nss-myhostname

systemd
nss-myhostname 8
nss-myhostname libnss_myhostname.so.2 Hostname resolution for the locally configured system hostname

`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