Iptables rule-set so that a docker container can access a service on a host IP

Question

I have troubles accessing a host private interface (ip) from a docker container. I'm fairly certain that it's related to my Iptables rules (or perhaps routing). When I add the --net=host flag to docker run, everything works as expected. Similarly when I specify that the INPUT policy is following a liberal -P INPUT ACCEPT, things also work as I would expect. However these are undesirable and unsafe options i'd like to avoid.

Since it's not specific to my services (DNS) I've excluded that from the problem, since searching for that in combination with docker yields in a different (popular) problem area, adding noise to the search results.

Also linking of Docker containers is not a viable option, because certain containers need to be run with the --net=host option, preventing linking and I want to create a consistent situation where possible.

I have the following Iptables rules. A combination of CoreOS, Digital Ocean and Docker I assume.

-P INPUT DROP
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-N DOCKER
-A INPUT -i lo -j ACCEPT
-A INPUT -i eth1 -j ACCEPT
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p icmp -m icmp --icmp-type 0 -j ACCEPT
-A INPUT -p icmp -m icmp --icmp-type 3 -j ACCEPT
-A INPUT -p icmp -m icmp --icmp-type 11 -j ACCEPT
-A FORWARD -o docker0 -j DOCKER
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT

My (relevant) host interfaces:

3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    inet 10.129.112.210/16 brd 10.129.255.255 scope global eth1
       valid_lft forever preferred_lft forever
4: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default
    inet 172.17.42.1/16 scope global docker0
       valid_lft forever preferred_lft forever

And I run a docker container:

$ docker run --rm -it --dns=10.129.112.210 debian:jessie # Specifying the DNS is so that the public DNS servers aren't used.

At this point I want to be able to use a local service, bound on 10.129.112.210:53. So that the following should yield a reply:

$ ping google.com
^C
$ ping user.skydns.local
^C

When I run the same command from my host:

$ ping photo.skydns.localPING photo.skydns.local (10.129.112.206) 56(84) bytes of data.
64 bytes from 10.129.112.206: icmp_seq=1 ttl=64 time=0.790 ms
^C

My resolv.conf

$ cat /etc/resolv.conf
nameserver 10.129.112.210
nameserver 127.0.0.1
nameserver 8.8.8.8
nameserver 8.8.4.4

The point here is not to access public hosts, but rather internal ones, using the local DNS service available on the host (via another docker instance).

To illustrate it even further (My ascii art design skills surpass my iptables fu, so that should say enough at this point):

 ______________________________________________
|  __________________________           Host   |
| |   Docker DNS container   |                 |
|  ``````````````````````|```                  |
|                        |                     |
|     ,----------,---( private n. interface )  |
|     |          |                             |
|     |          |   ( public  n. interface )---
|     |          |                             |
|     |          |   ( loopbck n. interface )  |
|     |          |                             |
|     |          |                             |
|     |        __|_______________________      |
|     |       | Docker service container |     |
|     |        ``````````````````````````      |
|     |                                        |
|     |                                        |
| [ Local host service using DNS. ]            |
|                                              |
|______________________________________________|

  private (host) network interface: eth1 (10.129.0.0/16)
  Docker network interface: docker0 (172.17.0.0/16)

I've searched, read and applied different example Iptables configurations, but I know too little of the more "advanced" Iptables rules to understand whats going on and thus to get the desired result.

Output of iptables -t nat -nL:

Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination
DOCKER     all  --  0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type LOCAL

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
DOCKER     all  --  0.0.0.0/0           !127.0.0.0/8          ADDRTYPE match dst-type LOCAL

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
MASQUERADE  all  --  172.17.0.0/16        0.0.0.0/0

Chain DOCKER (2 references)
target     prot opt source               destination

Output of cat /proc/sys/net/ipv4/ip_forward:

1

Answer 1

Container communicates with host using docker0 interface. To allow traffic from container add:

-A INPUT -i docker0 -j ACCEPT

Answer 2

I've encountered very similar situation but adding -A INPUT -i docker0 -j ACCEPT will open all accesses over my eth0 interface of docker host to containers which is absolutely not what I intended.

And since I noticed that my container just had limited access(say only port 22) to host interface instead of totally shut down from host network, I reviewed my iptables rules and found a rule in chain IN_public_allow which should be responsible for this. The rule is -A IN_public_allow -p tcp -m tcp --dport 22 -m conntrack --ctstate NEW -j ACCEPT. So I added similar rules to allow my container to access other host ports desired, which I think could be a bit more precise way to open host network access to containers.