hostname(5)

.SH "NAME" hostname - Local hostname configuration file

.SH "SYNOPSIS"

.PP /etc/hostname

.SH "DESCRIPTION"

.PP 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.

.PP 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.

.PP 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.

.PP 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 sunny-red-europa-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.

.PP 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.

.PP 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.

.PP 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.

.PP Added in version 262.

.PP 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.

.SH "HOSTNAME SEMANTICS"

.PP systemd(1) and the associated tools will obtain the hostname in the following ways:

.PP 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.

.SH "HISTORY"

.PP The simple configuration file format of /etc/hostname originates from Debian GNU/Linux.

.SH "SEE ALSO"

.PP systemd(1), sethostname(2), hostname(1), hostname(7), machine-id(5), machine-info(5), hostnamectl(1), systemd-hostnamed.service(8), systemd-firstboot(1)