• NAME
  
  res_init, res_query, res_search, res_querydomain, res_mkquery, res_send, dn_comp, dn_expand - resolver routines
• SYNOPSIS
  

  
   #include <netinet/in.h>
   #include <arpa/nameser.h>
   #include <resolv.h>
   extern struct state _res;
  
   int res_init(void);
  
   int res_query(const char * dname , int  class , int  type ,
      unsigned char * answer , int  anslen );    
    
  
       int res_search(const char * dname , int  class , int  type ,   
      unsigned char * answer , int  anslen );    
    
  
       int res_querydomain(const char * name , const char * domain ,   
      int  class , int  type , unsigned char * answer ,    int  anslen );   
    
  
       int res_mkquery(int  op , const char * dname , int  class ,   
      int  type , char * data , int  datalen , struct rrec * newrr ,    char * buf , int  buflen );   
    
  
       int res_send(const char * msg , int  msglen , char * answer ,   
      int  anslen );    
    
  
       int dn_comp(unsigned char * exp_dn , unsigned char * comp_dn ,   
      int  length , unsigned char ** dnptrs , unsigned char * exp_dn ,    unsigned char ** lastdnptr );   
    
  
       int dn_expand(unsigned char * msg , unsigned char * eomorig ,   
      unsigned char * comp_dn , char * exp_dn ,    int  length );   
    
  
     Link with -lresolv.
  
  
    

• DESCRIPTION
  
  These functions make queries to and interpret the responses from Internet
  domain name servers.
  
  
  
  The
  res_init ()
  function reads the configuration files (see
  resolv.conf(5) ) to get the default domain name, search order 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_init ()
  is normally executed by the first call to one of the
  other functions.
  
  
  
  The
  res_query ()
  function queries 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_search ()
  function makes a query and waits for the response
  like
  res_query (),
  but in addition implements the default and search
  rules controlled by
  RES_DEFNAMES and
  RES_DNSRCH (see description of
  _res options below).
  
  
  
  The
  res_querydomain ()
  function makes a query using
  res_query ()
  on the concatenation of name and domain.
  
  
  
  The following functions are lower-level routines used by
  res_query ().
  
  
  
  The
  res_mkquery ()
  function constructs a query message in buf
  of length buflen for the domain name dname.
  The query type
  op is usually
  QUERY ,
  but can be any of the types defined in
  <arpa/nameser.h>.
  newrr is currently unused.
  
  
  
  The
  res_send ()
  function sends a pre-formatted query given in
  msg of length msglen and returns the answer in answer
  which is of length anslen.
  It will call
  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 global configuration and state information
  contained in the structure _res, which is defined in
  <resolv.h>.
  The only field that is normally manipulated by the
  user is _res.options.
  This field can contain the bitwise "OR"
  of the following options:
  

  RES_INIT

  True if
  res_init ()
  has been called.
  

  RES_DEBUG

  Print debugging messages.
  

  RES_AAONLY

  Accept authoritative answers only.
  res_send ()
  continues until
  it finds an authoritative answer or returns an error.  [Not currently
  implemented].
  

  RES_USEVC

  Use TCP connections for queries rather than UDP datagrams.
  

  RES_PRIMARY

  Query primary domain name server only.
  

  RES_IGNTC

  Ignore truncation errors.
  Don't retry with TCP.  [Not currently
  implemented].
  

  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, i.e., 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].
  

• RETURN VALUE
  
  The
  res_init ()
  function returns 0 on success, or -1 if an error
  occurs.
  
  
  
  The
  res_query (),
  res_search (),
  res_querydomain (),
  res_mkquery ()
  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.
  
• FILES
  

  
  /etc/resolv.conf          resolver configuration file
  /etc/host.conf            resolver configuration file
  

• CONFORMING TO
  
  4.3BSD.
• SEE ALSO
  
  gethostbyname(3) , resolv.conf(5) , resolver(5) , hostname(7) , named(8)
• COLOPHON
  
  This page is part of release 3.19 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

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