# resolver

## 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 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**.
    


## 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**(),    │ Thread safety │ MT-Safe locale │
           │ **res_nquery**(), **res_nsearch**(),  │               │                │
           │ **res_nquerydomain**(),           │               │                │
           │ **res_nsend**()                   │               │                │
           ├───────────────────────────────┼───────────────┼────────────────┤
           │ **res_nmkquery**(), **dn_comp**(),    │ Thread safety │ MT-Safe        │
           │ **dn_expand**()                   │               │                │
           └───────────────────────────────┴───────────────┴────────────────┘
    


## STANDARDS



           None.
    


* * *