RESOLVER(3) BSD Library Functions Manual RESOLVER(3)NAME
res_query, res_search, res_mkquery, res_send, res_init, dn_comp,
dn_expand, dn_skipname, ns_get16, ns_get32, ns_put16, ns_put32 — resolver
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 u_char *data, int datalen, const u_char *newrr_in, u_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, char *exp_dn, int length);
dn_skipname(const u_char *comp_dn, const u_char *eom);
ns_get16(const u_char *src);
ns_get32(const u_char *src);
ns_put16(u_int src, u_char *dst);
ns_put32(u_long src, u_char *dst);
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
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 imple‐
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., do not
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_NOALIASES This option turns off the user level aliasing feature con‐
trolled by the “HOSTALIASES” environment variable. Net‐
work daemons should set this option.
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
resolver(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 over‐
ride 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 resolver(5). Ini‐
tialization normally occurs on the first call to one of the following
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-qual‐
ified 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. The newrr_in argument is currently unused but is
intended for making 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 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 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
The dn_skipname() function skips over a compressed domain name, which
starts at a location pointed to by comp_dn. The compressed name is con‐
tained in a query or reply message; eom is a pointer to the end of the
message. The size of compressed name is returned or -1 if there was an
The ns_get16() function gets a 16-bit quantity from a buffer pointed to
The ns_get32() function gets a 32-bit quantity from a buffer pointed to
The ns_put16() function puts a 16-bit quantity src to a buffer pointed to
The ns_put32() function puts a 32-bit quantity src to a buffer pointed to
This implementation of the resolver is thread-safe, but it will not func‐
tion properly if the programmer attempts to declare his or her own _res
structure in an attempt to replace the per-thread version referred to by
The following compile-time option can be specified to change the default
behavior of resolver routines when necessary.
RES_ENFORCE_RFC1034 If this symbol is defined during compile-time,
res_search() will enforce RFC 1034 check, namely,
disallow using of underscore character within host
names. This is used by the standard host lookup
routines like gethostbyname(3). For compatibility
reasons this option is not enabled by default.
The res_init() function will return 0 on success, or -1 in a threaded
program if per-thread storage could not be allocated.
The res_mkquery(), res_search(), and res_query() functions return the
size of the response on success, or -1 if an error occurs. The integer
h_errno may be checked to determine the reason for error. See
gethostbyname(3) for more information.
/etc/resolv.conf The configuration file, see resolver(5).
SEE ALSOgethostbyname(3), resolver(5), hostname(7), named(8)
RFC1032, RFC1033, RFC1034, RFC1035, RFC974
Name Server Operations Guide for BIND.
The res_query function appeared in 4.3BSD.
BSD May 29, 2009 BSD