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 rea‐
sonable 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
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 sin‐
gle-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 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. This envi‐
ronment 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 com‐
mand. 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 mecha‐
nism. 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 mak‐
ing 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 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 previ‐
ously-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 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 error.
/etc/resolv.conf see resolver(5)SEE ALSOgethostbyname(3), named(8), resolver(5), hostname(7),
RFC1032, RFC1033, RFC1034, RFC1035, RFC974,
SMM:11 Name Server Operations Guide for BIND
4th Berkeley Distribution November 11, 1993 RESOLVER(3)