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