mdw@git.distorted.org.uk Git - mLib/blob - sel/bres.3

   1 .\" -*-nroff-*-
   2 .TH bres 3 "1 October 1999" "Straylight/Edgeware" "mLib utilities library"
   3 .SH NAME
   4 bres \- background name resolver
   5 .\" @bres_abort
   6 .\" @bres_byname
   7 .\" @bres_byaddr
   8 .\" @bres_exec
   9 .\" @bres_init
  10 .SH SYNOPSIS
  11 .nf
  12 .B "#include <mLib/bres.h>"
  13
  14 .ds mT \fBvoid bres_byname(
  15 .BI "\*(mTbres_client *" rc ", const char *" name ,
  16 .BI "\h'\w'\*(mT'u'void (*" func ")(struct hostent *" h ", void *" p ),
  17 .BI "\h'\w'\*(mT'u'void *" p );
  18 .ds mT \fBvoid bres_byaddr(
  19 .BI "\*(mTbres_client *" rc ", struct inaddr " addr ,
  20 .BI "\h'\w'\*(mT'u'void (*" func ")(struct hostent *" h ", void *" p ),
  21 .BI "\h'\w'\*(mT'u'void *" p );
  22 .BI "void bres_abort(bres_client *" rc );
  23 .BI "void bres_exec(const char *" file );
  24 .BI "void bres_init(sel_state *" sel );
  25 .fi
  26 .SH DESCRIPTION
  27 The
  28 .B bres.h
  29 header file declares types and functions for doing translation between
  30 host names and IP addresses in the background.
  31 .PP
  32 The system must be initialized before use by a call to
  33 .BR bres_init ,
  34 passing it the address of an I/O multiplexor (see
  35 .BR sel (3)).
  36 .PP
  37 A resolver task is stored in an object of type
  38 .BR bres_client ,
  39 the storage for which is allocated by the caller.  The object is a
  40 structure, and its contents are unspecified.  The object is initialized
  41 by one of the name resolution functions
  42 .B bres_byname
  43 and
  44 .BR bres_byaddr .
  45 Each function is passed the following arguments:
  46 .TP
  47 .BI "bres_client *" rc
  48 Pointer to the client block to initialize and store the resolver job's
  49 state.
  50 .TP
  51 .BI "struct in_addr " addr "\fR (\fBbres_byaddr\fR)"
  52 .sp -1
  53 .TP
  54 .BI "const char *" name "\fR (\fBbres_byname\fR)"
  55 The IP address or hostname to resolve.
  56 .TP
  57 .BI "void (*" func ")(struct hostent *" h ", void *" p )
  58 A handler function to call when the resolver job is complete.
  59 .TP
  60 .BI "void *" p
  61 A pointer argument to pass to the handler function.
  62 .PP
  63 The
  64 .B bres_client
  65 block must not be discarded until either the job is complete (i.e., the
  66 handler function has been called) or
  67 .B bres_abort
  68 is called on it.
  69 .PP
  70 The handler function is passed either the address of a
  71 .B "struct hostent"
  72 structure describing the resolved host, or a null pointer indicating
  73 failure.  The
  74 .B hostent
  75 structure is as returned by the standard
  76 .BR gethostbyname (3)
  77 and
  78 .BR gethostbyaddr (3)
  79 functions.  This isn't the most convenient format for the results, but
  80 it does have the benefit of being standard.  Similarly, errors are
  81 reported through the global
  82 .B h_errno
  83 variable.
  84 .PP
  85 The function
  86 .B bres_abort
  87 cancels a running resolver job.  When it returns, the client structure
  88 is safe to discard.
  89 .PP
  90 There are two versions of
  91 .BR bres .
  92 The standard one uses a pool of server processes.  Incoming resolver
  93 jobs are passed to an available server, or a new server is started if
  94 all are busy.  There is a maximum number of servers, and jobs are queued
  95 once this limit is reached.  Old servers which have been idle for a
  96 period of time are killed off.  Servers are also killed if they start
  97 misbehaving or their jobs are aborted.
  98 .PP
  99 By default, servers are started simply by calling
 100 .BR fork (2).
 101 This can cause undesirably high memory usage in large programs.  The
 102 function
 103 .B bres_exec
 104 requests the resolver system to
 105 .BR exec (2)
 106 a small dedicated server program to perform name lookups to reduce
 107 memory consumption.  The argument to
 108 .B bres_exec
 109 is the full pathname of the server program, or null to accept the
 110 default set at library configuration time (which is usually correct).
 111 .PP
 112 The other implementation of
 113 .B bres
 114 uses the
 115 .B adns
 116 library to do asynchronous resolution.  It can cope with many more
 117 simultaneous resolver jobs, and doesn't use up external processes.  If
 118 you're using the
 119 .BR adns -based
 120 resolver, then the
 121 .B bres_exec
 122 function does nothing at all.
 123 .PP
 124 For security reasons, when an address is resolved, the hostname received
 125 is verified by performing a forward lookup.  If the forward lookup fails
 126 to return the expected IP address, an error is reported.
 127 .SH "SEE ALSO"
 128 .BR gethostbyname (3),
 129 .BR gethostbyaddr (3),
 130 .BR sel (3),
 131 .BR mLib (3).
 132 .SH "AUTHOR"
 133 Mark Wooding, <mdw@distorted.org.uk>