From: Lennart Poettering <lennart@poettering.net>
Date: Wed, 10 Dec 2025 01:03:47 +0000 (+0900)
Subject: hostname: explain a bit why hostnames should be single DNS labels, not fqdns
X-Git-Tag: v259-rc3~10
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=4362d942c4e1bcefb76e928e2f42ebe207a4ed03;p=thirdparty%2Fsystemd.git

hostname: explain a bit why hostnames should be single DNS labels, not fqdns
---

diff --git a/man/hostname.xml b/man/hostname.xml
index 76acb8d7a5c..20f00057fdc 100644
--- a/man/hostname.xml
+++ b/man/hostname.xml
@@ -38,10 +38,12 @@
 
     <para>The file should contain a single newline-terminated hostname string. Comments (lines starting with
     a <literal>#</literal>) are ignored. The hostname should be composed of up to 64 7-bit ASCII lower-case
-    alphanumeric characters or hyphens forming a valid DNS domain name. It is recommended that this name
-    contains only a single label, i.e. without any dots. Invalid characters will be filtered out in an
-    attempt to make the name valid, but obviously it is recommended to use a valid name and not rely on this
-    filtering.</para>
+    alphanumeric characters or hyphens forming a valid DNS domain name. It is strongly recommended that this
+    name contains only a single DNS label, i.e does not contain any dots. This recommendation reflects both
+    usual expectations of applications, and the fact that the Linux kernel limits the length of the system
+    hostname to 64 (i.e. close to the maximum DNS label length of 63) rather than 255 (the maximum DNS domain
+    name length). When applied, invalid characters will be filtered out in an attempt to make the name valid,
+    but obviously it is recommended to use a valid name and not rely on this filtering.</para>
 
     <para id="question-mark-hostname-pattern">If the question mark character <literal>?</literal> appears in
     the hostname, it is automatically substituted by a hexadecimal character derived from the