resolver(3) Library Functions Manual resolver(3)
NAME
res_ninit, res_nquery, res_nsearch, res_nquerydomain, res_nmkquery,
res_nsend, res_nclose, res_init, res_query, res_search, res_querydo-
main, 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(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);
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glibc are not described) and likely out of date.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 re-
sponses 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 in-
terfaces 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 ad-
dress(es). If no server is given, the local host is tried. If no do-
main 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 us-
ing 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 mes-
sage 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 al-
ready 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 mes-
sage. 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 func-
tions). 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 de-
fault.
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 op-
tion 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 dep-
recated, 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 sin-
gle 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 gethostby-
name(3). [Enabled by default].
RES_INSECURE1
Accept a response from a wrong server. This can be used to de-
tect potential security hazards, but you need to compile glibc
with debugging enabled and use RES_DEBUG option (for debug pur-
pose only).
RES_INSECURE2
Accept a response which contains a wrong query. This can be
used to detect potential security hazards, but you need to com-
pile 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 warn-
ing; applications should use getaddrinfo(3), rather than geth-
ostbyname(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 unimple-
mented 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 re-
solving 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_nquery-
domain(), res_querydomain(), res_nmkquery(), res_mkquery(),
res_nsend(), and res_send() functions return the length of the re-
sponse, or -1 if an error occurs.
The dn_comp() and dn_expand() functions return the length of the com-
pressed 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.
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 at-
tributes(7).
┌─────────────────────────────────────┬───────────────┬────────────────┐
│Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼────────────────┤
│res_ninit(), res_nclose(), │ Thread safety │ MT-Safe locale │
│res_nquery(), res_nsearch(), │ │ │
│res_nquerydomain(), res_nsend() │ │ │
├─────────────────────────────────────┼───────────────┼────────────────┤
│res_nmkquery(), dn_comp(), │ Thread safety │ MT-Safe │
│dn_expand() │ │ │
└─────────────────────────────────────┴───────────────┴────────────────┘
STANDARDS
4.3BSD.
SEE ALSO
gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)
The GNU C library source file resolv/README.
Linux man-pages 6.03 2023-02-05 resolver(3)
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