RESOLVER(3) BSD Programmer's Manual RESOLVER(3)
res_query, res_search, res_mkquery, res_send, res_init, dn_comp,
dn_expand - resolver routines
res_query(char *dname, int class, int type, u_char *answer, int anslen);
res_search(char *dname, int class, int type, u_char *answer, int anslen);
res_mkquery(int op, char *dname, int class, int type, char *data,
int datalen, struct rrec *newrr, char *buf, int buflen);
res_send(char *msg, int msglen, char *answer, int anslen);
dn_comp(char *exp_dn, char *comp_dn, int length, char **dnptrs,
dn_expand(u_char *msg, u_char *eomorig, u_char *comp_dn, u_char *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 reason-
able defaults and can be ignored. Options stored in _res.options are de-
fined 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 implement-
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 de-
fault. (res_send() does not do iterative queries and ex-
pects 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 rou-
tine 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 Inter-
net 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 nor-
mally occurs on the first call to one of the following routines.
The res_query() function provides an interface to the server query mecha-
nism. It constructs a query, sends it to the local server, awaits a re-
sponse, 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 up-
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 lo-
cal name server, and handle timeouts and retries. The length of the re-
ply 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 begin-
ning 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 com-
pressed. 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 uncom-
pressed 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 er-
/etc/resolv.conf The configuration file see resolver(5).
gethostbyname(3), named(8), resolver(5), hostname(7),
RFC1032, RFC1033, RFC1034, RFC1035, RFC974
Name Server Operations Guide for BIND.
The res_query function appeared in 4.3BSD.
4.3 Berkeley Distribution June 4, 1993 3