New Daemon - Docker — ContextQMD

iptables for a new Daemon

When the daemon starts, it creates custom chains, and rules for the default bridge network.

Table filter:

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination         

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 DOCKER-USER  all  --  any    any     anywhere             anywhere            
2        0     0 DOCKER-FORWARD  all  --  any    any     anywhere             anywhere            

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination         

Chain DOCKER (1 references)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 DROP       all  --  !docker0 docker0  anywhere             anywhere            

Chain DOCKER-BRIDGE (1 references)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 DOCKER     all  --  any    docker0  anywhere             anywhere            

Chain DOCKER-CT (1 references)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 ACCEPT     all  --  any    docker0  anywhere             anywhere             ctstate RELATED,ESTABLISHED

Chain DOCKER-FORWARD (1 references)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 DOCKER-CT  all  --  any    any     anywhere             anywhere            
2        0     0 DOCKER-INTERNAL  all  --  any    any     anywhere             anywhere            
3        0     0 DOCKER-BRIDGE  all  --  any    any     anywhere             anywhere            
4        0     0 ACCEPT     all  --  docker0 any     anywhere             anywhere            

Chain DOCKER-INTERNAL (1 references)
num   pkts bytes target     prot opt in     out     source               destination         

Chain DOCKER-USER (1 references)
num   pkts bytes target     prot opt in     out     source               destination

<details> <summary>iptables commands</summary>

-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-N DOCKER
-N DOCKER-BRIDGE
-N DOCKER-CT
-N DOCKER-FORWARD
-N DOCKER-INTERNAL
-N DOCKER-USER
-A FORWARD -j DOCKER-USER
-A FORWARD -j DOCKER-FORWARD
-A DOCKER ! -i docker0 -o docker0 -j DROP
-A DOCKER-BRIDGE -o docker0 -j DOCKER
-A DOCKER-CT -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A DOCKER-FORWARD -j DOCKER-CT
-A DOCKER-FORWARD -j DOCKER-INTERNAL
-A DOCKER-FORWARD -j DOCKER-BRIDGE
-A DOCKER-FORWARD -i docker0 -j ACCEPT

</details>

The FORWARD chain's policy shown above is ACCEPT. However:

For IPv4, setupIPv4Forwarding sets the POLICY to DROP if the sysctl net.ipv4.ip_forward was not set to '1', and the daemon set it itself when an IPv4-enabled bridge network was created.
For IPv6, similar, but for sysctls "/proc/sys/net/ipv6/conf/default/forwarding" and "/proc/sys/net/ipv6/conf/all/forwarding".

The FORWARD chain rules, explained in the order they appear in the output above, are:

Unconditional jump to DOCKER-USER. This is set up by libnetwork, in setupUserChain. Docker won't add rules to the DOCKER-USER chain, it's only for user-defined rules. It's (mostly) kept at the top of the by deleting it and re-creating after each new network is created, while traffic may be running for other networks.
Unconditional jump to DOCKER-FORWARD. This is set up by libnetwork, in setupIPChains.

Once the daemon has initialised, it doesn't touch these rules. Users are free to append rules to the FORWARD chain, and they'll run after DOCKER's rules (or to the DOCKER-USER chain, for rules that run before DOCKER's).

The DOCKER-FORWARD chain contains the first stage of Docker's filter rules. Initial rules are inserted at the top of the table, then not touched. Per-network rules are appended. The DOCKER-FORWARD chain rules, explained in the order they appear in the output above, are:

Unconditional jump to DOCKER-CT. Created during driver initialisation, in setupIPChains.
Unconditional jump to DOCKER-INTERNAL. Also created during driver initialisation, in setupIPChains.
Unconditional jump to DOCKER-BRIDGE. Also created during driver initialisation, in setupIPChains.
ACCEPT any packet leaving a network, set up when the network is created, in setupIPTablesInternal. Note that this accepts any packet leaving the network that's made it through the DOCKER and isolation chains, whether the destination is external or another network.

The DOCKER-CT chain is an early ACCEPT for any RELATED,ESTABLISHED traffic to a docker bridge. It contains a conntrack ACCEPT rule for each bridge network.

DOCKER-BRIDGE has a rule for each bridge network, to jump to the DOCKER chain.

The DOCKER chain implements per-port/protocol filtering for each container.

The DOCKER chain has a single DROP rule for the bridge network, to drop any packets routed to the network that have not originated in the network. Added by setDefaultForwardRule. This means there is no dependency on the filter-FORWARD chain's default policy. Even if it is ACCEPT, packets will be dropped unless container ports/protocols are published.

The DOCKER-INTERNAL chain is for --internal networks (bridge networks that have no external access), it's unused in this example.

Table nat:

Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 DOCKER     all  --  any    any     anywhere             anywhere             ADDRTYPE match dst-type LOCAL

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 DOCKER     all  --  any    any     anywhere            !loopback/8           ADDRTYPE match dst-type LOCAL

Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 MASQUERADE  all  --  any    !docker0  172.17.0.0/16        anywhere            

Chain DOCKER (2 references)
num   pkts bytes target     prot opt in     out     source               destination

<details> <summary>iptables commands</summary>

-P PREROUTING ACCEPT
-P INPUT ACCEPT
-P OUTPUT ACCEPT
-P POSTROUTING ACCEPT
-N DOCKER
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
-A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER
-A POSTROUTING -s 172.17.0.0/16 ! -o docker0 -j MASQUERADE

</details>