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
Resolver library (
libresolv,
-lresolv)
#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(res_state statep,
const char *dname, int class, int type,
unsigned char answer[.anslen], int anslen);
int res_nsearch(res_state statep,
const char *dname, int class, int type,
unsigned char answer[.anslen], int anslen);
int res_nquerydomain(res_state statep,
const char *name, const char *domain,
int class, int type, unsigned char answer[.anslen],
int anslen);
int res_nmkquery(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(res_state statep,
const unsigned char msg[.msglen], int msglen,
unsigned char answer[.anslen], int anslen);
int dn_comp(const char *exp_dn, unsigned char comp_dn[.length],
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[.length],
int length);
[[deprecated]] extern struct __res_state _res;
[[deprecated]] int res_init(void);
[[deprecated]]
int res_query(const char *dname, int class, int type,
unsigned char answer[.anslen], int anslen);
[[deprecated]]
int res_search(const char *dname, int class, int type,
unsigned char answer[.anslen], int anslen);
[[deprecated]]
int res_querydomain(const char *name, const char *domain,
int class, int type, unsigned char answer[.anslen],
int anslen);
[[deprecated]]
int res_mkquery(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(const unsigned char msg[.msglen], int msglen,
unsigned char answer[.anslen], int anslen);
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glibc are not described) and likely out of date.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. 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 lower-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—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 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 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 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 top-level domain
(TLD).
- RES_DEFAULT
- Default option which implies: RES_RECURSE,
RES_DEFNAMES, RES_DNSRCH, and RES_NOIP6DOTINT.
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 cause of the error.
- /etc/resolv.conf
- resolver configuration file
- /etc/host.conf
- resolver configuration file
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 |
MT-Safe locale |
| res_nmkquery (), dn_comp (), dn_expand () |
Thread safety |
MT-Safe |
4.3BSD.
gethostbyname(3),
resolv.conf(5),
resolver(5),
hostname(7),
named(8)
The GNU C library source file
resolv/README.