Manpages - nss-resolve.8

nss-resolve, libnss_resolve.so.2 - Hostname resolution via systemd-resolved.service

libnss_resolve.so.2

nss-resolve is a plug-in module for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc) enabling it to resolve hostnames via the systemd-resolved*(8) local network name resolution service. It replaces the *nss-dns plug-in module that traditionally resolves hostnames via DNS.

To activate the NSS module, add “resolve [!UNAVAIL=return]” to the line starting with “hosts:” in /etc/nsswitch.conf. Specifically, it is recommended to place “resolve” early in /etc/nsswitch.confs “hosts:” line. It should be before the “files” entry, since systemd-resolved supports /etc/hosts internally, but with caching. To the contrary, it should be after “mymachines”, to give hostnames given to local VMs and containers precedence over names received over DNS. Finally, we recommend placing “dns” somewhere after “resolve”, to fall back to nss-dns if systemd-resolved.service is not available.

Note that systemd-resolved will synthesize DNS resource records in a few cases, for example for “localhost” and the current local hostname, see *systemd-resolved*(8) for the full list. This duplicates the functionality of nss-myhostname*(8), but it is still recommended (see examples below) to keep *nss-myhostname configured in /etc/nsswitch.conf, to keep those names resolveable if systemd-resolved is not running.

Please keep in mind that nss-myhostname (and nss-resolve) also resolve in the other direction — from locally attached IP adresses to hostnames. If you rely on that lookup being provided by DNS, you might want to order things differently.

Communication between nss-resolve and systemd-resolved.service takes place via the /run/systemd/resolve/io.systemd.Resolve AF_UNIX socket.

Here is an example /etc/nsswitch.conf file that enables nss-resolve 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

*systemd*(1), *systemd-resolved*(8), *nss-systemd*(8), *nss-myhostname*(8), *nss-mymachines*(8), *nsswitch.conf*(5)