NSS-RESOLVE(8) nss-resolve NSS-RESOLVE(8)
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
SEE ALSO
systemd(1), systemd-resolved(8), nss-systemd(8), nss-myhostname(8),
nss-mymachines(8), nsswitch.conf(5), systemd.syntax(5)
systemd 252 NSS-RESOLVE(8)
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