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NAME | SYNOPSIS | DESCRIPTION | HOSTNAME SEMANTICS | HISTORY | SEE ALSO | NOTES | COLOPHON |
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HOSTNAME(5) hostname HOSTNAME(5)
hostname - Local hostname configuration file
/etc/hostname
The /etc/hostname file configures the name of the local system.
Unless overridden as described in the next section, systemd(1)
will set this hostname during boot using the sethostname(2) system
call.
The file should contain a single newline-terminated hostname
string. Comments (lines starting with a "#") 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 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.
If the question mark character "?" appears in the hostname, it is
automatically substituted by a hexadecimal character derived from
the machine-id(5) when applied, securely and deterministically by
cryptographic hashing. Example: "foobar-????-????" will
automatically expand to "foobar-92a9-061c" or similar, depending
on the local machine ID.
In addition, the token "$" is substituted by a word picked
deterministically from a word list, again derived from the
machine-id(5) by cryptographic hashing. Each "$" is positional:
the first "$" uses the word list file named 1, the second 2, and
so on. This allows human-friendly names, for example "$-$-$-????"
might expand to "wildly-happy-octopus-92a9". The word lists are
searched for in /etc/systemd/hostname-wordlist/,
/run/systemd/hostname-wordlist/,
/usr/local/lib/systemd/hostname-wordlist/ and
/usr/lib/systemd/hostname-wordlist/ (the first directory providing
a given list wins, lists are not merged); one word per line, with
empty lines and lines starting with "#" ignored.
The word for each token is derived deterministically from the
machine ID and recomputed on every boot (the lists are not loaded
wholesale: a word is chosen by hashing to a byte offset into the
file). Consequently the word lists must be kept stable: changing a
list (adding, removing, or reordering words) may change the name a
machine already has, so installations that rely on persistent
hostnames must not modify the lists after deployment. If a
referenced list is missing the name is treated as invalid and the
built-in fallback hostname is used. The combined name space is the
product of the list sizes, so collisions follow the birthday
bound; append a few "?" characters for extra entropy when
uniqueness across a large fleet matters.
Note that hostname can be at most 64 characters long. The word
lists and the pattern should be chosen so that the longest
possible expansion (the longest words from each list plus any
literal and "?" characters and separators) stays within this
limit. An expanded name that exceeds the limit is considered
invalid and the built-in fallback hostname is used.
Because the name only stays stable as long as the pattern and word
lists are unchanged, the preferred way to obtain a stable,
generated hostname is to provide the pattern through the
firstboot.hostname credential (see systemd.system-credentials(7))
rather than placing it directly in this file. Concretely, write
the pattern to /etc/credstore/firstboot.hostname (see the
description of LoadCredential= in systemd.exec(5) for the
credential store): when firstboot runs on the live system the
credential is resolved once and the concrete result is persisted
here, so the word lists may be updated afterwards without changing
the hostname. A pattern placed directly in /etc/hostname (or
provisioned into an offline image) is instead re-evaluated on
every boot, which only really makes sense for systems that keep
the wordlist and the pattern stable.
Added in version 262.
You may use hostnamectl(1) to change the value of this file during
runtime from the command line. Use systemd-firstboot(1) to
initialize it on mounted (but not booted) system images.
systemd(1) and the associated tools will obtain the hostname in
the following ways:
• If the kernel command line parameter systemd.hostname=
specifies a valid hostname, systemd(1) will use it to set the
hostname during early boot, see kernel-command-line(7),
• Otherwise, the "static" hostname specified by /etc/hostname as
described above will be used.
• Otherwise, a transient hostname may be set during runtime, for
example based on information in a DHCP lease, see
systemd-hostnamed.service(8). Both NetworkManager[1] and
systemd-networkd.service(8) allow this. Note that
systemd-hostnamed.service(8) gives higher priority to the
static hostname, so the transient hostname will only be used
if the static hostname is not configured.
• Otherwise, a fallback hostname configured at compilation time
will be used ("localhost").
Effectively, the static hostname has higher priority than a
transient hostname, which has higher priority than the fallback
hostname. Transient hostnames are equivalent, so setting a new
transient hostname causes the previous transient hostname to be
forgotten. The hostname specified on the kernel command line is
like a transient hostname, with the exception that it has higher
priority when the machine boots. Also note that those are the
semantics implemented by systemd tools, but other programs may
also set the hostname.
The simple configuration file format of /etc/hostname originates
from Debian GNU/Linux.
systemd(1), sethostname(2), hostname(1), hostname(7),
machine-id(5), machine-info(5), hostnamectl(1),
systemd-hostnamed.service(8), systemd-firstboot(1)
1. NetworkManager
https://developer.gnome.org/NetworkManager/stable/
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2026-08-04. (At that
time, the date of the most recent commit that was found in the
repository was 2026-08-03.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 262~devel HOSTNAME(5)
Pages that refer to this page: hostname(1), hostnamectl(1), systemd-firstboot(1), machine-id(5), machine-info(5), os-release(5), kernel-command-line(7), systemd.directives(7), systemd.index(7), systemd.system-credentials(7), systemd-hostnamed.service(8)
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HTML rendering created 2026-09-09 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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