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.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.
$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(8).
$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(8).
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