res_query(3N) res_query(3N)
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,
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 char *data, int datalen, const char *newrr,
char *buf, int buflen);
int res_send (const char *msg, int msglen, char *answer, int anslen);
int res_init (void);
int dn_comp (const u_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);
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 (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 authoritative
answer or finds an error. Currently this is not
implemented.
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res_query(3N) res_query(3N)
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; 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 appends 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 searches for hostnames
in the current domain and in parent domains; see
hostname(5). This is used by the standard host lookup
routine gethostbyname(3N). This option is enabled by
default.
The res_init routine reads the configuration file (if any; see
resolver(4)) 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. 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 fullyqualified
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.
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res_query(3N) res_query(3N)
The res_send routine sends a preformatted query and returns an answer.
It calls res_init if RES_INIT is not set, sends the query to the local
name server, and handles 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.
/etc/resolv.conf see resolver(4)
named(1M), gethostbyname(3N), resolver(4), hostname(5).
RFC1032, RFC1033, RFC1034, RFC1035, RFC974
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