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

Provided by: libnss-resolve_251.4-1ubuntu7_amd64

NAME

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

SYNOPSIS

       libnss_resolve.so.2

DESCRIPTION

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.conf's "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 addresses 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.

ENVIRONMENT VARIABLES

       $SYSTEMD_NSS_RESOLVE_VALIDATE
           Takes a boolean argument. When false, cryptographic validation of resource records via
           DNSSEC will be disabled. This may be useful for testing, or when system time is known
           to be unreliable.

       $SYSTEMD_NSS_RESOLVE_SYNTHESIZE
           Takes a boolean argument. When false, synthetic records, e.g. for the local host name,
           will not be returned. See section SYNTHETIC RECORDS in systemd-resolved.service(8) for
           more information. This may be useful to query the "public" resource records,
           independent of the configuration of the local machine.

       $SYSTEMD_NSS_RESOLVE_CACHE
           Takes a boolean argument. When false, the cache of previously queried records will not
           be used by systemd-resolved.

       $SYSTEMD_NSS_RESOLVE_ZONE
           Takes a boolean argument. When false, answers using locally registered public
           LLMNR/mDNS resource records will not be returned.

       $SYSTEMD_NSS_RESOLVE_TRUST_ANCHOR
           Takes a boolean argument. When false, answers using locally configured trust anchors
           will not be used.

       $SYSTEMD_NSS_RESOLVE_NETWORK
           Takes a boolean argument. When false, answers will be returned without using the
           network, i.e. either from local sources or the cache in systemd-resolved.

EXAMPLE

       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

NAME

SYNOPSIS

DESCRIPTION

ENVIRONMENT VARIABLES

EXAMPLE

SEE ALSO