resolver
Section: C Library Functions (3)
Updated: 202-0-08
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NAME
res_ninit, res_nquery, res_nsearch, res_nquerydomain, res_nmkquery, res_nsend,
res_nclose,
res_init, res_query, res_search, res_querydomain, res_mkquery, res_send,
dn_comp, dn_expand -
resolver routines
LIBRARY
Resolver library
(
libresolv,~
-lresolv)
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);
void res_nclose(res_state statep);
int res_nquery(int anslen;
res_state statep,
const char *dname,
int class, int type,
unsigned char answer[anslen], int anslen);
int res_nsearch(int anslen;
res_state statep,
const char *dname,
int class, int type,
unsigned char answer[anslen], int anslen);
int res_nquerydomain(int anslen;
res_state statep,
const char *name, const char *domain,
int class, int type,
unsigned char answer[anslen], int anslen);
int res_nmkquery(int datalen, int buflen;
res_state statep, int op,
const char *dname,
int class, int type,
const unsigned char data[datalen], int datalen,
const unsigned char *newrr,
unsigned char buf[buflen], int buflen);
int res_nsend(int msglen, int anslen;
res_state statep,
const unsigned char msg[msglen], int msglen,
unsigned char answer[anslen], int anslen);
int dn_comp(int length;
const char *exp_dn,
unsigned char comp_dn[length], int length,
unsigned char **dnptrs,
unsigned char **lastdnptr);
int dn_expand(int length;
const unsigned char *msg,
const unsigned char *eomorig,
const unsigned char *comp_dn,
char exp_dn[length], int length);
[[deprecated]] extern struct __res_state _res;
[[deprecated]] int res_init(void);
[[deprecated]]
int res_query(int anslen;
const char *dname,
int class, int type,
unsigned char answer[anslen], int anslen);
[[deprecated]]
int res_search(int anslen;
const char *dname,
int class, int type,
unsigned char answer[anslen], int anslen);
[[deprecated]]
int res_querydomain(int anslen;
const char *name, const char *domain,
int class, int type,
unsigned char answer[anslen], int anslen);
[[deprecated]]
int res_mkquery(int datalen, int buflen;
int op,
const char *dname,
int class, int type,
const unsigned char data[datalen], int datalen,
const unsigned char *newrr,
unsigned char buf[buflen], int buflen);
[[deprecated]]
int res_send(int msglen, int anslen;
const unsigned char msg[msglen], int msglen,
unsigned char answer[anslen], int anslen);
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 no-threa-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 pe-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.
Every call to
res_ninit()
requires a corresponding call to
res_nclose()
to free memory allocated by
res_ninit()
and subsequent calls to
res_nquery().
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 lowe-level routines used by
res_nquery()/
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 one of the following (typically
QUERY):
- QUERY
-
Standard query.
- IQUERY
-
Inverse query.
This option was removed in glibc 2.26,
since it has not been supported by DNS servers for a very long time.
- NS_NOTIFY_OP
-
Notify secondary of SOA (Start of Authority) change.
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 until glibc 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 until glibc 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[em]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 roun-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 (_), no-ASCII,
or control characters.
This option was present until glibc 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 until glibc 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 until glibc 2.24;
since glibc 2.25, it is deprecated, and its usage produces a warning.
- RES_USEBSTRING (glibc 2.3.4 to glibc 2.24)
-
Make reverse IPv6 lookups using the bi-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 backwar-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 up to and including glibc 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 glibc 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 to-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.
In the case of an error return from
res_nquery(),
res_query(),
res_nsearch(),
res_search(),
res_nquerydomain(),
or
res_querydomain(),
the global variable
h_errno
(see
gethostbyname(3))
can be consulted to determine the error.
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_nclose(),
res_nquery(),
res_nsearch(),
res_nquerydomain(),
res_nsend()
| Thread safety | M-Safe locale
|
|
res_nmkquery(),
dn_comp(),
dn_expand()
| Thread safety | M-Safe
|
STANDARDS
None.
HISTORY
4.3BSD.
SEE ALSO
gethostbyname(3),
resolv.conf(5),
resolver(5),
hostname(7),
named(8)
The GNU C library source file
resolv/README.
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- FILES
-
- ATTRIBUTES
-
- STANDARDS
-
- HISTORY
-
- SEE ALSO
-
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- Running on 
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