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Port-mapping plugin

plugins/meta/portmap/README.md

This plugin will forward traffic from one or more ports on the host to the container. It expects to be run as a chained plugin.

Usage

You should use this plugin as part of a network configuration list. It accepts the following configuration options:

  • snat - boolean, default true. If true or omitted, set up the SNAT chains
  • conditionsV4, conditionsV6 - array of strings. A list of arbitrary iptables matches to add to the per-container rule. This may be useful if you wish to exclude specific IPs from port-mapping

The plugin expects to receive the actual list of port mappings via the portMappings capability argument

So a sample standalone config list (with the file extension .conflist) might look like:

{
        "cniVersion": "0.3.1",
        "name": "mynet",
        "plugins": [
                {
                        "type": "ptp",
                        "ipMasq": true,
                        "ipam": {
                                "type": "host-local",
                                "subnet": "172.16.30.0/24",
                                "routes": [
                                        {
                                                "dst": "0.0.0.0/0"
                                        }
                                ]
                        }
                },
                {
                        "type": "portmap",
                        "capabilities": {"portMappings": true},
                        "snat": false,
                        "conditionsV4": ["!", "-d", "192.0.2.0/24"],
                        "conditionsV6": ["!", "-d", "fc00::/7"]
                }
        ]
}

Rule structure

The plugin sets up two sequences of chains and rules - one “primary” DNAT sequence to rewrite the destination, and one additional SNAT sequence that rewrites the source address for packets from localhost. The sequence is somewhat complex to minimize the number of rules non-forwarded packets must traverse.

DNAT

The DNAT rule rewrites the destination port and address of new connections. There is a top-level chain, CNI-HOSTPORT-DNAT which is always created and never deleted. Each plugin execution creates an additional chain for ease of cleanup. So, if a single container exists on IP 172.16.30.2 with ports 8080 and 8043 on the host forwarded to ports 80 and 443 in the container, the rules look like this:

PREROUTING, OUTPUT chains:

  • --dst-type LOCAL -j CNI-HOSTPORT-DNAT

CNI-HOSTPORT-DNAT chain:

  • ${ConditionsV4/6} -j CNI-DN-xxxxxx (where xxxxxx is a function of the ContainerID and network name)

CNI-DN-xxxxxx chain:

  • -p tcp --dport 8080 -j DNAT --to-destination 172.16.30.2:80
  • -p tcp --dport 8043 -j DNAT --to-destination 172.16.30.2:443

New connections to the host will have to traverse every rule, so large numbers of port forwards may have a performance impact. This won’t affect established connections, just the first packet.

SNAT

The SNAT rule enables port-forwarding from the localhost IP on the host. This rule rewrites (masquerades) the source address for connections from localhost. If this rule did not exist, a connection to localhost:80 would still have a source IP of 127.0.0.1 when received by the container, so no packets would respond. Again, it is a sequence of 3 chains. Because SNAT has to occur in the POSTROUTING chain, the packet has already been through the DNAT chain.

POSTROUTING:

  • -s 127.0.0.1 ! -d 127.0.0.1 -j CNI-HOSTPORT-SNAT

CNI-HOSTPORT-SNAT:

  • -j CNI-SN-xxxxx

CNI-SN-xxxxx:

  • -p tcp -s 127.0.0.1 -d 172.16.30.2 --dport 80 -j MASQUERADE
  • -p tcp -s 127.0.0.1 -d 172.16.30.2 --dport 443 -j MASQUERADE

Only new connections from the host, where the source address is 127.0.0.1 but not the destination will traverse this chain. It is unlikely that any packets will reach these rules without being SNATted, so the cost should be minimal.

Because MASQUERADE happens in POSTROUTING, it means that packets with source ip 127.0.0.1 need to pass a routing boundary. By default, that is not allowed in Linux. So, need to enable the sysctl net.ipv4.conf.IFNAME.route_localnet, where IFNAME is the name of the host-side interface that routes traffic to the container.

There is no equivalent to route_localnet for ipv6, so SNAT does not work for ipv6. If you need port forwarding from localhost, your container must have an ipv4 address.

Known issues

  • ipsets could improve efficiency
  • SNAT does not work with ipv6.