Section (3) resolver
Name
res_ninit, res_nclose, res_nquery, res_nsearch, res_nquerydomain, res_nmkquery, res_nsend, res_init, res_query, res_search, res_querydomain, res_mkquery, res_send, dn_comp, dn_expand — resolver routines
Synopsis
#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( |
res_state statep, |
const char *dname, | |
int class, | |
int type, | |
unsigned char *answer, | |
int anslen) ; |
int
res_nsearch( |
res_state statep, |
const char *dname, | |
int class, | |
int type, | |
unsigned char *answer, | |
int anslen) ; |
int
res_nquerydomain( |
res_state statep, |
const char *name, | |
const char *domain, | |
int class, | |
int type, | |
unsigned char *answer, | |
int anslen) ; |
int
res_nmkquery( |
res_state statep, |
int op, | |
const char *dname, | |
int class, | |
int type, | |
const unsigned char *data, | |
int datalen, | |
const unsigned char *newrr, | |
unsigned char *buf, | |
int buflen) ; |
int
res_nsend( |
res_state statep, |
const unsigned char *msg, | |
int msglen, | |
unsigned char *answer, | |
int anslen) ; |
int
dn_comp( |
const char *exp_dn, |
unsigned char *comp_dn, | |
int length, | |
unsigned char **dnptrs, | |
unsigned char **lastdnptr) ; |
int
dn_expand( |
const unsigned char *msg, |
const unsigned char *eomorig, | |
const unsigned char *comp_dn, | |
char *exp_dn, | |
int length) ; |
Deprecated extern struct __res_state _res;
int
res_init( |
void) ; |
int
res_query( |
const char *dname, |
int class, | |
int type, | |
unsigned char *answer, | |
int anslen) ; |
int
res_search( |
const char *dname, |
int class, | |
int type, | |
unsigned char *answer, | |
int anslen) ; |
int
res_querydomain( |
const char *name, |
const char *domain, | |
int class, | |
int type, | |
unsigned char *answer, | |
int anslen) ; |
int
res_mkquery( |
int op, |
const char *dname, | |
int class, | |
int type, | |
const unsigned char *data, | |
int datalen, | |
const unsigned char *newrr, | |
unsigned char *buf, | |
int buflen) ; |
int
res_send( |
const unsigned char *msg, |
int msglen, | |
unsigned char *answer, | |
int anslen) ; |
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DESCRIPTION
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This page is incomplete (various resolver functions provided by glibc are not described) and likely out of date.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_query
()/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
() orres_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 in glibc until version 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 in glibc until version 2.24; since glibc 2.25, it is deprecated, and its usage produces a warning.
RES_IGNTC
-
Ignore truncation errors. Don_zsingle_quotesz_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 in glibc until version 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 in glibc until version 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 in glibc until version 2.24; since glibc 2.25, it is deprecated, and its usage produces a warning.
RES_USEBSTRING
(glibc 2.3.4 to 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 ofip6.int
, which is deprecated since glibc 2.3.4. This option is present in glibc up to and including version 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 version 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
andRES_NOIP6DOTINT
.
RETURN VALUE
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 cause of the error.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
|
Thread safety | MT-Safe locale |
|
Thread safety | MT-Safe |
SEE ALSO
gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)
The GNU C library source file resolv/README
.
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright 1993 David Metcalfe (davidprism.demon.co.uk) and (C) Copyright 2015 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END References consulted: Linux libc source code Lewine_zsingle_quotesz_s _POSIX Programmer_zsingle_quotesz_s Guide_ (O_zsingle_quotesz_Reilly & Associates, 1991) 386BSD man pages Modified 1993-07-25 by Rik Faith (faithcs.unc.edu) Modified 2004-10-31 by aeb |