RESOLVER(3) | Library Functions Manual | RESOLVER(3) |
res_query
,
res_search
, res_mkquery
,
res_send
, res_init
,
dn_comp
, dn_expand
,
dn_skipname
, ns_get16
,
ns_get32
, ns_put16
,
ns_put32
— resolver
routines
DNS Resolver Library (libresolv, -lresolv)
#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,
u_char *answer, int anslen);
int
res_search
(const char *dname,
int class, int type,
u_char *answer, int anslen);
int
res_mkquery
(int op,
const char *dname, int class,
int type, const u_char *data,
int datalen, const u_char
*newrr_in, u_char *buf, int
buflen);
int
res_send
(const u_char *msg,
int msglen, u_char *answer,
int anslen);
int
res_init
();
int
dn_comp
(const char *exp_dn,
u_char *comp_dn, int length,
u_char **dnptrs, u_char
**lastdnptr);
int
dn_expand
(const u_char *msg,
const u_char *eomorig, const u_char
*comp_dn, char *exp_dn, int
length);
int
dn_skipname
(const
u_char *comp_dn, const
u_char *eom);
u_int
ns_get16
(const
u_char *src);
u_long
ns_get32
(const
u_char *src);
void
ns_put16
(u_int
src, u_char
*dst);
void
ns_put32
(u_long
src, u_char
*dst);
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
RES_AAONLY
res_send
() should continue until it finds an
authoritative answer or finds an error. Currently this is not
implemented.RES_USEVC
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_IGNTC
RES_RECURSE
res_send
() does not do iterative queries and
expects the name server to handle recursion.)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_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_NOALIASES
HOSTALIASES
” environment variable.
Network daemons should set this option.RES_USE_INET6
10.1.1.1
will be returned as
::ffff:10.1.1.1
. The option is meaningful with
certain kernel configuration only.RES_USE_EDNS0
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 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 resolver(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. Constants for
type and class 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
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.
The
dn_skipname
()
function skips over a compressed domain name, which starts at a location
pointed to by comp_dn. The compressed name is
contained in a query or reply message; eom is a
pointer to the end of the message. The size of compressed name is returned
or -1 if there was an error.
The
ns_get16
()
function gets a 16-bit quantity from a buffer pointed to by
src.
The
ns_get32
()
function gets a 32-bit quantity from a buffer pointed to by
src.
The
ns_put16
()
function puts a 16-bit quantity src to a buffer
pointed to by dst.
The
ns_put32
()
function puts a 32-bit quantity src to a buffer
pointed to by dst.
gethostbyname(3), resolver(5), hostname(7), named(8)
RFC1032, RFC1033, RFC1034, RFC1035, RFC974
Name Server Operations Guide for BIND.
The res_query
function appeared in
4.3BSD.
June 4, 1993 | macOS 14.6 |