res_init, res_query, res_search, res_querydomain,
res_mkquery, res_send, dn_comp, dn_expand - resolver rou-
extern struct state _res;
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 res_mkquery(int op, const char *dname, int class
int type, char *data, int datalen
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 *exp_dn
unsigned char **lastdnptr));
int dn_expand(unsigned char *msg, unsigned char *eomorig,
unsigned char *comp_dn, unsigned char *exp_dn,
These functions make queries to and interpret the responses
from Internet domain name servers.
The res_init() function reads the configuration files (see
resolv+(8)) 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
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 pre-formatted 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
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 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:
True if res_init() has been called.
Print debugging messages.
Accept authoritative answers only. res_send() contin-
ues until it fins an authoritative answer or returns an
error. [Not currently implemented].
Use TCP connections for queries rather than UDP
Query primary domain name server only.
Ignore truncation errors. Don't retry with TCP. [Not
Set the recursion desired bit in queries. Recursion is
carried out by the domain name server, not by
res_send(). [Enabled by default].
If set, res_search() will append the default domain
name to single component names, ie. those that do not
contain a dot. [Enabled by default].
Used with RES_USEVC to keep the TCP connection open
If set, res_search() will search for host names in the
current domain and in parent domains. This option is
used by gethostbyname(3). [Enabled by default].
The res_init() function returns 0 on success, or -1 if an
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.
/etc/resolv.conf resolver configuration file
/etc/host.conf resolver configuration file
gethostbyname(3), hostname(7), named(8),