NAME¶
res_ninit, res_nquery, res_nsearch, res_nquerydomain, res_nmkquery, res_nsend,
res_init, res_query, res_search, res_querydomain, res_mkquery, res_send,
dn_comp, dn_expand - resolver routines
SYNOPSIS¶
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
struct __res_state;
typedef struct __res_state *res_state;
int res_ninit(res_state statep);
int res_nquery(res_state statep,
const char *dname, int class, int type,
unsigned char *answer, int anslen);
int res_nsearch(res_state statep,
const char *dname, int class, int type,
unsigned char *answer, int anslen);
int res_nquerydomain(res_state statep,
const char *name, const char *domain,
int class, int type, unsigned char *answer,
int anslen);
int res_nmkquery(res_state statep,
int op, const char *dname, int class,
int type, const unsigned char *data, int datalen,
const unsigned char *newrr,
unsigned char *buf, int buflen);
int res_nsend(res_state statep,
const unsigned char *msg, int msglen,
unsigned char *answer, int anslen);
int dn_comp(const char *exp_dn, unsigned char *comp_dn,
int length, unsigned char **dnptrs,
unsigned char **lastdnptr);
int dn_expand(const unsigned char *msg,
const unsigned char *eomorig,
const unsigned char *comp_dn, char *exp_dn,
int length);
Deprecated¶
extern struct __res_state _res;
int res_init(void);
int res_query(const char *dname, int class, int type,
unsigned char *answer, int anslen);
int res_search(const char *dname, int class, int type,
unsigned char *answer, int anslen);
int res_querydomain(const char *name, const char *domain,
int class, int type, unsigned char *answer,
int anslen);
int res_mkquery(int op, const char *dname, int class,
int type, const unsigned char *data, int datalen,
const unsigned char *newrr,
unsigned char *buf, int buflen);
int res_send(const unsigned char *msg, int msglen,
unsigned char *answer, int anslen);
Link with
-lresolv.
DESCRIPTION¶
Note: This page is incomplete (various resolver functions provided by
glibc are not described) and likely out of date.
The functions described below make queries to and interpret the responses from
Internet domain name servers.
The API consists of a set of more modern, reentrant functions and an older set
of nonreentrant functions that have been superseded. The traditional resolver
interfaces such as
res_init() and
res_query() use some static
(global) state stored in the
_res structure, rendering these functions
non-thread-safe. BIND 8.2 introduced a set of new interfaces
res_ninit(),
res_nquery(), and so on, which take a
res_state as their first argument, so you can use a per-thread resolver
state.
The
res_ninit() and
res_init() functions read the configuration
files (see
resolv.conf(5)) to get the default domain name and name
server address(es). If no server is given, the local host is tried. If no
domain is given, that associated with the local host is used. It can be
overridden with the environment variable
LOCALDOMAIN.
res_ninit() or
res_init() is normally executed by the first call
to one of the other functions.
The
res_nquery() and
res_query() functions query the name server
for the fully qualified domain name
name of specified
type and
class. The reply is left in the buffer
answer of length
anslen supplied by the caller.
The
res_nsearch() and
res_search() functions make a query and
waits for the response like
res_nquery() and
res_query(), but in
addition they implement the default and search rules controlled by
RES_DEFNAMES and
RES_DNSRCH (see description of
_res
options below).
The
res_nquerydomain() and
res_querydomain() functions make a
query using
res_nquery()/
res_query() on the concatenation of
name and
domain.
The following functions are lower-level routines used by
res_query()/
res_query().
The
res_nmkquery() and
res_mkquery() functions construct a query
message in
buf of length
buflen for the domain name
dname. The query type
op is usually
QUERY, but can be any
of the types defined in
<arpa/nameser.h>.
newrr is
currently unused.
The
res_nsend() and
res_send() function send a preformatted query
given in
msg of length
msglen and returns the answer in
answer which is of length
anslen. They will call
res_ninit()/
res_init() if it has not already been called.
The
dn_comp() function compresses the domain name
exp_dn and
stores it in the buffer
comp_dn of length
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 of the array is specified by
lastdnptr. If
dnptr is NULL, domain names are not compressed. If
lastdnptr is NULL, the list of labels is not updated.
The
dn_expand() function expands the compressed domain name
comp_dn to a full domain name, which is placed in the buffer
exp_dn of size
length. The compressed name is contained in a
query or reply message, and
msg points to the beginning of the message.
The resolver routines use configuration and state information contained in a
__res_state structure (either passed as the
statep argument, or
in the global variable
_res, in the case of the older nonreentrant
functions). The only field of this structure that is normally manipulated by
the user is the
options field. This field can contain the bitwise
"OR" of the following options:
- RES_INIT
- True if res_ninit() or res_init() has been called.
- RES_DEBUG
- Print debugging messages. This option is available only if glibc was built
with debugging enabled, which is not the default.
- RES_AAONLY (unimplemented; deprecated in glibc 2.25)
- Accept authoritative answers only. res_send() continues until it
finds an authoritative answer or returns an error. This option was present
but unimplemented in glibc until version 2.24; since glibc 2.25, it is
deprecated, and its usage produces a warning.
- RES_USEVC
- Use TCP connections for queries rather than UDP datagrams.
