NAME
resolver, res_query, res_search, res_mkquery, res_send,
res_init, dn_comp, dn_expand - resolver routines
SYNOPSIS
#include <sys/types.h>
#include <net/gen/in.h>
#include <net/gen/nameser.h>
#include <net/gen/resolv.h>
res_query(dname, class, type, answer, anslen)
char *dname;
int class, type;
u_char *answer;
int anslen;
res_search(dname, class, type, answer, anslen)
char *dname;
int class, type;
u_char *answer;
int anslen;
res_mkquery(op, dname, class, type, data, datalen, newrr,
buf, buflen)
int op;
char *dname;
int class, type;
char *data;
int datalen;
struct rrec *newrr;
char *buf;
int buflen;
res_send(msg, msglen, answer, anslen)
char *msg;
int msglen;
char *answer;
int anslen;
res_init()
dn_comp(exp_dn, comp_dn, length, dnptrs, lastdnptr)
char *exp_dn, *comp_dn;
int length;
char **dnptrs, **lastdnptr;
dn_expand(msg, eomorig, comp_dn, exp_dn, length)
char *msg, *eomorig, *comp_dn, exp_dn;
int length;
DESCRIPTION
These routines are used for making, sending and interpreting
query and reply messages with Internet domain name servers.
Global configuration and state information that is used by
the resolver routines is kept in the structure _res. Most
of the values have reasonable defaults and can be ignored.
Options stored in _res.options are defined in resolv.h and
are as follows. Options are stored as a simple bit mask
containing the bitwise ``or'' of the options enabled.
RES_INIT
True if the initial name server address and default
domain name are initialized (i.e., res_init has been
called).
RES_DEBUG
Print debugging messages.
RES_AAONLY
Accept authoritative answers only. With this option,
res_send should continue until it finds an authorita-
tive answer or finds an error. Currently this is not
implemented.
RES_USEVC
Use TCP connections for queries instead of UDP
datagrams.
RES_STAYOPEN
Used with RES_USEVC to keep the TCP connection open
between queries. This is useful only in programs that
regularly do many queries. UDP should be the normal
mode used.
RES_IGNTC
Unused currently (ignore truncation errors, i.e., don't
retry with TCP).
RES_RECURSE
Set the recursion-desired bit in queries. This is the
default. (res_send does not do iterative queries and
expects the name server to handle recursion.)
RES_DEFNAMES
If set, res_search will append the default domain name
to single-component names (those that do not contain a
dot). This option is enabled by default.
RES_DNSRCH
If this option is set, res_search will search for host
names in the current domain and in parent domains; see
hostname(7). This is used by the standard host lookup
routine gethostbyname(3). This option is enabled by
default.
The res_init routine reads the configuration file (if any;
see resolver(5)) to get the default domain name, search list
and the Internet address of the local name server(s). If no
server is configured, the host running the resolver is
tried. The current domain name is defined by the hostname
if not specified in the configuration file; it can be over-
ridden by the environment variable LOCALDOMAIN. Initializa-
tion normally occurs on the first call to one of the follow-
ing routines.
The res_query function provides an interface to the server
query mechanism. It constructs a query, sends it to the
local server, awaits a response, and makes preliminary
checks on the reply. The query requests information of the
specified type and class for the specified fully-qualified
domain name dname . The reply message is left in the answer
buffer with length anslen supplied by the caller.
The res_search routine makes a query and awaits a response
like res_query, but in addition, it implements the default
and search rules controlled by the RES_DEFNAMES and
RES_DNSRCH options. It returns the first successful reply.
The remaining routines are lower-level routines used by
res_query. The res_mkquery function constructs a standard
query message and places it in buf. It returns the size of
the query, or -1 if the query is larger than buflen. The
query type op is usually QUERY, but can be any of the query
types defined in <arpa/nameser.h>. The domain name for the
query is given by dname. Newrr is currently unused but is
intended for making update messages.
The res_send routine sends a pre-formatted query and returns
an answer. It will call res_init if RES_INIT is not set,
send the query to the local name server, and handle timeouts
and retries. The length of the reply message is returned,
or -1 if there were errors.
The dn_comp function compresses the domain name exp_dn and
stores it in comp_dn. The size of the compressed name is
returned or -1 if there were errors. The size of the array
pointed to by comp_dn is given by length. The compression
uses an array of pointers dnptrs to previously-compressed
names in the current message. The first pointer points to
to the beginning of the message and the list ends with NULL.
The limit to the array is specified by lastdnptr. A side
effect of dn_comp is to update the list of pointers for
labels inserted into the message as the name is compressed.
If dnptr is NULL, names are not compressed. If lastdnptr is
NULL, the list of labels is not updated.
The dn_expand entry expands the compressed domain name
comp_dn to a full domain name The compressed name is con-
tained in a query or reply message; msg is a pointer to the
beginning of the message. The uncompressed name is placed
in the buffer indicated by exp_dn which is of size length.
The size of compressed name is returned or -1 if there was
an error.
FILES
/etc/resolv.conf see resolver(5)
SEE ALSO
gethostbyname(3), named(8), resolver(5), hostname(7),
RFC1032, RFC1033, RFC1034, RFC1035, RFC974,
SMM:11 Name Server Operations Guide for BIND