ch16_05.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

socket(Server, PF_INET, SOCK_STREAM, getprotobyname('tcp'));

# so we can restart our server quickly
setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, 1);

# build up my socket address
$my_addr = sockaddr_in($server_port, INADDR_ANY);
bind(Server, $my_addr)
    or die "Couldn't bind to port $server_port: $!\n";

# establish a queue for incoming connections
listen(Server, SOMAXCONN)
    or die "Couldn't listen on port $server_port: $!\n";

# accept and process connections
while (accept(Client, Server)) {
    # do something with new Client connection
}

close(Server);</pre>
</blockquote>

The client doesn't need to <tt class="literal">bind</tt> to any address, but the server
does.  We've specified its address as <tt class="literal">INADDR_ANY</tt>, which means
that clients can connect from any available network interface.  If you
want to sit on a particular interface (like the external side of a
gateway or firewall machine), use that interface's real address
instead.  (Clients can do this, too, but rarely need to.)</p>

<p>
<a name="INDEX-3095"></a><a name="INDEX-3096"></a>
If you want to know which machine connected to you, call
<tt class="literal">getpeername</tt> on the client connection.  This returns
an IP address, which you'll have to translate into a name on your own
(if you can):
<blockquote>
<pre class="programlisting">use Socket;
$other_end = getpeername(Client)
    or die "Couldn't identify other end: $!\n";
($port, $iaddr) = unpack_sockaddr_in($other_end);
$actual_ip = inet_ntoa($iaddr);
$claimed_hostname = gethostbyaddr($iaddr, AF_INET);</pre>
</blockquote>

This is trivially spoofable because the owner of that IP address
can set up their reverse tables to say anything they want.  For 
a small measure of additional confidence, translate back the 
other way again:
<blockquote>
<pre class="programlisting">@name_lookup = gethostbyname($claimed_hostname)
    or die "Could not reverse $claimed_hostname: $!\n";
@resolved_ips = map { inet_ntoa($_) } @name_lookup[ 4 .. $#name_lookup ];
$might_spoof = !grep { $actual_ip eq $_ } @resolved_ips;</pre>
</blockquote>
<a name="INDEX-3097"></a><a name="INDEX-3098"></a><a name="INDEX-3099"></a>
Once a client connects to your server, your server can do I/O both
to and from that client handle.  But while the server is so engaged,
it can't service any further incoming requests from other clients.
To avoid getting locked down to just one client at a time,
many servers immediately <tt class="literal">fork</tt> a clone of themselves to handle
each incoming connection.  (Others <tt class="literal">fork</tt> in advance, or multiplex
I/O between several clients using the <tt class="literal">select</tt> syscall.)
<blockquote>
<pre class="programlisting">REQUEST:
while (accept(Client, Server)) {
    if ($kidpid = fork) {
        close Client;         # parent closes unused handle
        next REQUEST;
    } 
    defined($kidpid)   or die "cannot fork: $!" ;
    
    close Server;             # child closes unused handle
    
    select(Client);           # new default for prints
    $| = 1;                   # autoflush
    
    # per-connection child code does I/O with Client handle
    $input = &lt;Client&gt;;
    print Client "output\n";  # or STDOUT, same thing
    
    open(STDIN, "&lt;&lt;&amp;Client")    or die "can't dup client: $!";
    open(STDOUT, "&gt;&amp;Client")    or die "can't dup client: $!";
    open(STDERR, "&gt;&amp;Client")    or die "can't dup client: $!";
    
