NSS-RESOLVE(8)

💅 NSS-RESOLVE(8) — the nss-resolve glow-up

okay so bestie, welcome to the nss-resolve manual page, straight out of the nss-resolve corner of the systemd world. This is nss-resolve, aka libnss_resolve.so.2, and she's literally the friend who hooks you up with hostname resolution through systemd-resolved.service. Let's get into it 🎀

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✨ NAME ✨

nss-resolve, libnss_resolve.so.2 — hostname resolution via systemd-resolved.service. That's the whole vibe. Iconic, honestly.

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🦋 SYNOPSIS 🦋

libnss_resolve.so.2

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🌸 DESCRIPTION 🌸

So nss-resolve is a plug-in module for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc), and what she does is enable glibc to resolve hostnames via the systemd-resolved(8) local network name resolution service. She's basically taking over from the nss-dns plug-in module that traditionally resolves hostnames via DNS. Slay, evolution 💅

To activate the NSS module, add "resolve [!UNAVAIL=return]" to the line starting with "hosts:" in /etc/nsswitch.conf. And 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. Order matters, bestie, no cap 🫶

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. It's giving plot twist, but it's real 🙈

Communication between nss-resolve and systemd-resolved.service takes place via the /run/systemd/resolve/io.systemd.Resolve AF_UNIX socket. That's the little private line they text on 💌

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🧚 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 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. Added in version 250.
• $SYSTEMD_NSS_RESOLVE_CACHE — takes a boolean argument. When false, the cache of previously queried records will not be used by systemd-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 in systemd-resolved(8). Added in version 250.

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⭐ EXAMPLE ⭐

Here is an example /etc/nsswitch.conf file that enables nss-resolve correctly. Copy it, screenshot it, put it on your moodboard, whatever works 💖

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

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🌈 SEE ALSO 🌈

systemd(1), systemd-resolved(8), nss-systemd(8), nss-myhostname(8), nss-mymachines(8), nsswitch.conf(5), systemd.syntax(7)

Go read those too, bestie — the lore is deep 👑

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and that's the page!! This absolutely iconic piece of documentation is from systemd 257.13, and she's dated NSS-RESOLVE(8) — periodt, go resolve some hostnames 💜🦋✨