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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | FILES | ATTRIBUTES | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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resolver(3) Library Functions Manual resolver(3)
res_ninit, res_nquery, res_nsearch, res_nquerydomain,
res_nmkquery, res_nsend, res_nclose, res_init, res_query,
res_search, res_querydomain, res_mkquery, res_send, dn_comp,
dn_expand - resolver routines
Resolver library (libresolv, -lresolv)
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
struct __res_state;
typedef struct __res_state *res_state;
int res_ninit(res_state statep);
void res_nclose(res_state statep);
int res_nquery(int anslen;
res_state statep,
const char *dname, int class, int type,
unsigned char answer[anslen], int anslen);
int res_nsearch(int anslen;
res_state statep,
const char *dname, int class, int type,
unsigned char answer[anslen], int anslen);
int res_nquerydomain(int anslen;
res_state statep,
const char *name, const char *domain,
int class, int type, unsigned char answer[anslen],
int anslen);
int res_nmkquery(int datalen, int buflen;
res_state statep,
int op, const char *dname, int class,
int type, const unsigned char data[datalen], int datalen,
const unsigned char *newrr,
unsigned char buf[buflen], int buflen);
int res_nsend(int msglen, int anslen;
res_state statep,
const unsigned char msg[msglen], int msglen,
unsigned char answer[anslen], int anslen);
int dn_comp(int length;
const char *exp_dn, unsigned char comp_dn[length],
int length, unsigned char **dnptrs,
unsigned char **lastdnptr);
int dn_expand(int length;
const unsigned char *msg,
const unsigned char *eomorig,
const unsigned char *comp_dn, char exp_dn[length],
int length);
[[deprecated]] extern struct __res_state _res;
[[deprecated]] int res_init(void);
[[deprecated]]
int res_query(int anslen;
const char *dname, int class, int type,
unsigned char answer[anslen], int anslen);
[[deprecated]]
int res_search(int anslen;
const char *dname, int class, int type,
unsigned char answer[anslen], int anslen);
[[deprecated]]
int res_querydomain(int anslen;
const char *name, const char *domain,
int class, int type, unsigned char answer[anslen],
int anslen);
[[deprecated]]
int res_mkquery(int datalen, int buflen;
int op, const char *dname, int class,
int type, const unsigned char data[datalen], int datalen,
const unsigned char *newrr,
unsigned char buf[buflen], int buflen);
[[deprecated]]
int res_send(int msglen, int anslen;
const unsigned char msg[msglen], int msglen,
unsigned char answer[anslen], int anslen);
Note: This page is incomplete (various resolver functions provided
by glibc are not described) and likely out of date.
The functions described below make queries to and interpret the
responses from Internet domain name servers.
The API consists of a set of more modern, reentrant functions and
an older set of nonreentrant functions that have been superseded.
The traditional resolver interfaces such as res_init() and
res_query() use some static (global) state stored in the _res
structure, rendering these functions non-thread-safe. BIND 8.2
introduced a set of new interfaces res_ninit(), res_nquery(), and
so on, which take a res_state as their first argument, so you can
use a per-thread resolver state.
The res_ninit() and res_init() functions read the configuration
files (see resolv.conf(5)) to get the default domain name and name
server address(es). If no server is given, the local host is
tried. If no domain is given, that associated with the local host
is used. It can be overridden with the environment variable
LOCALDOMAIN. res_ninit() or res_init() is normally executed by
the first call to one of the other functions. Every call to
res_ninit() requires a corresponding call to res_nclose() to free
memory allocated by res_ninit() and subsequent calls to
res_nquery().
The res_nquery() and res_query() functions query the name server
for the fully qualified domain name name of specified type and
class. The reply is left in the buffer answer of length anslen
supplied by the caller.
The res_nsearch() and res_search() functions make a query and
waits for the response like res_nquery() and res_query(), but in
addition they implement the default and search rules controlled by
RES_DEFNAMES and RES_DNSRCH (see description of _res options
below).
The res_nquerydomain() and res_querydomain() functions make a
query using res_nquery()/res_query() on the concatenation of name
and domain.
The following functions are lower-level routines used by
res_nquery()/res_query().
The res_nmkquery() and res_mkquery() functions construct a query
message in buf of length buflen for the domain name dname. The
query type op is one of the following (typically QUERY):
QUERY Standard query.
IQUERY Inverse query. This option was removed in glibc 2.26,
since it has not been supported by DNS servers for a very
long time.
NS_NOTIFY_OP
Notify secondary of SOA (Start of Authority) change.
newrr is currently unused.
The res_nsend() and res_send() function send a preformatted query
given in msg of length msglen and returns the answer in answer
which is of length anslen. They will call res_ninit()/res_init()
if it has not already been called.
The dn_comp() function compresses the domain name exp_dn and
stores it in the buffer comp_dn of length length. The compression
uses an array of pointers dnptrs to previously compressed names in
the current message. The first pointer points to the beginning of
the message and the list ends with NULL. The limit of the array
is specified by lastdnptr. If dnptr is NULL, domain names are not
compressed. If lastdnptr is NULL, the list of labels is not
updated.
The dn_expand() function expands the compressed domain name
comp_dn to a full domain name, which is placed in the buffer
exp_dn of size length. The compressed name is contained in a
query or reply message, and msg points to the beginning of the
message.
The resolver routines use configuration and state information
contained in a __res_state structure (either passed as the statep
argument, or in the global variable _res, in the case of the older
nonreentrant functions). The only field of this structure that is
normally manipulated by the user is the options field. This field
can contain the bitwise "OR" of the following options:
RES_INIT
True if res_ninit() or res_init() has been called.
