resolver(3)

NAME

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

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#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);

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 #include <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 (i.e., 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.
RES_USEVC Use TCP connections for queries instead of
UDP datagrams.
RES_STAYOPEN Used with RES_USEVC to keep the TCP connec
tion open: between queries. This is useful only in pro; grams that
regularly do many queries. UDP should be the; normal mode
used.
RES_IGNTC Unused currently (ignore truncation errors,
i.e., 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 host: names in the current domain and in parent do; mains; see
hostname(7). This is used by the standard; host lookup
routine gethostbyname(3). This option is en; abled by
default.
RES_NOALIASES This option turns off the user level aliasing
feature con: trolled by the ``HOSTALIASES'' environment; variable. Network daemons should set this option.
RES_USE_INET6 Enables support for IPv6-only applications.
This causes: IPv4 addresses to be returned as an IPv4; mapped address.
For example, 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 Enables support for OPT pseudo-RR for EDNS0
extension.: With the option, resolver code will attach; OPT pseudo-RR
into DNS queries, to inform of our receive; buffer size.
The option will allow DNS servers to take ad; vantage of
non-default receive buffer size, and to send; larger
replies. DNS query packets with EDNS0 exten; sion is not
compatible with non-EDNS0 DNS servers.
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 con
figured, 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 simi
lar to the
search command in the configuration file. Another environ
ment variable ``RES_OPTIONS'' can be set to override certain internal re
solver 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 re
solver(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 speci
fied 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 The domain name for the query is giv
en by dname.
The newrr_in argument is currently unused but is intended
for making update messages.
The res_send() routine sends a pre-formatted query and re
turns 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 giv
en by length. The compression uses an array of pointers dnptrs to previ
ously-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.

IMPLEMENTATION NOTES

This implementation of the resolver is thread-safe, but it
will not function properly if the programmer attempts to declare his or
her own _res
structure in an attempt to replace the per-thread version
referred to by that macro.

RETURN VALUES

The res_init() function will return 0 on success, or -1 in a
threaded program if per-thread storage could not be allocated.

FILES

/etc/resolv.conf The configuration file, see resolver(5).

HISTORY

The res_query function appeared in 4.3BSD.
BSD June 4, 1993

docs.sk

comprehensive documentation repository

See also

resolver(3)

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

IMPLEMENTATION NOTES

RETURN VALUES

FILES

SEE ALSO

HISTORY