resolver(3) - Linux manual page (original) (raw)
resolver(3) Library Functions Manual resolver(3)
NAME top
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 routinesLIBRARY top
Resolver library (_libresolv_, _-lresolv_)SYNOPSIS top
**#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 ***_expdn_**, unsigned char** _compdn_**[.**_length_**],**
**int** _length_**, unsigned char** _dnptrs_**,**
**unsigned char** _lastdnptr_**);**
**int dn_expand(const unsigned char ***_msg_**,**
**const unsigned char ***_eomorig_**,**
**const unsigned char ***_compdn_**, char** _expdn_**[.**_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 top
**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 _resstate_ 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)](../man5/resolv.conf.5.html)) 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 _expdn_ and
stores it in the buffer _compdn_ 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
_compdn_ to a full domain name, which is placed in the buffer
_expdn_ 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 __resstate_ 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)](../man3/gethostbyname.3.html). [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)](../man3/gethostbyname.3.html) 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)](../man3/getaddrinfo.3.html), rather than [gethostbyname(3)](../man3/gethostbyname.3.html).
**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**.RETURN VALUE top
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 _herrno_ (see
[gethostbyname(3)](../man3/gethostbyname.3.html)) can be consulted to determine the error.FILES top
_/etc/resolv.conf_
resolver configuration file
_/etc/host.conf_
resolver configuration fileATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌───────────────────────────────┬───────────────┬────────────────┐
│ **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 top
None.HISTORY top
4.3BSD.SEE ALSO top
[gethostbyname(3)](../man3/gethostbyname.3.html), [resolv.conf(5)](../man5/resolv.conf.5.html), [resolver(5)](../man5/resolver.5.html), [hostname(7)](../man7/hostname.7.html),
**named**(8)
The GNU C library source file _resolv/README_.COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 resolver(3)
Pages that refer to this page:hostname(1), pmdanetcheck(1), gethostbyname(3), hosts(5), resolv.conf(5)