RES_DEBUG
Print debugging messages. This option is available only if
glibc was built with debugging enabled, which is not the
default.
RES_AAONLY (unimplemented; deprecated in glibc 2.25)
Accept authoritative answers only. res_send() continues
until it finds an authoritative answer or returns an error.
This option was present but unimplemented until glibc 2.24;
since glibc 2.25, it is deprecated, and its usage produces
a warning.
RES_USEVC
Use TCP connections for queries rather than UDP datagrams.
RES_PRIMARY (unimplemented; deprecated in glibc 2.25)
Query primary domain name server only. This option was
present but unimplemented until glibc 2.24; since glibc
2.25, it is deprecated, and its usage produces a warning.
RES_IGNTC
Ignore truncation errors. Don't retry with TCP.
RES_RECURSE
Set the recursion desired bit in queries. Recursion is
carried out by the domain name server, not by res_send().
[Enabled by default].
RES_DEFNAMES
If set, res_search() will append the default domain name to
single component names—that is, those that do not contain a
dot. [Enabled by default].
RES_STAYOPEN
Used with RES_USEVC to keep the TCP connection open between
queries.
RES_DNSRCH
If set, res_search() will search for hostnames in the
current domain and in parent domains. This option is used
by gethostbyname(3). [Enabled by default].
RES_INSECURE1
Accept a response from a wrong server. This can be used to
detect potential security hazards, but you need to compile
glibc with debugging enabled and use RES_DEBUG option (for
debug purpose only).
RES_INSECURE2
Accept a response which contains a wrong query. This can
be used to detect potential security hazards, but you need
to compile glibc with debugging enabled and use RES_DEBUG
option (for debug purpose only).
RES_NOALIASES
Disable usage of HOSTALIASES environment variable.
RES_USE_INET6
Try an AAAA query before an A query inside the
gethostbyname(3) function, and map 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, and its usage produces a warning; applications
should use getaddrinfo(3), rather than gethostbyname(3).
RES_ROTATE
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.
RES_NOCHECKNAME (unimplemented; deprecated in glibc 2.25)
Disable the modern BIND checking of incoming hostnames and
mail names for invalid characters such as underscore (_),
non-ASCII, or control characters. This option was present
until glibc 2.24; since glibc 2.25, it is deprecated, and
its usage produces a warning.
RES_KEEPTSIG (unimplemented; deprecated in glibc 2.25)
Do not strip TSIG records. This option was present but
unimplemented until glibc 2.24; since glibc 2.25, it is
deprecated, and its usage produces a warning.
RES_BLAST (unimplemented; deprecated in glibc 2.25)
Send each query simultaneously and recursively to all
servers. This option was present but unimplemented until
glibc 2.24; since glibc 2.25, it is deprecated, and its
usage produces a warning.
RES_USEBSTRING (glibc 2.3.4 to glibc 2.24)
Make reverse IPv6 lookups 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.
RES_NOIP6DOTINT (glibc 2.24 and earlier)
Use ip6.arpa zone in IPv6 reverse lookup instead of
ip6.int, which is deprecated since glibc 2.3.4. This
option is present up to and including glibc 2.24, where it
is enabled by default. In glibc 2.25, this option was
removed.
RES_USE_EDNS0 (since glibc 2.6)
Enables support for the DNS extensions (EDNS0) described in
RFC 2671.
RES_SNGLKUP (since glibc 2.10)
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).
RES_SNGLKUPREOP
When RES_SNGLKUP option is enabled, opens a new socket for
the each request.
RES_USE_DNSSEC
Use DNSSEC with OK bit in OPT record. This option implies
RES_USE_EDNS0.
RES_NOTLDQUERY
Do not look up unqualified name as a top-level domain
(TLD).
RES_DEFAULT
Default option which implies: RES_RECURSE, RES_DEFNAMES,
RES_DNSRCH, and RES_NOIP6DOTINT.
The res_ninit() and res_init() functions return 0 on success, or
-1 if an error occurs.
The res_nquery(), res_query(), res_nsearch(), res_search(),
res_nquerydomain(), res_querydomain(), res_nmkquery(),
res_mkquery(), res_nsend(), and res_send() functions return the
length of the response, or -1 if an error occurs.
The dn_comp() and dn_expand() functions return the length of the
compressed name, or -1 if an error occurs.
In the case of an error return from res_nquery(), res_query(),
res_nsearch(), res_search(), res_nquerydomain(), or
res_querydomain(), the global variable h_errno (see
gethostbyname(3)) can be consulted to determine the error.
/etc/resolv.conf
resolver configuration file
/etc/host.conf
resolver configuration file
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────┼───────────────┼────────────────┤
│ res_ninit(), res_nclose(), │ Thread safety │ MT-Safe locale │
│ res_nquery(), res_nsearch(), │ │ │
│ res_nquerydomain(), │ │ │
│ res_nsend() │ │ │
├───────────────────────────────┼───────────────┼────────────────┤
│ res_nmkquery(), dn_comp(), │ Thread safety │ MT-Safe │
│ dn_expand() │ │ │
└───────────────────────────────┴───────────────┴────────────────┘
None.
4.3BSD.
gethostbyname(3), resolv.conf(5), resolver(5), hostname(7),
named(8)
The GNU C library source file resolv/README.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 resolver(3)
Pages that refer to this page: hostname(1), pmdanetcheck(1), gethostbyname(3), hosts(5), resolv.conf(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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