diff --git a/howto/new-machine.md b/howto/new-machine.md
index 8e1d7f147e93f1b397f2c327972a7776e09322ad..15f4490a7a7ed9340c0161f9acb8cf32c5ac1562 100644
--- a/howto/new-machine.md
+++ b/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: