I am using Docker 0.7.0 to create containers on RedHat Enterprise Linux 6.5. When firewall is turned off containers can talk to outside world, but when firewall is on, container cannot be accessed from outside.

This is how I am running the docker and mapping a port from host to container

$ docker run -i  -t -p 3838:3838  shiny  "shiny-server"

Without firewall, I can access Node.js server running inside a container on port 3838 from outside network as http://servername:3838, but not with firewall turned on.

These are my default firewall rules –

# Firewall configuration written by system-config-firewall
# Manual customization of this file is not recommended.
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 443 -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 -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited

I have tried opening a port 3838 by adding a rule as below, but it does not work

-A INPUT -m state --state NEW -m tcp -p tcp --dport 3838 -j ACCEPT

Docker is creating a virtual NAT on the host machine, I am feeling that somehow firewall is blocking the packet forwarding from eth0 to docker 0

I need help in configuring iptables so that docker containers can be accessed from outside network, without need to turn off the entire firewall.

This is the output of $ifconfig (I have masked the server IP)

docker0   Link encap:Ethernet  HWaddr 00:00:00:00:00:00  
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::87d:8dff:fed0:f16d/64 Scope:Link
          RX packets:408321 errors:0 dropped:0 overruns:0 frame:0
          TX packets:681809 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:126511933 (120.6 MiB)  TX bytes:924200959 (881.3 MiB)

eth0      Link encap:Ethernet  HWaddr 00:25:64:A8:5B:8F  
          inet addr:XXX.XXX.XXX.XXX  Bcast:XXX.XXX.XXX.XXX  Mask:
          inet6 addr: XXXX::XXX:XXXX:XXXX:XXXX/64 Scope:Link
          RX packets:29786186 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1137982 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:4209047011 (3.9 GiB)  TX bytes:234657696 (223.7 MiB)

lo        Link encap:Local Loopback  
          inet addr:  Mask:
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:8444 errors:0 dropped:0 overruns:0 frame:0
          TX packets:8444 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:4701771 (4.4 MiB)  TX bytes:4701771 (4.4 MiB)

Output of $docker version :

Client version: 0.7.0
Go version (client): go1.1.2
Git commit (client): 0ff9bc1/0.7.0
Server version: 0.7.0
Git commit (server): 0ff9bc1/0.7.0
Go version (server): go1.1.2
Last stable version: 0.7.2, please update docker

Output of $docker info:

Containers: 321
Images: 278
Driver: devicemapper
 Pool Name: docker-8:17-13239310-pool
 Data file: /var/lib/docker/devicemapper/devicemapper/data
 Metadata file: /var/lib/docker/devicemapper/devicemapper/metadata
 Data Space Used: 56464.5 Mb
 Data Space Total: 102400.0 Mb
 Metadata Space Used: 59.5 Mb
 Metadata Space Total: 2048.0 Mb

I believe you also have to allow the packets in on the FORWARD chain. You'll also need to make sure the ALLOW rules you're adding come before the REJECT rules, since iptables works on a first-match-wins basis.

  • Could you please elaborate how would I do this? I have followed this tutorial, but have been unsuccessful centos.org/docs/4/html/rhel-sg-en-4/s1-firewall-ipt-fwd.html. BTW, I created a similar setup on ubuntu and changed DEFAULT_FORWRARD_POLICY from "DROP" to "ALLOW" and I seems to be able to access docker from outside. But I am unable to figure out how to do same with RedHat/CentOS – Ajinkya Kulkarni Jan 3 '14 at 1:03

I had a similar problem, and the solution was missing Masquarading - also that would not explain why it works for you with no filter rules at all.

How about you try to add the following rules:

-A FORWARD -d -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -s -i docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT
-A FORWARD -o docker0 -j REJECT --reject-with icmp-port-unreachable
-A FORWARD -i docker0 -j REJECT --reject-with icmp-port-unreachable

-A POSTROUTING -s ! -d -p tcp -j MASQUERADE --to-ports 1016-65535
-A POSTROUTING -s ! -d -p udp -j MASQUERADE --to-ports 1016-65535
  • Thanks @liquidat. Actually this issues is now fixed with a recent Docker release. I am not sure about the details, but may be they did what you just suggested. – Ajinkya Kulkarni Jun 12 '14 at 16:49
  • Ah, good to know. Still, I would be happy if you would give me at least an upvote, I'm still earning my reputation here ;-) – liquidat Jun 13 '14 at 12:44

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