--- title: Port-mapping plugin description: "plugins/meta/portmap/README.md" date: 2020-11-02 toc: true draft: false weight: 200 --- 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 * `masqAll` - boolean, default false. If false or omitted, the `snat` rule set up on loopback & hairpin traffic, else will `snat` all source traffic. * `markMasqBit` - int, (0-31), default 13. The mark bit to use for masquerading (see section SNAT). Cannot be set when `externalSetMarkChain` is used. (Only used by the "iptables" backend.) * `externalSetMarkChain` - string, default nil. If you already have a Masquerade mark chain (e.g. Kubernetes), specify it here. This will use that instead of creating a separate chain. When this is set, `markMasqBit` must be unspecified. (Only used by the "iptables" backend.) * `conditionsV4`, `conditionsV6` - array of strings. A list of arbitrary `iptables` or `nft` matches to add to the per-container rule. This may be useful if you wish to exclude specific IPs from port-mapping * `backend` - string. The backend ("iptables" or "nftables") to use for rules. Defaults to "iptables", unless iptables is unavailable, or nftables-specific configuration is provided (e.g., in `conditionsV4`). The plugin expects to receive the actual list of port mappings via the `portMappings` [capability argument](https://github.com/containernetworking/cni/blob/master/CONVENTIONS.md) A sample standalone config list for Kubernetes (with the file extension .conflist) might look like: ```json { "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}, } ] } ``` (Note that `"externalSetMarkChain": "KUBE-MARK-MASQ"` is [not recommended] with recent releases of Kubernetes, since that chain is considered private to kube-proxy, and may change in the future (and does not exist when using kube-proxy in "nftables" mode).) [not recommended]: https://kubernetes.io/blog/2022/09/07/iptables-chains-not-api/ A configuration file with all options set: ```json { "type": "portmap", "backend": "iptables", "capabilities": {"portMappings": true}, "snat": true, "markMasqBit": 13, "externalSetMarkChain": "CNI-HOSTPORT-SETMARK", "conditionsV4": ["!", "-d", "192.0.2.0/24"], "conditionsV6": ["!", "-d", "fc00::/7"] } ``` Or using the "nftables" backend: ```json { "type": "portmap", "backend": "nftables", "capabilities": {"portMappings": true}, "snat": true, "conditionsV4": ["ip", "daddr", "!=", "192.0.2.0/24"], "conditionsV6": ["ip6", "daddr", "!=", "fc00::/7"] } ``` ## Rule structure (iptables) The plugin sets up two sequences of chains and rules - one "primary" DNAT sequence to rewrite the destination, and one additional SNAT sequence that will masquerade traffic as needed. ### 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 172.16.30.2/24 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} -p tcp --destination-ports 8080,8043 -j CNI-DN-xxxxxx` (where xxxxxx is a function of the ContainerID and network name) `CNI-HOSTPORT-SETMARK` chain: - `-j MARK --set-xmark 0x2000/0x2000` `CNI-DN-xxxxxx` chain: - `-p tcp -s 172.16.30.0/24 --dport 8080 -j CNI-HOSTPORT-SETMARK` (masquerade hairpin traffic) - `-p tcp -s 127.0.0.1 --dport 8080 -j CNI-HOSTPORT-SETMARK` (masquerade localhost traffic) - `-p tcp --dport 8080 -j DNAT --to-destination 172.16.30.2:80` (rewrite destination) - `-p tcp -s 172.16.30.0/24 --dport 8043 -j CNI-HOSTPORT-SETMARK` - `-p tcp -s 127.0.0.1 --dport 8043 -j CNI-HOSTPORT-SETMARK` - `-p tcp --dport 8043 -j DNAT --to-destination 172.16.30.2:443` ### SNAT (Masquerade) Some packets also need to have the source address rewritten: * connections from localhost * Hairpin traffic back to the container. * Plugins whose traffic does not go through the default net namespace e.g., ipvlan,macvlan,etc. (need `masqAll` option) In the DNAT chain, a bit is set on the mark for packets that need snat. This chain performs that masquerading. By default, bit 13 is set, but this is configurable. If you are using other tools that also use the iptables mark, you should make sure this doesn't conflict. `POSTROUTING`: - `-j CNI-HOSTPORT-MASQ` `CNI-HOSTPORT-MASQ`: - `--mark 0x2000 -j MASQUERADE` ## Rule structure (nftables) The organization is slightly simpler than in the iptables case. All rules are created in the `cni_hostport` table (of the `ip` or `ip6` family, as appropriate). ### DNAT The DNAT rule rewrites the destination port and address of new connections. DNAT rules are added to the `hostports` or `hostip_hostports` chains of the `cni_hostport` table, depending on whether the mapping is for all host IPs or only for a single host IP. So, if a single container exists on IP 172.16.30.2/24 with ports 8080 and 8043 on the host forwarded to ports 80 and 443 in the container, the rules look like this: ``` table ip cni_hostport { comment "CNI portmap plugin" chain input { type nat hook input priority dstnat; jump hostports } chain output { type nat hook output priority dstnat; fib daddr type local jump hostports } chain hostports { ip protocol tcp th dport 8080 dnat ip addr . port to 172.16.30.2 . 80 ip protocol tcp th dport 8043 dnat ip addr . port to 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 (Masquerade) Some packets also need to have the source address rewritten: * connections from localhost * Hairpin traffic back to the container. * Plugins whose traffic does not go through the default net namespace e.g., ipvlan,macvlan,etc. (need `masqAll` option) Unlike the iptables backend, the nftables backend figures out the packets that need to be masqueraded without using the packet mark or an external chain. Continuing the above example: ``` table ip cni_hostport { comment "CNI portmap plugin" chain masquerading { type nat hook postrouting priority srcnat; # Hairpin traffic ip saddr 127.16.30.2 ip daddr 172.16.30.2 masquerade # Localhost hostports ip saddr 127.0.0.1 ip daddr 10.0.0.2 masquerade } } ``` ## Known issues ### Efficiency Each new connection to the host will have to traverse every rule in the chain, so large numbers of port forwards may have a performance impact. (This won't affect established connections, just the first packet.) In theory, it should be possible to use nftables sets (or ipsets with iptables) to address this problem, but for complicated technical reasons, this doesn't quite work. ### Localhost hostports Because MASQUERADE happens in POSTROUTING, packets with source ip 127.0.0.1 need to first pass a routing boundary before being masqueraded. By default, that is not allowed in Linux. So, the plugin needs 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 connections to ::1 will not be portmapped for ipv6. If you need port forwarding from localhost, your container must have an ipv4 address.