darwin.builder: use port 31022 by default

doc/builders/special/darwin-builder.section.md

@@ -4,9 +4,6 @@
 
 This requires macOS version 12.4 or later.
 
-This also requires that port 22 on your machine is free (since Nix does not
-permit specifying a non-default SSH port for builders).
-
 You will also need to be a trusted user for your Nix installation.  In other
 words, your `/etc/nix/nix.conf` should have something like:
 
@@ -50,12 +47,21 @@ To delegate builds to the remote builder, add the following options to your
 ```
 # - Replace ${ARCH} with either aarch64 or x86_64 to match your host machine
 # - Replace ${MAX_JOBS} with the maximum number of builds (pick 4 if you're not sure)
-builders = ssh-ng://builder@localhost ${ARCH}-linux /etc/nix/builder_ed25519 ${MAX_JOBS} - - - c3NoLWVkMjU1MTkgQUFBQUMzTnphQzFsWkRJMU5URTVBQUFBSUpCV2N4Yi9CbGFxdDFhdU90RStGOFFVV3JVb3RpQzVxQkorVXVFV2RWQ2Igcm9vdEBuaXhvcwo=
+builders = ssh-ng://builder@linux-builder ${ARCH}-linux /etc/nix/builder_ed25519 ${MAX_JOBS} - - - c3NoLWVkMjU1MTkgQUFBQUMzTnphQzFsWkRJMU5URTVBQUFBSUpCV2N4Yi9CbGFxdDFhdU90RStGOFFVV3JVb3RpQzVxQkorVXVFV2RWQ2Igcm9vdEBuaXhvcwo=
 
 # Not strictly necessary, but this will reduce your disk utilization
 builders-use-substitutes = true
 ```
 
+To allow Nix to connect to a builder not running on port 22, you will also need to create a new file at `/etc/ssh/ssh_config.d/100-linux-builder.conf`:
+
+```
+Host linux-builder
+  Hostname localhost
+  HostKeyAlias linux-builder
+  Port 31022
+```
+
 … and then restart your Nix daemon to apply the change:
 
 ```ShellSession

nixos/modules/profiles/macos-builder.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, ... }:
 
 let
   keysDirectory = "/var/keys";
@@ -67,9 +67,9 @@ in
        '';
     };
     hostPort = mkOption {
-      default = 22;
+      default = 31022;
       type = types.int;
-      example = 31022;
+      example = 22;
       description = ''
         The localhost host port to forward TCP to the guest port.
       '';