Index · Directives systemd 257~rc3

Name

nss-myhostname, libnss_myhostname.so.2 — Hostname resolution for the locally configured system hostname

Synopsis

libnss_myhostname.so.2

Description

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 "file" 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.

Example

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.

See Also

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