Networking config always goes in your system configuration. This can be done declaratively as shown in the following sections or through non-declarative tools such as NetworkManager.

Configuration

Static IP for network adapter

The following example configures a static IPv4 and IPv6 address and a default gateway for the interface ens3

networking = {
  interfaces.ens3 = {
    ipv6.addresses = [{
      address = "2a01:4f8:1c1b:16d0::1";
      prefixLength = 64;
    }];
    ipv4.addresses = [{
      address = "192.0.2.2";
      prefixLength = 24;
    }];
  };
  defaultGateway = {
    address = "192.0.2.1";
    interface = "ens3";
  };
  defaultGateway6 = {
    address = "fe80::1";
    interface = "ens3";
  };
};

Hosts file

To edit /etc/hosts just add something like this to your configuration.nix:

networking.hosts = {
  "127.0.0.2" = ["other-localhost"];
  "192.0.2.1" = ["mail.example.com" "imap.example.com"];
};

Port forwarding

In this example we're going to forward the port 80 via NAT from our internal network interface ens3 to the host 10.100.0.3 on our external interface wg0.

networking = {
  nftables = {
    enable = true;
    ruleset = ''
        table ip nat {
          chain PREROUTING {
            type nat hook prerouting priority dstnat; policy accept;
            iifname "ens3" tcp dport 80 dnat to 10.100.0.3:80
          }
        }
    '';
  };
  firewall = {
    enable = true;
    allowedTCPPorts = [ 80 ];
  };
  nat = {
    enable = true;
    internalInterfaces = [ "ens3" ];
    externalInterface = "wg0";
    forwardPorts = [
      {
        sourcePort = 80;
        proto = "tcp";
        destination = "10.100.0.3:80";
      }
    ];
  };
};

For IPv6 port forwarding, the example would look like this. Incoming connections on the address 2001:db8:: and port 80 will be forwarded to [fe80::1234:5678:9abc:def0]:80.

networking = {
  nftables = {
    enable = true;
    ruleset = ''
        table ip6 nat {
          chain PREROUTING {
            type nat hook prerouting priority dstnat; policy accept;
            iifname "ens3" ip6 daddr [2001:db8::] tcp dport 80 dnat to [fe80::1234:5678:9abc:def0]:80
          }
        }
    '';
  };
  firewall = {
    enable = true;
    allowedTCPPorts = [ 80 ];
  };
  nat = {
    enable = true;
    internalInterfaces = [ "ens3" ];
    externalInterface = "wg0";
    enableIPv6 = true;
    internalIPv6s = [ "2001:db8::/64" ];
    externalIPv6 = "fe80::1234:5678:9abc:def0";
    forwardPorts = [
      {
        sourcePort = 80;
        proto = "tcp";
        destination = "fe80::1234:5678:9abc:def0]:80";
      }
    ];
  };
};

IPv6

Prefix delegation with fixed DUID

Sometimes the hosting provider manages IPv6 networks via a so-called DUID or clientid. This snippet is required to make the network routable:

{ config, pkgs, ... }:

let
  # Get this from your hosting provider
  clientid = "00:11:22:33:44:55:66:77:88:99";
  interface = "enp2s0";
  subnet =  "56";
  network = "2001:bbb:3333:1111::/${subnet}";
  own_ip =  "2001:bbb:3333:1111::1/${subnet}";
in {
  # ... snip ...

  networking.enableIPv6 = true;
  networking.useDHCP = true;
  networking.dhcpcd.persistent = true;
  networking.dhcpcd.extraConfig = ''
    clientid "${clientid}"
    noipv6rs
    interface ${interface}
    ia_pd 1/${network} ${interface}
    static ip6_address=${own_ip}
  '';
  environment.etc."dhcpcd.duid".text = clientid;

}

Source: gleber gist for online.net IPv6 config in NixOS

Note: Recent versions of dhcpcd move the duid file to /var/db/dcpcd/duid. For that to work, you have to replace the above environment.etc line with something like:

systemd.services.dhcpcd.preStart = ''
  cp ${pkgs.writeText "duid" "<ID>"} /var/db/dhcpcd/duid
'';

IPv6-mostly

For IPv6 mostly networks the situation in Linux is a little bit dire. A 464XLAT CLAT implementation on the client device has to be running.

For example run clatd:

{
  services.clatd.enable = true;
}

Caveats:

disable IPv4 manually for DHCPv4 clients that do not accept Option 108 (IPv6-Only Preferred Option)
set NAT64 prefix manually, if client doesn't support RA/PREF64 (RFC 8781) or DNS64 (RFC 7050):

{
  services.clatd.settings = {
    plat-prefix = "64:ff9b::/96";
  };
}

clatd needs to be restarted, if the network has changed

Sources:

VLANs

Refer to networking.vlans in the manual.

Below is a complete networking example showing two interfaces, one with VLAN trunk tagging and one without.

enp2s1 is a normal network interface at 192.168.1.2 with no VLAN information.

enp2s0 is the virtual LAN trunk with two tagged VLANs, vlan100 and vlan101.

vlan100 is in the 10.1.1.X network and vlan101 is in the 10.10.10.X network.

The hostID should be unique among your machines, as mentioned in the manual.

Complete networking section example:

    networking = {
      hostId = "deadb33f";
      hostName = "nixos";
      domain = "example.com";
      dhcpcd.enable = false;
      interfaces.enp2s1.ipv4.addresses = [{
        address = "192.168.1.2";
        prefixLength = 28;
      }];
      vlans = {
        vlan100 = { id=100; interface="enp2s0"; };
        vlan101 = { id=101; interface="enp2s0"; };
      };
      interfaces.vlan100.ipv4.addresses = [{
        address = "10.1.1.2";
        prefixLength = 24;
      }];
      interfaces.vlan101.ipv4.addresses = [{
        address = "10.10.10.3";
        prefixLength = 24;
      }];
      defaultGateway = "192.168.1.1";
      nameservers = [ "1.1.1.1" "8.8.8.8" ];
    };