notice what happens when we pick a name too long

howto/new-machine.md

@@ -81,6 +81,12 @@ taken by the installer:
 
         fab -H root@38.229.82.108 host.rewrite-hosts chi-node-05.torproject.org 38.229.82.108
 
+    WARNING: The short hostname (e.g. `foo` in `foo.example.com`) MUST
+    NOT be longer than 21 characters, as that will crash the backup
+    server because its label will be too long:
+    
+        Sep 24 17:14:45 bacula-director-01 bacula-dir[1467]: Config error: name torproject-static-gitlab-shim-source.torproject.org-full.${Year}-${Month:p/2/0/r}-${Day:p/2/0/r}_${Hour:p/2/0/r}:${Minute:p/2/0/r} length 130 too long, max is 127
+
  3. a public IP address has been set and the host is available over
     SSH on that IP address. this can be fixed with: