res_query, res_search, res_mkquery, res_send, res_init,
dn_expand - resolver routines
res_query(char *dname, int class, int type, u_char *answer,
res_search(char *dname, int class, int type, u_char *answer,
res_mkquery(int op, char *dname, int class, int type, char
int datalen, struct rrec *newrr, char *buf, int
res_send(char *msg, int msglen, char *answer, int anslen);
dn_comp(char *exp_dn, char *comp_dn, int length, char
dn_expand(u_char *msg, u_char *eomorig, u_char *comp_dn,
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
routines is kept in the structure _res. Most of the values
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
name are initialized (i.e., res_init() has
RES_DEBUG Print debugging messages.
RES_AAONLY Accept authoritative answers only. With this
res_send() should continue until it finds an
answer or finds an error. Currently this is
RES_USEVC Use TCP connections for queries instead of
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
RES_IGNTC Unused currently (ignore truncation errors,
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
to single-component names (those that do not
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
routine gethostbyname(3). This option is enabled by default.
RES_USE_INET6 Enables support for IPv6-only applications.
IPv4 addresses to be returned as an IPv4
For example, 10.1.1.1 will be returned as
The option is not meaningful on OpenBSD.
The res_init() routine reads the configuration file (if any;
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
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
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
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 res_query() function provides an interface to the server
It constructs a query, sends it to the local server,
awaits a response,
and makes preliminary checks on the reply. The
information of the specified type and class for the specified fully qualified
domain name dname. The reply message is left in the
with length anslen supplied by the caller.
The res_search() routine makes a query and awaits a response
res_query(), but in addition, it implements the default and
controlled by the RES_DEFNAMES and RES_DNSRCH options. It
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
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
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
If dnptr is NULL, names are not compressed. If
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
message; msg is a pointer to the beginning of the message.
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
/etc/resolv.conf configuration file see resolv.conf(5).
gethostbyname(3), resolv.conf(5), hostname(7), named(8)
RFC 1032, RFC 1033, RFC 1034, RFC 1035, RFC 1535, RFC 974
Name Server Operations Guide for BIND.
The res_query function appeared in 4.3BSD.
OpenBSD 3.6 June 4, 1993
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