resolver(3)
NAME
res_init, res_query, res_search, res_querydomain,
res_mkquery, res_send, dn_comp, dn_expand - resolver rou
tines
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);
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+(8)) 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 associ
ated 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 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 com
pressed. 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 con
tained 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_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 implemented].
RES_RECURSE
Set the recursion desired bit in queries. Recur
sion 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
BSD 4.3
SEE ALSO
gethostbyname(3), hostname(7), named(8), resolv+(8)
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