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RESOLVER(3) Linux Programmer's Manual RESOLVER(3)
NAME
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>
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 *exp_dn,
unsigned char **lastdnptr);
int dn_expand(unsigned char *msg, unsigned char *eomorig,
unsigned char *comp_dn, unsigned char *exp_dn,
int length);
Link with -lresolv.
DESCRIPTION
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 LOCALDO-
MAIN. res_init() is normally executed by the first call to one of the other func-
tions.
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 pre-formatted 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 nor-
mally manipulated by the user is _res.options. This field can contain the bitwise
''or'' of the following options:
RES_INIT
True if res_init() has been called.
RES_DEBUG
Print debugging messages.
RES_AAONLY
Accept authoritative answers only. res_send() continues until it fins an
authoritative answer or returns an error. [Not currently implemented].
RES_USEVC
Use TCP connections for queries rather than UDP datagrams.
RES_PRIMARY
Query primary domain name server only.
RES_IGNTC
Ignore truncation errors. Don't retry with TCP. [Not currently imple-
mented].
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, ie. 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 host names in the current domain and in
parent domains. This option is used by gethostbyname(3). [Enabled by
default].
RETURN VALUE
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.
FILES
/etc/resolv.conf resolver configuration file
/etc/host.conf resolver configuration file
CONFORMING TO
4.3BSD
SEE ALSO
gethostbyname(3), resolver(5), hostname(7), resolv.conf(5), named(8)
BSD 2004-10-31 RESOLVER(3)
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On Apache/2.2.3 (CentOS)
Under GNU General Public License
2008-10-12 23:29 @127.0.0.1 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)