resolv.conf(5)

💖 resolv.conf — resolver configuration file 💖

okay bestie, gather round, because this one is iconic — it's the resolv.conf manual page, from the File Formats Manual, and it's literally the file that tells your whole machine how to do DNS. no cap. 💅

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✨ SYNOPSIS ✨

/etc/resolv.conf

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🦋 DESCRIPTION 🦋

So the resolver is a bunch of routines living inside the C library, and their whole job is getting you access to the Internet Domain Name System (DNS). slay. 🌐

The resolver configuration file holds the info that the resolver routines read the very first time they get invoked by a process. It's designed to be human readable (we love that for us 🫶) and it's just a list of keywords with values, and those values give you all the different kinds of resolver information.

Heads up: this config file is treated as a trusted source of DNS information — peep the trust-ad option below for the deets. 👀

And if this file just… doesn't exist? 😭 Then only the name server on the local machine gets queried, and the search list contains the local domain name determined from the hostname. that's it. that's the fallback.

Here are all the different configuration options, bestie. buckle up, there's a lot.

👑 nameserver — Name server IP address

The Internet address of a name server that the resolver should query. It's either an IPv4 address (in dot notation) or an IPv6 address in colon (and possibly dot) notation as per RFC 2373. ✨

You can list up to MAXNS name servers (currently 3, see <res_state.h>), one per keyword. If there's more than one server, the resolver library queries them in the order listed. If there are no nameserver entries at all, the default is to use the name server on the local machine.

The algorithm, for the curious: try a name server, and if the query times out, try the next, until you're out of name servers, then repeat trying all the name servers until a maximum number of retries are made. it's giving persistence. 💀

🔎 search — Search list for host-name lookup.

By default, the search list has exactly one entry: the local domain name. It's determined from the local hostname returned by gethostname(2); the local domain name is taken to be everything after the first .. And finally, if the hostname doesn't contain a . at all, the root domain is assumed as the local domain name.

You can change all that by listing the desired domain search path after the search keyword, with spaces or tabs separating the names. Resolver queries that have fewer than ndots dots in them (default is 1) will be attempted using each component of the search path in turn until a match is found. 🎯

For environments with multiple subdomains, please read options ndots:n below to avoid man-in-the-middle attacks and unnecessary traffic for the root-dns-servers. not me being dramatic, this is literally the manpage talking. 😭

Note that this process may be slow and will generate a lot of network traffic if the servers for the listed domains aren't local, and that queries will time out if no server is available for one of the domains.

If there are multiple search directives, only the search list from the last instance is used. last one wins, periodt. ✋

In glibc 2.25 and earlier, the search list is limited to six domains with a total of 256 characters. Since glibc 2.26, the search list is unlimited. glow up. 🌈

The domain directive is an obsolete name for the search directive that handles one search list entry only.

🗂️ sortlist

This option lets addresses returned by gethostbyname(3) get sorted. A sortlist is specified by IP-address-netmask pairs. The netmask is optional and defaults to the natural netmask of the net. The IP address and optional network pairs are separated by slashes. Up to 10 pairs may be specified. 🧚

Here's an example:

sortlist 130.155.160.0/255.255.240.0 130.155.0.0

⚙️ options

options lets certain internal resolver variables be modified. The syntax is:

options option ...

where option is one of the following:

🐛 debug

Sets RES_DEBUG in _res.options (effective only if glibc was built with debug support; see resolver(3)).

🔢 ndots:n

Sets a threshold for the number of dots which must appear in a name given to res_query(3) (see resolver(3)) before an initial absolute query will be made. The default for n is 1, meaning that if there are any dots in a name, the name will be tried first as an absolute name before any search list elements are appended to it. The value for this option is silently capped to 15. 🙈

⏱️ timeout:n

Sets the amount of time the resolver will wait for a response from a remote name server before retrying the query via a different name server. Careful bestie — this may not be the total time taken by any resolver API call, and there's no guarantee that a single resolver API call maps to a single timeout. Measured in seconds, the default is RES_TIMEOUT (currently 5, see <resolv.h>). The value for this option is silently capped to 30.

🔁 attempts:n

Sets the number of times the resolver will send a query to its name servers before giving up and returning an error to the calling application. The default is RES_DFLRETRY (currently 2, see <resolv.h>). The value for this option is silently capped to 5.

🌀 rotate

Sets RES_ROTATE in _res.options, which causes round-robin selection of name servers from among those listed. This has the effect of spreading the query load among all listed servers, rather than having all clients try the first listed server first every time. fairness icon. 👑

🚫 no-aaaa (since glibc 2.36)

Sets RES_NOAAAA in _res.options, which suppresses AAAA queries made by the stub resolver, including AAAA lookups triggered by NSS-based interfaces such as getaddrinfo(3). Only DNS lookups are affected: IPv6 data in hosts(5) is still used, getaddrinfo(3) with AI_PASSIVE will still produce IPv6 addresses, and configured IPv6 name servers are still used. To produce correct Name Error (NXDOMAIN) results, AAAA queries are translated to A queries. This option is intended preliminary for diagnostic purposes, to rule out that AAAA DNS queries have adverse impact. It is incompatible with EDNS0 usage and DNSSEC validation by applications. ⚠️

🧼 no-check-names

Sets RES_NOCHECKNAME in _res.options, which disables the modern BIND checking of incoming hostnames and mail names for invalid characters such as underscore (_), non-ASCII, or control characters.

