NAME¶
res_init, res_query, res_search, res_querydomain, res_mkquery, res_send,
dn_comp, dn_expand - resolver routines
SYNOPSIS¶
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
extern struct state _res;
int res_init(void);
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_querydomain(const char *name, const char *domain,
int class, int type, unsigned char *answer,
int anslen);
int res_mkquery(int op, const char *dname, int class,
int type, char *data, int datalen, struct rrec *newrr,
char *buf, int buflen);
int res_send(const char *msg, int msglen, char *answer,
int dn_comp(unsigned char *exp_dn, unsigned char *comp_dn,
int length, unsigned char **dnptrs, unsigned char **lastdnptr);
int dn_expand(unsigned char *msg, unsigned char *eomorig,
unsigned char *comp_dn, char *exp_dn,
int length);
Link with
-lresolv.
DESCRIPTION¶
These functions make queries to and interpret the responses from Internet domain
name servers.
The
res_init() function reads the configuration files (see
resolv.conf(5)) to get the default domain name, search order and name server
address(es). If no server is given, the local host is tried. If no domain is
given, that associated with the local host is used. It can be overridden with
the environment variable
LOCALDOMAIN.
res_init() is normally
executed by the first call to one of the other functions.
The
res_query() function queries the name server for the fully qualified
domain name
name of specified
type and
class. The reply
is left in the buffer
answer of length
anslen supplied by the
caller.
The
res_search() function makes a query and waits for the response like
res_query(), but in addition implements the default and search rules
controlled by
RES_DEFNAMES and
RES_DNSRCH (see description of
_res options below).
The
res_querydomain() function makes a query using
res_query() on
the concatenation of
name and
domain.
The following functions are lower-level routines used by
res_query().
The
res_mkquery() function constructs a query message in
buf of
length
buflen for the domain name
dname. The query type
op is usually
QUERY, but can be any of the types defined in
<arpa/nameser.h>.
newrr is currently unused.
The
res_send() function sends a preformatted query given in
msg of
length
msglen and returns the answer in
answer which is of
length
anslen. It will call
res_init(), if it has not already
been called.
The
dn_comp() function compresses the domain name
exp_dn and
stores it in the buffer
comp_dn of length
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 of the array is specified by
lastdnptr. If
dnptr is NULL, domain names are not compressed. If
lastdnptr is NULL, the list of labels is not updated.
The
dn_expand() function expands the compressed domain name
comp_dn to a full domain name, which is placed in the buffer
exp_dn of size
length. The compressed name is contained in a
query or reply message, and
msg points to the beginning of the message.
The resolver routines use global configuration and state information contained
in the structure
_res, which is defined in
<resolv.h>. The
only field that is normally manipulated by the user is
_res.options.
This field can contain the bitwise "OR" of the following options:
- RES_INIT
- True if res_init() has been called.
- RES_DEBUG
- Print debugging messages.
- RES_AAONLY
- Accept authoritative answers only. res_send()
continues until it finds an authoritative answer or returns an error. [Not
currently implemented].
- RES_USEVC
- Use TCP connections for queries rather than UDP
datagrams.
- RES_PRIMARY
- Query primary domain name server only.
- RES_IGNTC
- Ignore truncation errors. Don't retry with TCP. [Not
currently implemented].
- RES_RECURSE
- Set the recursion desired bit in queries. Recursion is
carried out by the domain name server, not by res_send(). [Enabled
by default].
- RES_DEFNAMES
- If set, res_search() will append the default domain
name to single component names, i.e., those that do not contain a dot.
[Enabled by default].
- RES_STAYOPEN
- Used with RES_USEVC to keep the TCP connection open
between queries.
- RES_DNSRCH
- If set, res_search() will search for hostnames in
the current domain and in parent domains. This option is used by
gethostbyname(3). [Enabled by default].
This list is not complete. You can find some other flags described in
resolv.conf(5).
RETURN VALUE¶
The
res_init() function returns 0 on success, or -1 if an error occurs.
The
res_query(),
res_search(),
res_querydomain(),
res_mkquery() and
res_send() functions return the length of the
response, or -1 if an error occurs.
The
dn_comp() and
dn_expand() functions return the length of the
compressed name, or -1 if an error occurs.
FILES¶
/etc/resolv.conf resolver configuration file
/etc/host.conf resolver configuration file
4.3BSD.
SEE ALSO¶
gethostbyname(3),
resolv.conf(5),
resolver(5),
hostname(7),
named(8)
COLOPHON¶
This page is part of release 3.44 of the Linux
man-pages project. A
description of the project, and information about reporting bugs, can be found
at
http://www.kernel.org/doc/man-pages/.
