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:

    • 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.

EXAMPLE

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.

SEE ALSO

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