res_init, res_query, res_search, res_querydomain, res_mkquery, res_send, dn_comp, dn_expand — resolver routines
#include <netinet/in.h> #include <arpa/nameser.h> #include <resolv.h> extern struct 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, | |
| char *data, | |
| int datalen, | |
| struct rrec *newrr, | |
| char *buf, | |
int buflen); |
int
res_send( |
const char *msg, |
| int msglen, | |
| char *answer, | |
int anslen); |
int
dn_comp( |
unsigned char *exp_dn, |
| unsigned char *comp_dn, | |
| int length, | |
| unsigned char **dnptrs, | |
unsigned char **lastdnptr); |
int
dn_expand( |
unsigned char *msg, |
| unsigned char *eomorig, | |
| unsigned char *comp_dn, | |
| char *exp_dn, | |
int length); |
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Note |
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Link with |
These functions make queries to and interpret the responses from Internet domain name servers.
The res_init() function
reads the configuration files (see resolv.conf(5)) to get the
default domain name, search order 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_init() is normally executed by the
first call to one of the other functions.
The res_query() function
queries 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_search() function
makes a query and waits for the response like res_query(), but in addition implements the
default and search rules controlled by RES_DEFNAMES and RES_DNSRCH (see description of _res options below).
The res_querydomain()
function makes a query using res_query() on the concatenation of
name and domain.
The following functions are lower-level routines used by
res_query().
The res_mkquery() function
constructs a query message in buf of length buflen for the domain name
dname. The query type
op is usually
QUERY, but can be any of the
types defined in <arpa/nameser.h>
newrr is currently
unused.
The res_send() function
sends a preformatted query given in msg of length msglen and returns the answer
in answer which is of
length anslen. It
will call 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 global configuration and state
information contained in the structure _res, which is defined in <resolv.h>
The only field that is normally manipulated by the user is
_res.options. This
field can contain the bitwise "OR" of the following
options:
RES_INITTrue if res_init() has
been called.
RES_DEBUGPrint debugging messages.
RES_AAONLYAccept authoritative answers only. res_send() continues until it finds
an authoritative answer or returns an error. [Not
currently implemented].
RES_USEVCUse TCP connections for queries rather than UDP datagrams.
RES_PRIMARYQuery primary domain name server only.
RES_IGNTCIgnore truncation errors. Don't retry with TCP. [Not currently implemented].
RES_RECURSESet the recursion desired bit in queries. Recursion
is carried out by the domain name server, not by
res_send(). [Enabled by
default].
RES_DEFNAMESIf set, res_search()
will append the default domain name to single component
names, i.e., those that do not contain a dot. [Enabled
by default].
RES_STAYOPENUsed with RES_USEVC to
keep the TCP connection open between queries.
RES_DNSRCHIf 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].
This list is not complete. You can find some other flags described in resolv.conf(5).
The res_init() function
returns 0 on success, or −1 if an error occurs.
The res_query(),
res_search(), res_querydomain(), res_mkquery() 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.
This page is part of release 3.41 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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Copyright 1993 David Metcalfe (davidprism.demon.co.uk) 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. References consulted: Linux libc source code Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) 386BSD man pages Modified 1993-07-25 by Rik Faith (faithcs.unc.edu) Modified 2004-10-31 by aeb |