RESOLVER(3) OpenBSD Programmer's Manual RESOLVER(3)NAME
res_query, res_search, res_mkquery, res_send, res_init, dn_comp,
dn_expand - resolver routines
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, 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);
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 <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 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, i.e., 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() 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 domains; see
hostname(7). This is used by the standard host lookup
routine gethostbyname(3). This option is enabled by
default.
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 not meaningful on OpenBSD.
RES_USE_EDNS0 Enables support for OPT pseudo-RR for EDNS0 extension.
With this 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 advantage of
non-default receive buffer size, and to send larger
replies. DNS query packets with EDNS0 extension is not
compatible with non-EDNS0 DNS servers.
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.
FILES
/etc/resolv.conf The configuration file.
SEE ALSOgethostbyname(3), resolv.conf(5), hostname(7), named(8)
RFC 974, RFC 1032, RFC 1033, RFC 1034, RFC 1035, RFC 1535
Name Server Operations Guide for BIND.
HISTORY
The res_query function appeared in 4.3BSD.
OpenBSD 4.9 May 31, 2007 OpenBSD 4.9