- RES_PRIMARY (unimplemented; deprecated in glibc 2.25)
- Query primary domain name server only. This option was present but
unimplemented in glibc until version 2.24; since glibc 2.25, it is
deprecated, and its usage produces a warning.
- RES_IGNTC
- Ignore truncation errors. Don't retry with TCP.
- RES_RECURSE
- Set the recursion desired bit in queries. Recursion is carried out by the
domain name server, not by res_send(). [Enabled by default].
- RES_DEFNAMES
- If set, res_search() will append the default domain name to single
component names—that is, those that do not contain a dot. [Enabled
by default].
- RES_STAYOPEN
- Used with RES_USEVC to keep the TCP connection open between
queries.
- RES_DNSRCH
- If set, res_search() will search for hostnames in the current
domain and in parent domains. This option is used by
gethostbyname(3). [Enabled by default].
- RES_INSECURE1
- Accept a response from a wrong server. This can be used to detect
potential security hazards, but you need to compile glibc with debugging
enabled and use RES_DEBUG option (for debug purpose only).
- RES_INSECURE2
- Accept a response which contains a wrong query. This can be used to detect
potential security hazards, but you need to compile glibc with debugging
enabled and use RES_DEBUG option (for debug purpose only).
- RES_NOALIASES
- Disable usage of HOSTALIASES environment variable.
- RES_USE_INET6
- Try an AAAA query before an A query inside the gethostbyname(3)
function, and map IPv4 responses in IPv6 "tunneled form" if no
AAAA records are found but an A record set exists. Since glibc 2.25, this
option is deprecated, and its usage produces a warning; applications
should use getaddrinfo(3), rather than
gethostbyname(3).
- RES_ROTATE
- Causes round-robin selection of name servers from among those listed. This
has the effect of spreading the query load among all listed servers,
rather than having all clients try the first listed server first every
time.
- RES_NOCHECKNAME (unimplemented; deprecated in glibc 2.25)
- Disable the modern BIND checking of incoming hostnames and mail names for
invalid characters such as underscore (_), non-ASCII, or control
characters. This option was present in glibc until version 2.24; since
glibc 2.25, it is deprecated, and its usage produces a warning.
- RES_KEEPTSIG (unimplemented; deprecated in glibc 2.25)
- Do not strip TSIG records. This option was present but unimplemented in
glibc until version 2.24; since glibc 2.25, it is deprecated, and its
usage produces a warning.
- RES_BLAST (unimplemented; deprecated in glibc 2.25)
- Send each query simultaneously and recursively to all servers. This option
was present but unimplemented in glibc until version 2.24; since glibc
2.25, it is deprecated, and its usage produces a warning.
- RES_USEBSTRING (glibc 2.3.4 to 2.24)
- Make reverse IPv6 lookups using the bit-label format described in RFC
2673; if this option is not set (which is the default), then nibble format
is used. This option was removed in glibc 2.25, since it relied on a
backward-incompatible DNS extension that was never deployed on the
Internet.
- RES_NOIP6DOTINT (glibc 2.24 and earlier)
- Use ip6.arpa zone in IPv6 reverse lookup instead of ip6.int,
which is deprecated since glibc 2.3.4. This option is present in glibc up
to and including version 2.24, where it is enabled by default. In glibc
2.25, this option was removed.
- RES_USE_EDNS0 (since glibc 2.6)
- Enables support for the DNS extensions (EDNS0) described in RFC 2671.
- RES_SNGLKUP (since glibc 2.10)
- By default, glibc performs IPv4 and IPv6 lookups in parallel since version
2.9. Some appliance DNS servers cannot handle these queries properly and
make the requests time out. This option disables the behavior and makes
glibc perform the IPv6 and IPv4 requests sequentially (at the cost of some
slowdown of the resolving process).
- RES_SNGLKUPREOP
- When RES_SNGLKUP option is enabled, opens a new socket for the each
request.
- RES_USE_DNSSEC
- Use DNSSEC with OK bit in OPT record. This option implies
RES_USE_EDNS0.
- RES_NOTLDQUERY
- Do not look up unqualified name as a top-level domain (TLD).
- RES_DEFAULT
- Default option which implies: RES_RECURSE, RES_DEFNAMES,
RES_DNSRCH and RES_NOIP6DOTINT.
RETURN VALUE¶
The
res_ninit() and
res_init() functions return 0 on success, or
-1 if an error occurs.
The
res_nquery(),
res_query(),
res_nsearch(),
res_search(),
res_nquerydomain(),
res_querydomain(),
res_nmkquery(),
res_mkquery(),
res_nsend(), and
res_send() functions return the length of the response, or -1 if an
error occurs.
The
dn_comp() and
dn_expand() functions return the length of the
compressed name, or -1 if an error occurs.
FILES¶
/etc/resolv.conf resolver configuration file
/etc/host.conf resolver configuration file
ATTRIBUTES¶
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
res_ninit (), res_nquery (), res_nsearch (), res_nquerydomain (),
res_nsend () |
Thread safety |
MT-Safe locale |
res_nmkquery (), dn_comp (), dn_expand () |
Thread safety |
MT-Safe |
4.3BSD.
SEE ALSO¶
gethostbyname(3),
resolv.conf(5),
resolver(5),
hostname(7),
named(8)
The GNU C library source file
resolv/README.
COLOPHON¶
This page is part of release 4.10 of the Linux
man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at
https://www.kernel.org/doc/man-pages/.