man 8 nss-myhostname

NSS-MYHOSTNAME(8)               nss-myhostname               NSS-MYHOSTNAME(8)

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:

       o   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).

       o   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.

       o   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.

       o   The hostname "_outbound" is resolved to the local IPv4 and IPv6
           addresses that are most likely used for communication with other
           hosts. This is 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:         compat systemd
           group:          compat [SUCCESS=merge] systemd
           shadow:         compat 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 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)

systemd 252                                                  NSS-MYHOSTNAME(8)