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 routines
Standard C Library (libc, -lc)
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
int
res_query
(const char *dname,
int class, int type,
u_char *answer, int anslen);
int
res_search
(const char *dname,
int class, int type,
u_char *answer, int anslen);
int
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, int
buflen);
int
res_send
(const u_char *msg,
int msglen, u_char *answer,
int anslen);
int
res_init
(void);
int
dn_comp
(const char *exp_dn,
u_char *comp_dn, int length,
u_char **dnptrs, u_char
**lastdnptr);
int
dn_expand
(const u_char *msg,
const u_char *eomorig, const u_char
*comp_dn, char *exp_dn, int
length);
int
dn_skipname
(const
u_char *comp_dn, const
u_char *eom);
u_int
ns_get16
(const
u_char *src);
u_long
ns_get32
(const
u_char *src);
void
ns_put16
(u_int
src, u_char
*dst);
void
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 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 used.
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 default.
RES_NOALIASES
- This option turns off the user level aliasing feature controlled by the
“
HOSTALIASES
” environment variable.
Network 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 only.
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 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
resolver(5).
Initialization normally occurs on the first call to one of the following
routines.
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. 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 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 error.
The dn_skipname
() function skips over a
compressed domain name, which starts at a location pointed to by
comp_dn. The compressed name is contained 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
error.
The ns_get16
() function gets a 16-bit
quantity from a buffer pointed to by src.
The ns_get32
() function gets a 32-bit
quantity from a buffer pointed to by src.
The ns_put16
() function puts a 16-bit
quantity src to a buffer pointed to by
dst.
The ns_put32
() function puts a 32-bit
quantity src to a buffer pointed to by
dst.
This implementation of the resolver is thread-safe, but it will not function
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 that macro.
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).
The res_query
function appeared in
4.3BSD.