserved specifically for user provided rules that won't be overwritten by the docker engine when it restarts.
conntrack: port forwarding can publish on one port and forward to another in the container. You can have multiple containers all listening on port 80 with different published ports on the host.
To use these, the resulting iptables rule looks like:
iptables -I DOCKER-USER -i eth0 -s 10.0.0.0/24 -p tcp \
-m conntrack --ctorigdstport 8080 -j ACCEPT
iptables -I DOCKER-USER -i eth0 ! -s 10.0.0.0/24 -p tcp \
-m conntrack --ctorigdstport 8080 -j DROP
This handles requests to the published port 8080/tcp (that's on the host, the container could be listening on 80 or any other port), and only accepts the requests from the 10.0.0.0/24 subnet. Everything outside of that subnet is dropped.
Note that the DOCKER-USER table has a default rule to immediately return, so all changes should be inserted before that default rule in the table.
Given: Debian stretch, docker 18.06, and a docker process created via
docker run ... -p 5678:1234 ...
Required: Access to the docker container restricted to multiple external subnetworks.
Solution: (using some example subnetworks)
iptables -I DOCKER-USER -p tcp --dport 1234 -j REJECT
iptables -I DOCKER-USER -s 18.204.0.0/16 -p tcp --dport 1234 -j RETURN
iptables -I DOCKER-USER -s 34.192.0.0/16 -p tcp --dport 1234 -j RETURN
iptables -I DOCKER-USER -s 35.153.0.0/16 -p tcp --dport 1234 -j RETURN
iptables -I DOCKER-USER -s 13.56.63.0/24 -p tcp --dport 1234 -j RETURN
Persist the changed rules using iptables-save:
iptables-save > /etc/iptables/rules.v4
Result:
iptables -L DOCKER-USER -n -v --line-numbers
Chain DOCKER-USER (1 references)
pkts bytes target prot opt in out source destination
0 0 RETURN tcp -- * * 13.56.63.0/24 0.0.0.0/0 tcp dpt:1234
0 0 RETURN tcp -- * * 35.153.0.0/16 0.0.0.0/0 tcp dpt:1234
0 0 RETURN tcp -- * * 34.192.0.0/16 0.0.0.0/0 tcp dpt:1234
0 0 RETURN tcp -- * * 18.204.0.0/16 0.0.0.0/0 tcp dpt:1234
0 0 REJECT tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:1234 reject-with icmp-port-unreachable
Background 1: Rules
All rules are added to the chain DOCKER-USER as recommended by the current docker documentation.
The first rule targets REJECT and will end up as the last rule, since the other rules are added on top (option -I without a position number corresponds to adding a rule at position 1). All packages with destination port 1234 reaching this rule will be rejected.
The other rules target RETURN, i.e. a package with destination port 1234 and source IP from one of the given subnetworks will be returned to the calling chain, which is the FORWARD chain.
iptables -L FORWARD -n -v
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
16471 4568K DOCKER-USER all -- * * 0.0.0.0/0 0.0.0.0/0
16413 4565K DOCKER-ISOLATION-STAGE-1 all -- * * 0.0.0.0/0 0.0.0.0/0
7173 2060K ACCEPT all -- * docker0 0.0.0.0/0 0.0.0.0/0 ctstate RELATED,ESTABLISHED
45 2340 DOCKER all -- * docker0 0.0.0.0/0 0.0.0.0/0
From the FORWARD chain, it will be processed by the DOCKER chain, where it is forwarded as desired to the docker container:
iptables -L DOCKER -n -v
Chain DOCKER (1 references)
pkts bytes target prot opt in out source destination
45 2340 ACCEPT tcp -- !docker0 docker0 0.0.0.0/0 172.17.0.2 tcp dpt:1234
Background 2: Port value
We don't use the external port value 5678 in the forward rule, because the destination port is changed via a rule automatically created by docker and applied before the forward chain is executed. See top section of /etc/iptables/rules.v4:
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
...
-A DOCKER ! -i docker0 -p tcp -m tcp --dport 5678 -j DNAT --to-destination 172.17.0.2:1234