resolver, res_query, res_search, res_mkquery, res_send, res_init,
dn_comp, dn_expand - resolver routines
res_query(dname, class, type, answer, anslen)
int class, type;
res_search(dname, class, type, answer, anslen)
int class, type;
res_mkquery(op, dname, class, type, data, datalen, newrr, buf, buflen)
int class, type;
struct rrec *newrr;
res_send(msg, msglen, answer, anslen)
dn_comp(exp_dn, comp_dn, length, dnptrs, lastdnptr)
char *exp_dn, *comp_dn;
char **dnptrs, **lastdnptr;
dn_expand(msg, eomorig, comp_dn, exp_dn, length)
char *msg, *eomorig, *comp_dn, exp_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 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.
True if the initial name server address and default domain name are
initialized (i.e., res_init has been called).
Print debugging messages.
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.
Use TCP connections for queries instead of UDP datagrams.
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.
Unused currently (ignore truncation errors, i.e., don't retry with
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.)
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.
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.
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 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 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 <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 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(5)
gethostbyname(3), named(8), resolver(5), hostname(7),
RFC1032, RFC1033, RFC1034, RFC1035, RFC974,
SMM:11 Name Server Operations Guide for BIND