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.
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" either between "resolve"
and "traditional" modules like "dns", or after them. In the first version, well-known
names like "localhost" and the machine hostname are given higher priority than the
external configuration. This is recommended when the external DNS servers and network are not absolutely
trusted. In the second version, external configuration is given higher priority and
nss-myhostname only provides a fallback mechanism. This might be suitable in closely
controlled networks, for example on a company LAN.
Here is an example /etc/nsswitch.conf file that enables
nss-myhostname correctly:
passwd: compat systemd group: compat [SUCCESS=merge] systemd shadow: compat # Either (untrusted network, see above): hosts: mymachines resolve [!UNAVAIL=return] files myhostname dns # Or (only trusted networks): hosts: mymachines resolve [!UNAVAIL=return] files dns myhostname networks: files protocols: db files services: db files ethers: db files rpc: db files netgroup: nis
To test, use glibc's getent 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.