resolver(3N)
resolver: res_mkquery, res_query, res_search, res_send, res_init, dn_comp, dn_expand --
resolver routines
Synopsis
cc [options] file
-lresolv -lsocket -lnsl
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
int
res_mkquery(op, dname, class, type, data, datalen, newrr, buf, buflen)
int op;
const char *dname;
int class, type;
const u_char *data;
int datalen;
const u_char *newrr;
u_char *buf;
int buflen;
int
res_query(dname, class, type, answer, anslen)
const char *dname;
int class, type;
u_char *answer;
int anslen;
int
res_search(dname, class, type, answer, anslen)
const char *dname;
int class, type;
u_char *answer;
int anslen;
int
res_send(msg, msglen, answer, anslen)
const u_char *msg;
int msglen;
u_char *answer;
int anslen;
int
res_init()
int
dn_comp(exp_dn, comp_dn, length, dnptrs, lastdnptr)
const char *exp_dn;
u_char *comp_dn;
int length;
u_char **dnptrs, **lastdnptr;
int
dn_expand(msg, eomorig, comp_dn, exp_dn, length)
const u_char *msg;
const u_char *eomorig;
const u_char *comp_dn;
char *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 (for example, res_init has been called).
RES_DEBUG-
Print debugging messages.
RES_HOSTFIRST-
Cause
gethostbyname
and
gethostbyaddr
to query
/etc/hosts
prior to querying the DNS.
RES_AAONLY-
Accept authoritative answers only.
With this option, res_send
should continue until it finds an authoritative 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,
for example, do not 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 hostnames in the current domain and in
parent domains; see
hostname(1bsd).
This is used by the standard host lookup routine
gethostbyname(3N).
This option is enabled by default.
RES_NOALIASES-
This option turns off the user level aliasing feature controlled
by the HOSTALIASES environment variable. Network daemons should
set this option.
The res_init routine
reads the initialization file, if any, (see
resolv.conf(4tcp))
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 command
if not specified in the configuration file;
it can be overridden by the environment variable LOCALDOMAIN.
This environment variable may contain several blank-separated
tokens if you wish to override the
search list
on a per-process basis. This is similar to the
search
command in the configuration file.
Another environment variable (RES_OPTIONS) can be set to
override certain internal resolver options which are otherwise
set by changing fields in the
_res
structure or are inherited from the configuration file's
options
command. The syntax of the RES_OPTIONS environment variable
is explained in
resolv.conf(4tcp).
Initialization normally occurs on the first call
to one of the other resolver 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 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 contained 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-
References
gethostbyname(3N),
hostname(1bsd),
named(1Mtcp),
resolv.conf(4tcp)
RFC 974, RFC 1032, RFC 1033, RFC 1034, RFC 1035, RFC 1123
Notices
Linking against libresolv before libsocket
and libnsl forces DNS resolution
only to be used for host lookups.
© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004