Usernet Portmap - Docker

Container on a user-defined network, with a published port

Adding a network running a container with a mapped port, equivalent to:

docker network create \
  -o com.docker.network.bridge.name=bridge1 \
  --subnet 192.0.2.0/24 --gateway 192.0.2.1 bridge1
docker run --network bridge1 -p 8080:80 --name c1 busybox

The ip docker-bridges table is updated as follows:

table ip docker-bridges {
	map filter-forward-in-jumps {
		type ifname : verdict
		elements = { "docker0" : jump filter-forward-in__docker0,
			     "bridge1" : jump filter-forward-in__bridge1 }
	}

	map filter-forward-out-jumps {
		type ifname : verdict
		elements = { "docker0" : jump filter-forward-out__docker0,
			     "bridge1" : jump filter-forward-out__bridge1 }
	}

	map nat-postrouting-in-jumps {
		type ifname : verdict
		elements = { "docker0" : jump nat-postrouting-in__docker0,
			     "bridge1" : jump nat-postrouting-in__bridge1 }
	}

	map nat-postrouting-out-jumps {
		type ifname : verdict
		elements = { "docker0" : jump nat-postrouting-out__docker0,
			     "bridge1" : jump nat-postrouting-out__bridge1 }
	}

	chain filter-FORWARD {
		type filter hook forward priority filter; policy accept;
		oifname vmap @filter-forward-in-jumps
		iifname vmap @filter-forward-out-jumps
	}

	chain nat-OUTPUT {
		type nat hook output priority dstnat; policy accept;
		ip daddr != 127.0.0.0/8 fib daddr type local counter jump nat-prerouting-and-output
	}

	chain nat-POSTROUTING {
		type nat hook postrouting priority srcnat; policy accept;
		iifname vmap @nat-postrouting-out-jumps
		oifname vmap @nat-postrouting-in-jumps
	}

	chain nat-PREROUTING {
		type nat hook prerouting priority dstnat; policy accept;
		fib daddr type local counter jump nat-prerouting-and-output
	}

	chain nat-prerouting-and-output {
		iifname != "bridge1" tcp dport 8080 counter dnat to 192.0.2.2:80 comment "DNAT"
	}

	chain raw-PREROUTING {
		type filter hook prerouting priority raw; policy accept;
		ip daddr 192.0.2.2 iifname != "bridge1" counter drop comment "DROP DIRECT ACCESS"
	}

	chain filter-forward-in__docker0 {
		ct state established,related counter accept
		iifname "docker0" counter accept comment "ICC"
		counter drop comment "UNPUBLISHED PORT DROP"
	}

	chain filter-forward-out__docker0 {
		ct state established,related counter accept
		counter accept comment "OUTGOING"
	}

	chain nat-postrouting-in__docker0 {
	}

	chain nat-postrouting-out__docker0 {
		oifname != "docker0" ip saddr 172.17.0.0/16 counter masquerade comment "MASQUERADE"
	}

	chain filter-forward-in__bridge1 {
		ct state established,related counter accept
		iifname "bridge1" counter accept comment "ICC"
		ip daddr 192.0.2.2 tcp dport 80 counter accept
		counter drop comment "UNPUBLISHED PORT DROP"
	}

	chain filter-forward-out__bridge1 {
		ct state established,related counter accept
		counter accept comment "OUTGOING"
	}

	chain nat-postrouting-in__bridge1 {
	}

	chain nat-postrouting-out__bridge1 {
		oifname != "bridge1" ip saddr 192.0.2.0/24 counter masquerade comment "MASQUERADE"
	}
}

The new network has its own set of chains, similar to the chains for docker0 (which doesn't have any containers), but ...

Published port

In chain filter-forward-in__bridge1, there's a rule to open the container's published port:

	chain filter-forward-in__bridge1 {
		ct state established,related counter accept
		iifname "bridge1" counter accept comment "ICC"
		ip daddr 192.0.2.2 tcp dport 80 counter accept
		counter drop comment "UNPUBLISHED PORT DROP"
	}

A rule in raw-PREROUTING makes sure the container's published port cannot be accessed from outside the host, because the network has the default gateway mode "nat":

	chain raw-PREROUTING {
		type filter hook prerouting priority raw; policy accept;
		ip daddr 192.0.2.2 iifname != "bridge1" counter drop comment "DROP DIRECT ACCESS"
	}

The nat-prerouting-and-output chain has a rule to DNAT from host port 8080 to the container's port 80:

	chain nat-prerouting-and-output {
		iifname != "bridge1" tcp dport 8080 counter dnat to 192.0.2.2:80 comment "DNAT"
	}