router: route imgur via WireGuard tunnel

services/imgur-vpn.nix

@@ -0,0 +1,74 @@
+# services/imgur-vpn.nix — route imgur out a WireGuard tunnel, network-wide
+#
+# imgur geo-blocks UK IPs, so LAN clients need a non-UK exit for imgur only.
+# imgur.com / i.imgur.com / api.imgur.com all resolve to two Fastly anycast
+# /24s; we send just those prefixes down the tunnel and leave everything else
+# on eno1.
+#
+# ponytail: static prefixes, not a DNS-driven nftset. Other Fastly customers
+# sharing these /24s ride the tunnel too — harmless, just a longer path. If
+# that ever matters, add dnsmasq `nftset=` + fwmark policy routing instead.
+#
+# Setup (one-off, on the mediaserver):
+#   1. Fill in peerPublicKey / peerEndpoint / tunnelAddress below from your
+#      VPN provider's WireGuard config (pick a non-UK exit).
+#   2. Write the private key, no trailing newline, readable by systemd-network:
+#        install -m600 -o systemd-network -g systemd-network \
+#          /dev/stdin /var/secrets/wg-imgur.key <<< '<PRIVATE_KEY>'
+#   3. Rebuild. Verify:  curl --interface wg-imgur https://api.imgur.com/
+{ config, lib, ... }:
+let
+  # --- provider details: fill these in ---
+  peerPublicKey = "@PEER_PUBLIC_KEY@";
+  peerEndpoint  = "@HOST_OR_IP@:51820";
+  tunnelAddress = "@TUNNEL_IP@/32";   # address the provider assigned us
+
+  # imgur's Fastly anycast prefixes.
+  imgurNets = [ "199.232.192.0/24" "199.232.196.0/24" ];
+in
+{
+  config = lib.mkIf (config.networking.hostName == "FredOS-Mediaserver") {
+
+    systemd.network = {
+      netdevs."30-wg-imgur" = {
+        netdevConfig = {
+          Name = "wg-imgur";
+          Kind = "wireguard";
+        };
+        # Key read at runtime — keeps flake eval pure (hosts build from Forgejo).
+        wireguardConfig.PrivateKeyFile = "/var/secrets/wg-imgur.key";
+        wireguardPeers = [{
+          PublicKey = peerPublicKey;
+          Endpoint = peerEndpoint;
+          # Doubles as the tunnel's crypto-routing scope: only imgur goes in.
+          AllowedIPs = imgurNets;
+          PersistentKeepalive = 25;
+        }];
+      };
+
+      networks."30-wg-imgur" = {
+        matchConfig.Name = "wg-imgur";
+        address = [ tunnelAddress ];
+        # Plain main-table routes: destination-based, so no fwmark/ip-rule
+        # machinery and no rp_filter breakage on the reply path.
+        routes = map (n: { Destination = n; }) imgurNets;
+        linkConfig.RequiredForOnline = "no";
+      };
+    };
+
+    # LAN sources must be NAT'd to the tunnel address, and TCP MSS clamped —
+    # the tunnel's 1420 MTU otherwise black-holes full-size segments.
+    networking.nftables.tables.router-nat.content = lib.mkAfter ''
+      chain wg_imgur_postrouting {
+        type nat hook postrouting priority 101; policy accept;
+        oifname "wg-imgur" masquerade
+      }
+    '';
+    networking.nftables.tables.filter.content = lib.mkAfter ''
+      chain wg_imgur_mss {
+        type filter hook forward priority 0; policy accept;
+        oifname "wg-imgur" tcp flags syn tcp option maxseg size set rt mtu
+      }
+    '';
+  };
+}