nixos: add userspace outbound proxy options

Add per-instance `proxy` (socks/http/both) and `proxyListenAddress`.
"both" shares one address; tailscaled splits SOCKS from HTTP. Assert
addresses are unique across instances and install `tailscale-proxies`
to list them.

nix/nixos/instance.nix

@@ -71,6 +71,38 @@ in {
       '';
     };
 
+    proxy = mkOption {
+      type = types.nullOr (types.enum ["off" "socks" "http" "both"]);
+      default = null;
+      example = "socks";
+      description = ''
+        Run an outbound proxy for accessing the tailnet without a TUN
+        interface (the usual access method in userspace networking mode,
+        also works in tun mode).
+
+        `null` or `"off"` disables it. `"socks"` runs a SOCKS5 server,
+        `"http"` an outbound HTTP proxy, `"both"` runs both on the same
+        `proxyListenAddress` (tailscaled routes SOCKS-looking connections
+        to SOCKS and the rest to HTTP).
+
+        Run `tailscale-proxies` on the host to list the active proxy
+        addresses across all instances.
+
+        See <https://tailscale.com/kb/1112/userspace-networking>
+      '';
+    };
+
+    proxyListenAddress = mkOption {
+      type = types.str;
+      default = "localhost:1055";
+      example = "0.0.0.0:1055";
+      description = ''
+        `[ip]:port` the proxy binds to. Both proxies share this address
+        when `proxy = "both"`. Defaults to localhost to keep it off the
+        wire. Must be unique across enabled instances.
+      '';
+    };
+
     permitCertUid = mkOption {
       type = types.nullOr types.nonEmptyStr;
       default = null;

nix/nixos/service.nix

@@ -91,6 +91,10 @@ self: {
       "--statedir=/var/lib/${meta.stateDir}"
       "--socket=${meta.socketPath}"
     ]
+    ++ optional (cfg.proxy == "socks" || cfg.proxy == "both")
+    "--socks5-server=${cfg.proxyListenAddress}"
+    ++ optional (cfg.proxy == "http" || cfg.proxy == "both")
+    "--outbound-http-proxy-listen=${cfg.proxyListenAddress}"
     ++ cfg.extraDaemonFlags;
 
   # Build auth key query parameters string.
@@ -315,6 +319,42 @@ self: {
       '';
     };
 
+  # Whether an instance runs a proxy (null and "off" both mean disabled).
+  proxyEnabled = cfg: cfg.proxy != null && cfg.proxy != "off";
+
+  # All enabled instances running a proxy, as {instance,proxy,address} records.
+  proxyList = let
+    entry = enabled: name: cfg:
+      optional (enabled && proxyEnabled cfg) {
+        instance = name;
+        inherit (cfg) proxy;
+        address = cfg.proxyListenAddress;
+      };
+  in
+    entry singularCfg.enable "default" singularCfg
+    ++ concatMap (n: entry pluralCfg.${n}.enable n pluralCfg.${n})
+    (attrNames pluralCfg);
+
+  proxyListJson = pkgs.writeText "tailscale-proxies.json" (builtins.toJSON proxyList);
+
+  # `tailscale-proxies [--json]`: list active proxy addresses across instances.
+  proxyListPkg = pkgs.writeShellScriptBin "tailscale-proxies" ''
+    if [ "$1" = "--json" ]; then
+      cat ${proxyListJson}
+      echo
+      exit 0
+    fi
+    ${pkgs.util-linux}/bin/column -t <<'EOF'
+    INSTANCE PROXY ADDRESS
+    ${concatStringsSep "\n" (map (p: "${p.instance} ${p.proxy} ${p.address}") proxyList)}
+    EOF
+    cat <<'EOF'
+
+    # SOCKS5: socks5://ADDRESS   HTTP: http://ADDRESS
+    # e.g. curl --proxy socks5://localhost:1055 https://host.your-tailnet.ts.net
+    EOF
+  '';
+
   # Generate the JSON config file for tailscale serve set-config --all.
   # Prepends "svc:" to each service name and omits `advertised` when null
   # to match the Go opt.Bool semantics (unset = advertised by default).
@@ -426,7 +466,30 @@ in {
               }
             ]
             else []
-        ) (builtins.attrNames pluralCfg);
+        ) (builtins.attrNames pluralCfg)
+        ++ (let
+          addrOf = enabled: c:
+            optional
+            (enabled
+              && proxyEnabled c
+              && !(lib.hasSuffix ":0" c.proxyListenAddress))
+            c.proxyListenAddress;
+          proxyAddrs =
+            addrOf singularCfg.enable singularCfg
+            ++ concatMap (n: addrOf pluralCfg.${n}.enable pluralCfg.${n})
+            (attrNames pluralCfg);
+          dupes = lib.unique (lib.filter (
+            a: lib.count (x: x == a) proxyAddrs > 1
+          ) proxyAddrs);
+        in [
+          {
+            assertion = proxyAddrs == lib.unique proxyAddrs;
+            message = ''
+              Multiple Tailscale instances share a proxy listen address: ${concatStringsSep ", " dupes}.
+              Each instance with a proxy enabled needs a unique `proxyListenAddress`.
+            '';
+          }
+        ]);
 
       # ── Systemd services (singular + plural merged) ──
       systemd.services = mkMerge [
@@ -448,6 +511,7 @@ in {
               ([cfg.package] ++ attrValues (mkCliWrapper name cfg))
           )
           pluralCfg))
+        (mkIf (proxyList != []) [proxyListPkg])
       ];
 
       # ── DHCP exclusion (TUN mode only) ──