nss-myhostname - Man Page

libnss_myhostname​.so​.2

nss-myhostname is a plug-in module for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc), primarily providing hostname resolution for the locally configured system hostname as returned by gethostname(2)​. The precise hostnames resolved by this module are:

• The local, configured hostname is resolved to all locally configured IP addresses ordered by their scope, or — if none are configured — the IPv4 address 127​.0​.0​.2 (which is on the local loopback) and the IPv6 address ::1 (which is the local host)​.
• The hostnames "localhost" and "localhost​.localdomain" (as well as any hostname ending in "​.localhost" or "​.localhost​.localdomain") are resolved to the IP addresses 127​.0​.0​.1 and ::1​.
• The hostname "_gateway" is resolved to all current default routing gateway addresses, ordered by their metric​. This assigns a stable hostname to the current gateway, useful for referencing it independently of the current network configuration state​.
• The hostname "_outbound" is resolved to the local IPv4 and IPv6 addresses that are most likely used for communication with other hosts​. This is the preferred source addresses of default gateways if specified, or determined by requesting a routing decision to the configured default gateways from the kernel and then using the local IP addresses selected by this decision​. This hostname is only available if there is at least one local default gateway configured​. This assigns a stable hostname to the local outbound IP addresses, useful for referencing them independently of the current network configuration state​.

Various software relies on an always-resolvable local hostname​. When using dynamic hostnames, this is traditionally achieved by patching /etc/hosts at the same time as changing the hostname​. This is problematic since it requires a writable /etc/ file system and is fragile because the file might be edited by the administrator at the same time​. With nss-myhostname enabled, changing /etc/hosts is unnecessary, and on many systems, the file becomes entirely optional​.

To activate the NSS modules, add "myhostname" to the line starting with "hosts:" in /etc/nsswitch​.conf​.

It is recommended to place "myhostname" after "files" and before "dns"​. This resolves well-known hostnames like "localhost" and the machine hostnames locally​. It is consistent with the behaviour of nss-resolve, and still allows overriding via /etc/hosts​.

Please keep in mind that nss-myhostname (and nss-resolve) also resolve in the other direction — from locally attached IP addresses to hostnames​. If you rely on that lookup being provided by DNS, you might want to order things differently​.

Here is an example /etc/nsswitch​.conf file that enables nss-myhostname correctly:

passwd:         files systemd
group:          files [SUCCESS=merge] systemd
shadow:         files systemd
gshadow:        files systemd

hosts:          mymachines resolve [!UNAVAIL=return] files myhostname dns
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis

To test, use glibc's getent(1) tool:

$ getent ahosts `hostname`
::1       STREAM omega
::1       DGRAM
::1       RAW
127​.0​.0​.2       STREAM
127​.0​.0​.2       DGRAM
127​.0​.0​.2       RAW

In this case, the local hostname is omega​.

systemd(1), nss-systemd(8), nss-resolve(8), nss-mymachines(8), nsswitch.conf(5), getent(1)

nss-mymachines(8), nss-resolve(8), nss-systemd(8), org.freedesktop.hostname1(5), systemd.directives(7), systemd.index(7).

The man page libnss_myhostname.so.2(8) is an alias of nss-myhostname(8).