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nftables-nat.nix
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nftables-nat.nix
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{ config, lib, pkgs, ... }:
let
inherit (lib) mkIf concatMapStrings optionalString elemAt isInt;
cfg = config.networking.nat;
dest = if cfg.externalIP == null then "masquerade" else "snat to ${cfg.externalIP}";
oifExternal = optionalString (cfg.externalInterface != null)
''oifname "${cfg.externalInterface}"'';
iptablesPortsToNftables = range:
if isInt range then toString range
else let m = builtins.match "([0-9]+):([0-9]+)" range;
in if m == null then range # assume a single port, rely in input validation.
else "${elemAt m 0}-${elemAt m 1}";
in
{
config = mkIf config.networking.nat.enable {
build.debug.nftables.extraCommands = ''
table ip nat {
chain nixos-nat-pre {
# We can't match on incoming interface in POSTROUTING, so
# mark packets coming from the internal interfaces.
${concatMapStrings (iface: ''
iifname "${iface}" counter meta mark set 1
'') cfg.internalInterfaces}
}
chain nixos-nat-post {
# NAT the marked packets.
${optionalString (cfg.internalInterfaces != []) ''
${oifExternal} meta mark 1 counter ${dest}
''}
# NAT packets coming from the internal IPs.
${concatMapStrings (range: ''
${oifExternal} ip saddr ${range} counter ${dest}
'') cfg.internalIPs}
}
}
# NAT from external ports to internal ports.
${concatMapStrings (fwd:
let nftSourcePort = iptablesPortsToNftables fwd.sourcePort; in
''
add rule ip nat nixos-nat-pre \
iifname "${cfg.externalInterface}" ${fwd.proto} dport ${nftSourcePort} \
counter dnat to ${fwd.destination}
${concatMapStrings (loopbackip:
let
m = builtins.match "([0-9.]+):([0-9-]+)" fwd.destination;
destinationIP = if (m == null) then throw "bad ip:ports `${fwd.destination}'" else elemAt m 0;
destinationPorts = if (m == null) then throw "bad ip:ports `${fwd.destination}'" else elemAt m 1;
in ''
# Allow connections to ${loopbackip}:${nftSourcePort} from the host itself
add rule ip nat output \
ip daddr ${loopbackip} ${fwd.proto} dport ${nftSourcePort} \
counter dnat to ${fwd.destination}
# Allow connections to ${loopbackip}:${nftSourcePort} from other hosts behind NAT
add rule ip nat nixos-nat-pre \
ip daddr ${loopbackip} ${fwd.proto} dport ${nftSourcePort} \
counter dnat to ${fwd.destination}
add rule ip nat nixos-nat-post \
ip daddr ${destinationIP} ${fwd.proto} dport ${iptablesPortsToNftables destinationPorts} \
counter snat to ${loopbackip}
'') fwd.loopbackIPs}
'') cfg.forwardPorts}
${optionalString (cfg.dmzHost != null) ''
add rule ip nat nixos-nat-pre \
iifname "${cfg.externalInterface}" \
counter dnat to ${cfg.dmzHost}
''}
# Append our chains to the nat tables
add rule ip nat prerouting counter jump nixos-nat-pre
add rule ip nat postrouting counter jump nixos-nat-post
'';
};
}