🥀 inet6

Sets RES_USE_INET6 in _res.options. This has the effect of trying an AAAA query before an A query inside the gethostbyname(3) function, and of mapping IPv4 responses in IPv6 "tunneled form" if no AAAA records are found but an A record set exists. Since glibc 2.25, this option is deprecated — applications should use getaddrinfo(3), rather than gethostbyname(3). 💀

👻 ip6-bytestring (since glibc 2.3.4 to glibc 2.24)

Sets RES_USEBSTRING in _res.options. This causes reverse IPv6 lookups to be made using the bit-label format described in RFC 2673; if this option is not set (which is the default), then nibble format is used. This option was removed in glibc 2.25, since it relied on a backward-incompatible DNS extension that was never deployed on the Internet. rip. 🪦

🌗 ip6-dotint/no-ip6-dotint (glibc 2.3.4 to glibc 2.24)

Clear/set RES_NOIP6DOTINT in _res.options. When this option is clear (ip6-dotint), reverse IPv6 lookups are made in the (deprecated) ip6.int zone; when this option is set (no-ip6-dotint), reverse IPv6 lookups are made in the ip6.arpa zone by default. These options are available up to glibc 2.24, where no-ip6-dotint is the default. Since ip6-dotint support long ago ceased to be available on the Internet, these options were removed in glibc 2.25. 💅

🧩 edns0 (since glibc 2.6)

Sets RES_USE_EDNS0 in _res.options. This enables support for the DNS extensions described in RFC 2671. ✨

🚦 single-request (since glibc 2.10)

Sets RES_SNGLKUP in _res.options. By default, glibc performs IPv4 and IPv6 lookups in parallel since glibc 2.9. Some appliance DNS servers cannot handle these queries properly and make the requests time out. 😭 This option disables the behavior and makes glibc perform the IPv6 and IPv4 requests sequentially (at the cost of some slowdown of the resolving process).

🔌 single-request-reopen (since glibc 2.9)

Sets RES_SNGLKUPREOP in _res.options. The resolver uses the same socket for the A and AAAA requests. Some hardware mistakenly sends back only one reply. When that happens the client system will sit and wait for the second reply. 🫠 Turning this option on changes this behavior so that if two requests from the same port are not handled correctly it will close the socket and open a new one before sending the second request.

🔍 no-tld-query (since glibc 2.14)

Sets RES_NOTLDQUERY in _res.options. This option causes res_nsearch() to not attempt to resolve an unqualified name as if it were a top level domain (TLD). This option can cause problems if the site has ``localhost'' as a TLD rather than having localhost on one or more elements of the search list. This option has no effect if neither RES_DEFNAMES or RES_DNSRCH is set.

📡 use-vc (since glibc 2.14)

Sets RES_USEVC in _res.options. This option forces the use of TCP for DNS resolutions. yeet the UDP, we're going reliable today. 🔥

🛑 no-reload (since glibc 2.26)

Sets RES_NORELOAD in _res.options. This option disables automatic reloading of a changed configuration file. 🙅

• trust-ad (since glibc 2.31) — sets RES_TRUSTAD in _res.options. 💅 this one is all about the AD bit behavior of the stub resolver. So like: if a validating resolver sets the AD bit in a response, that's it saying "hey bestie, this data was verified according to the DNSSEC protocol." But here's the tea — to actually rely on that AD bit, your local system has to trust both the DNSSEC-validating resolver and the network path to it, which is exactly why an explicit opt-in is required. No vibes-based trust here. 🙅‍♀️
  • If trust-ad is active, the stub resolver sets the AD bit in outgoing DNS queries (to enable AD bit support), and preserves the AD bit in responses. ✨
  • Without this option, the AD bit is not set in queries, and it is always removed from responses before they are returned to the application. 😭
  • Which means: applications can trust the AD bit in responses if the trust-ad option has been set correctly. That "if" is doing a lot of heavy lifting, so pay attention. 👀
  • 🕰️ In glibc 2.30 and earlier, the AD is not set automatically in queries, and is passed through unchanged to applications in responses. Different era, different rules, iykyk.

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The search keyword of a system's resolv.conf file can be overridden on a per-process basis by setting the environment variable LOCALDOMAIN to a space-separated list of search domains. 🦋

The options keyword of a system's resolv.conf file can be amended on a per-process basis by setting the environment variable RES_OPTIONS to a space-separated list of resolver options as explained above under options. 🌸

The keyword and value must appear on a single line — no wrapping, no spreading it out like a text message — and the keyword (e.g., nameserver) must start the line. The value follows the keyword, separated by white space.

Lines that contain a semicolon (;) or hash character (#) in the first column are treated as comments. 💬 (yes, comment your config, it's giving responsible main character energy)

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🗂️ FILES 🗂️

• /etc/resolv.conf
• <resolv.h>

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👀 SEE ALSO 👀

• gethostbyname(3)
• resolver(3)
• host.conf(5)
• hosts(5)
• nsswitch.conf(5)
• hostname(7)
• named(8)

Name Server Operations Guide for BIND

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okay that's the whole page bestie, you ate that 🫶 this lil masterpiece comes from the 4th Berkeley Distribution, dated 2026-08-22, and lives under resolv.conf(5). go configure something iconic 💖✨