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>>
res_query(dname, class, type, answer, anslen)
const char *dname;
int class, type;
u_char *answer;
int anslen;
res_search(dname, class, type, answer, anslen)
const char *dname;
int class, type;
u_char *answer;
int anslen;
res_mkquery(op, dname, class, type, data, datalen, newrr,
buf, buflen)
int op;
const char *dname;
int class, type;
const char *data;
int datalen;
struct rrec *newrr;
u_char *buf;
int buflen;
res_send(msg, msglen, answer, anslen)
const u_char *msg;
int msglen;
u_char *answer;
int anslen;
res_init()
dn_comp(exp_dn, comp_dn, length, dnptrs, lastdnptr)
const char *exp_dn;
u_char *comp_dn;
int length;
u_char **dnptrs, **lastdnptr;
dn_expand(msg, eomorig, comp_dn, exp_dn, length)
const u_char *msg, *eomorig, *comp_dn;
char *exp_dn;
int length;
herror(const char *s)
hstrerror(int err)
DESCRIPTION
These routines are used for making, sending and interpret-
ing 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_NOALIASES
This option turns off the user level aliasing fea-
ture controlled by the HOSTALIASES environment
variable. Network daemons should set this option.
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 host-
name 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
resolver(5). Initialization normally occurs on the first
call to one of the other resolver 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-qual-
ified 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.
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 han-
dle 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 com-
pression 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 con-
tained 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 external variable h_errno is set whenever an error
occurs during resolver operation. The following defini-
tions are given in <<netdb.h>>:
#define NETDB_INTERNAL -1 /* see errno */
#define NETDB_SUCCESS 0 /* no problem */
#define HOST_NOT_FOUND 1 /* Authoritative Answer Host not found */
#define TRY_AGAIN 2 /* Non-Authoritive not found, or SERVFAIL */
#define NO_RECOVERY 3 /* Nonrecoverable: FORMERR, REFUSED, NOTIMP */
#define NO_DATA 4 /* Valid name, no data for requested type */
The herror function writes a message to the diagnostic
output consisting of the string parameter s, the constant
string ": ", and a message corresponding to the value of
h_errno.
The hstrerror function returns a string which is the mes-
sage text corresponding to the value of the err parameter.
FILES
/etc/resolv.conf see resolver(5)
SEE ALSO
gethostbyname(3) named(8) resolver(5) hostname(7)
RFC1032, RFC1033, RFC1034, RFC1035, RFC974,
SMM:11 Name Server Operations Guide for BIND