res_query, res_search, res_mkquery, res_send, res_init, dn_comp,
dn_expand - resolver routines
Standard C Library (libc, -lc)
res_query(const char *dname, int class, int type, u_char *answer,
res_search(const char *dname, int class, int type, u_char *answer,
res_mkquery(int op, const char *dname, int class, int type,
const char *data, int datalen, struct rrec *newrr, char *buf,
res_send(const u_char *msg, int msglen, u_char *answer, int anslen);
dn_comp(const char *exp_dn, u_char *comp_dn, int length, u_char **dnptrs,
dn_expand(const u_char *msg, const u_char *eomorig,
const u_char *comp_dn, u_char *exp_dn, int length);
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 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
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
RES_USE_INET6 Enables support for IPv6-only applications. This causes
IPv4 addresses to be returned as an IPv4 mapped address.
For example, 10.1.1.1 will be returned as ::ffff:10.1.1.1.
The option is meaningful with certain kernel configuration
RES_USE_EDNS0 Enables support for OPT pseudo-RR for EDNS0 extension.
With the option, resolver code will attach OPT pseudo-RR
into DNS queries, to inform of our receive buffer size.
The option will allow DNS servers to take advantage of
non-default receive buffer size, and to send larger
replies. DNS query packets with EDNS0 extension is not
compatible with non-EDNS0 DNS servers.
The res_init() routine reads the configuration file (if any; see
resolv.conf(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 hostname 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 resolv.conf(5).
Initialization normally occurs on the first call to one of the following
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-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
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 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 contained 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
/etc/resolv.conf The configuration file see resolv.conf(5).
gethostbyname(3), resolv.conf(5), hostname(7), named(8)
RFC974, RFC1032, RFC1033, RFC1034, RFC1035, RFC1535
Name Server Operations Guide for BIND.
The res_query function appeared in 4.3BSD.
BSD June 4, 1993 BSD
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