RES_INIT(3) | Library Functions Manual | RES_INIT(3) |
res_query
,
res_search
, res_mkquery
,
res_send
, res_init
,
dn_comp
, dn_expand
—
resolver routines
#include
<sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
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_mkquery
(int op,
const char *dname, int class,
int type, const unsigned char
*data, int datalen, const
unsigned char *newrr, unsigned char *buf,
int buflen);
int
res_send
(const unsigned char
*msg, int msglen, unsigned char
*answer, int anslen);
int
res_init
(void);
int
dn_comp
(const char *exp_dn,
unsigned char *comp_dn, int
length, unsigned char **dnptrs,
unsigned char **lastdnptr);
int
dn_expand
(const unsigned char
*msg, const unsigned char *eomorig,
const unsigned char *comp_dn, char
*exp_dn, int length);
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
res_init
()
has been called).RES_DEBUG
DEBUG
. By default on
OpenBSD this option does nothing.RES_AAONLY
res_send
() should continue until it finds an
authoritative answer or finds an error. On OpenBSD
this option does nothing.RES_USEVC
RES_PRIMARY
RES_IGNTC
RES_RECURSE
res_send
() does not do iterative queries and
expects the name server to handle recursion.) This option is enabled by
default.RES_DEFNAMES
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_STAYOPEN
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_DNSRCH
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.RES_INSECURE_1
RES_INSECURE_2
RES_NOALIASES
HOSTALIASES
feature.RES_USE_INET6
RES_USE_EDNS0
RES_USE_DNSSEC
The
res_init
()
routine reads the configuration file (if any; see
resolv.conf(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 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(5).
Initialization normally occurs on the first call to one of the following
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. Values for the
class and type fields are
defined in
<arpa/nameser.h>
.
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
dnptrs 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.
M. Stahl, Domain Administrators Guide, RFC 1032, November 1987.
M. Lottor, Domain Administrators Operations Guide, RFC 1033, November 1987.
P. Mockapetris, Domain Names – Concepts and Facilities, RFC 1034, November 1987.
P. Mockapetris, Domain Names – Implementation and Specification, RFC 1035, November 1987.
J. Klensin, Simple Mail Transfer Protocol, RFC 5321, October 2008.
The functions res_mkquery
(),
res_send
(), res_init
(),
dn_comp
(), and dn_expand
()
appeared in 4.3BSD. The functions
res_query
() and res_search
()
appeared in 4.3BSD-Tahoe.
February 27, 2017 | OpenBSD-6.2 |