---
name: "resolver"
section: "5"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/resolv.conf"
---

# resolver(5)

resolver configuration file

Part of [resolv.conf(5)](/resolv.conf/5.md).

## NAME

**resolver** – resolver configuration file

## SYNOPSIS

**resolv.conf**

## DESCRIPTION

The [resolver(3)](/resolver/3.md) is a set of routines in the C library which provide access to the Internet Domain Name System. The resolver configuration file contains information that is read by the resolver routines the first time they are invoked by a process.

The file is read line by line. Anything after a ‘#’ or ‘;’ character is considered a comment and discarded. Blank lines are ignored. The first word on each non-blank line is a keyword which determines how the rest of the line is interpreted. Available keywords are:

- **nameserver** *address*

  IPv4 or IPv6 address of a name server that the resolver should query. Up to MAXNS (currently 3) name servers may be listed on separate lines. If there are multiple servers, the resolver library queries them in the order listed, unless RES\_ROTATE is in effect( See the **rotate** option below). If no **nameserver** entries are present, the default is to use the name server on the local machine. (The algorithm used is to try a name server, and if the query times out, try the next, until out of name servers, then repeat trying all the name servers until a maximum number of retries are made).

- **domain** *name*

  Set both the local domain name and the search list to *name*. Most queries for names within this domain can use short names relative to the local domain. If no domain name has been set, it is derived from the local host name returned by [gethostname(3)](/gethostname/3.md); the domain part is taken to be everything after the first ‘.’. Finally, if the host name does not contain a domain part, the root domain is assumed.

- **search** *name1* \[name2 ...\]

  Set the domain search list. Additionally, sets the local domain to *name1*. Most resolver queries will be attempted using each component of the search path in turn until a match is found. Note that this process may be slow and will generate a lot of network traffic if the servers for the listed domains are not local, and that queries will time out if no server is available for one of the domains.

  The search list is currently limited to six domains with a total of 256 characters.

- **sortlist** *addr1*\[/*plen1*\] \[addr2\[/*plen2*\] ...\]

  Sort order for query results in the form of a list of *addr*/*mask* or *addr*/*plen* entries, for instance:

  ```
  sortlist 198.51.100.0/24 203.0.113.0/255.255.255.0 2001:db8::/32
  ```

  Masks are only supported for IPv4. If the mask or prefix length is omitted, it defaults to historical class-based addressing for IPv4 and to 64 for IPv6.

  Query results that match one of the sort list entries are sorted accordingly and ahead of query results that don't. If multiple results match a single sort list entry, the order is unspecified.

  If multiple **sortlist** lines are encountered, entries are appended to the list in the order in which they were encountered, up to a maximum of ten.

- **options**

  Options allows certain internal resolver variables to be modified. The following options are supported:

  - **debug**

    Set RES\_DEBUG in *\_res.options*.

  - **usevc**

    Set RES\_USEVC to use TCP instead of UDP for queries.

  - **ndots**:*n*

    Set a threshold for the number of dots which must appear in a name given to **res\_query**() (see [resolver(3)](/resolver/3.md)) 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.

  - **timeout**:*n*

    Set the initial amount of time the resolver will wait for a response from a remote name server before retrying the query via a different name server. The resolver may wait longer during subsequent retries of the current query since an exponential back-off is applied to the timeout value. Measured in seconds, the default is RES\_TIMEOUT, the allowed maximum is RES\_MAXRETRANS (see ⟨resolv.h⟩).

  - **attempts**:*n*

    Set the number of times the resolver will send a query to each of its name servers before giving up and returning an error to the calling application. The default is RES\_DFLRETRY, the allowed maximum is RES\_MAXRETRY (see ⟨resolv.h⟩).

  - **edns0**

    Set RES\_USE\_EDNS0. Attach an OPT pseudo-RR for the EDNS0 extension, as specified in RFC 2671. This allows the resolver to advertise a larger UDP receive buffer size, permitting responses larger than the original 512-byte limit.

  - **inet6**

    Set RES\_USE\_INET6. Causes the resolver to look up AAAA records before A records and to map IPv4 responses into IPv6 addresses. The use of this option is discouraged.

  - **insecure1**

    Set RES\_INSECURE1. This disables the check that the response was received from the same server to which the query was sent. Use of this option is a security risk and is not recommended.

  - **insecure2**

    Set RES\_INSECURE2. This disables the check that the response contains a query matching the one that was sent. Use of this option is a security risk and is not recommended.

  - **no-check-names**

    Set RES\_NOCHECKNAME. This disables the check of incoming host names for invalid characters such as underscore, non-ASCII, or control characters.

  - **no-debug**

    Clears RES\_DEBUG. See **debug** above.

  - **no-rotate**

    Clears RES\_ROTATE. See **rotate** below.

  - **no-tld-query**

    Set RES\_NOTLDQUERY. This tells the resolver not to attempt to resolve a top level domain name, that is, a name that contains no dots. Use of this option does not prevent the resolver from obeying the standard **domain** and **search** rules with the given name.

  - **rotate**

    Set RES\_ROTATE. This causes the resolver to round-robin among the configured name servers, distributing the query load instead of always trying the first listed server.

  - **reload-period**:*n*

    The resolver checks the modification time of */etc/resolv.conf* if more than *n* seconds have elapsed since the last check. If the file has changed, it is automatically reloaded. The default for *n* is two seconds. Setting it to zero disables the file check.

    Options may also be passed as a space- or tab-separated list in the RES\_OPTIONS environment variable. This environment variable is applied after the configuration file has been read.

The **domain** and **search** keywords are mutually exclusive. If more than one instance of these keywords is present, the last instance will override all previous instances.

The keyword and value must appear on a single line, and the keyword (for example, **nameserver**) must start the line. The value follows the keyword, separated by white space.

## ENVIRONMENT

- LOCALDOMAIN

  If set, overrides the local domain and search list set in the resolver configuration file.

- RES\_OPTIONS

  If set, its value is split into words and interpreted as resolver options as described for the **options** keyword in the resolver configuration file.

## FILES

- */etc/resolv.conf*

  resolver configuration file

## EXAMPLES

```
# Set the local domain to example.com and the search list to
# example.com, example.net, example.org
search example.com example.net example.org
# Set the local domain and search list to example.com,
# completely overriding the previous line
domain example.com
# Use this if you are running a local caching forwarding (or
# recursing) resolver like local-unbound(8)
nameserver 127.0.0.1
# IP address of the local or ISP name service
nameserver 198.51.100.53
nameserver 203.0.113.53
# Enable EDNS0, which tells the name server that we are capable of
# handling large responses.  This is required for DNSSEC.
options edns0
```

## SEE ALSO

[getaddrinfo(3)](/getaddrinfo/3.md), [gethostbyname(3)](/gethostbyname/3.md), [gethostname(3)](/gethostname/3.md), [resolver(3)](/resolver/3.md), [hostname(7)](/hostname/7.md), [resolvconf(8)](/resolvconf/8.md)

## HISTORY

The **resolv.conf** file format appeared in 4.3BSD.