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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, sincesystemd-resolvedsupports/etc/hostsinternally, 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 tonss-dns if systemd-resolved.service is not available.

Note thatsystemd-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 thatnss-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 betweennss-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.

Added in version 250.

$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 insystemd-resolved.service(8) for more information. This may be useful to query the "public" resource records, independent of the configuration of the local machine.

Added in version 250.

$SYSTEMD_NSS_RESOLVE_CACHE

Takes a boolean argument. When false, the cache of previously queried records will not be used bysystemd-resolved(8).

Added in version 250.

$SYSTEMD_NSS_RESOLVE_ZONE

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

Added in version 250.

$SYSTEMD_NSS_RESOLVE_TRUST_ANCHOR

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

Added in version 250.

$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 insystemd-resolved(8).

Added in version 250.

EXAMPLE

Here is an example/etc/nsswitch.conffile that enablesnss-resolve correctly:

passwd:         files systemd
group:          files [SUCCESS=merge] systemd
shadow:         files 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(7)

systemd 257.7