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:
• 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 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)
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