|RES_INIT(3)||Library Functions Manual||RES_INIT(3)|
res_query(const char *dname,
int class, int type,
unsigned char *answer, int
res_search(const char *dname,
int class, int type,
unsigned char *answer, int
const char *dname, int class,
int type, const unsigned char
*data, int datalen, const
unsigned char *newrr, unsigned char *buf,
res_send(const unsigned char
*msg, int msglen, unsigned char
*answer, int anslen);
dn_comp(const char *exp_dn,
unsigned char *comp_dn, int
length, unsigned char **dnptrs,
unsigned char **lastdnptr);
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
Most of the values have reasonable defaults and can be ignored. Options
_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() has been called).
DEBUG. By default on OpenBSD this option does nothing.
res_send() should continue until it finds an authoritative answer or finds an error. Currently this is not implemented.
res_send() does not do iterative queries and expects the name server to handle recursion.) This option is enabled by default.
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_USEVCto 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_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_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
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
Initialization normally occurs on the first call to one of the following
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
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
options. It returns the first successful reply.
The remaining routines are lower-level routines used by
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
res_send() routine sends a
pre-formatted query and returns an answer. It will call
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
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
NULL, names are not
compressed. If lastdnptr is
NULL, the list of labels is not updated.
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.
res_query function appeared in
|December 16, 2015||OpenBSD-5.9|