    # run the calculator, just as an example
    system("bc -l");     # or whatever you'd like, so long as
                         # it doesn't have shell escapes!
    print "done\n";      # still to client
    
    close Client;
    exit;  # don't let the child back to accept!
}</pre>
</blockquote>
<a name="INDEX-3100"></a><a name="INDEX-3101"></a>
This server clones off a child with <tt class="literal">fork</tt> for each incoming
request.  That way it can handle many requests at once, as long as
you can create more processes.  (You might want to limit this.)
Even if you don't <tt class="literal">fork</tt>, the <tt class="literal">listen</tt> will allow up to <tt class="literal">SOMAXCONN</tt>
(usually five or more) pending connections.  Each connection uses
up some resources, although not as much as a process.  Forking
servers have to be careful about cleaning up after their expired
children (called "zombies" in Unix-speak) because otherwise they'd
quickly fill up your process table.  The <tt class="literal">REAPER</tt> code discussed
in the section <a href="ch16_01.htm#ch16-sect-signals">Section 16.1, "Signals"</a> will take care of that for you, or you
may be able to assign <tt class="literal">$SIG{CHLD} = 'IGNORE'</tt>.</p>

<p>
<a name="INDEX-3102"></a>
Before running another command, we connect the standard input and
output (and error) up to the client connection.  This way any command
that reads from <tt class="literal">STDIN</tt> and writes to <tt class="literal">STDOUT</tt> can also talk to the
remote machine.  Without the reassignment, the command couldn't find
the client handle--which by default gets closed across the <tt class="literal">exec</tt>
boundary, anyway.</p>

<p>
<a name="INDEX-3103"></a><a name="INDEX-3104"></a><a name="INDEX-3105"></a>
When you write a networking server, we strongly suggest that you
use the <tt class="userinput"><b>-T</b></tt> switch to enable taint checking even if you aren't
running setuid or setgid.  This is always a good idea for servers
and any other program that runs on behalf of someone else (like all
CGI scripts), because it lessens the chances that people from the
outside will be able to compromise your system.  See the section
<a href="ch23_01.htm#ch23-sect-hid">Section 16.1, "Handling Insecure Data"</a> in <a href="ch23_01.htm">Chapter 23, "Security"</a> for much
more about all this.</p>

<p>
<a name="INDEX-3106"></a><a name="INDEX-3107"></a>
One additional consideration when writing Internet programs:  many
protocols specify that the line terminator should be <tt class="literal">CRLF</tt>, which can be
specified various ways:  <tt class="literal">"\015\12"</tt>, or <tt class="literal">"\xd\xa"</tt>, or even
<tt class="literal">chr(13).chr(10)</tt>.  As of version 5.6 of Perl, saying <tt class="literal">v13.10</tt> also
produces the same string.  (On many machines, you can also use
<tt class="literal">"\r\n"</tt> to mean <tt class="literal">CRLF</tt>, but don't use <tt class="literal">"\r\n"</tt> if you want to be
portable to Macs, where the meanings of <tt class="literal">\r</tt> and <tt class="literal">\n</tt> are reversed!)
Many Internet programs will in fact accept a bare <tt class="literal">"\012"</tt> as a line
terminator, but that's because Internet programs usually try to be
liberal in what they accept and strict in what they emit.  (Now if only
we could get people to do the same...)</p>

<a name="INDEX-3108"></a><a name="INDEX-3109"></a><a name="INDEX-3110"></a>






<h3 class="sect2">16.5.3. Message Passing</h3>

<p>
<a name="INDEX-3111"></a><a name="INDEX-3112"></a><a name="INDEX-3113"></a>
As we mentioned earlier, UDP communication involves much lower overhead
but provides no reliability, since there are no promises that messages
will arrive in a proper order--or even that they will arrive at all.
UDP is often said to stand for Unreliable Datagram Protocol.</p>

<p>Still, UDP offers some advantages over TCP, including the ability to
broadcast or multicast to a whole bunch of destination hosts at once
(usually on your local subnet).  If you find yourself getting overly concerned
about reliability and starting to build checks into your message system,
then you probably should just use TCP to start with.  True, it costs
more to set up and tear down a TCP connection, but if you can amortize that
over many messages (or one long message), it doesn't much matter.</p>

<p>
<a name="INDEX-3114"></a>
Anyway, here's an example of a UDP program.  It contacts the UDP time
port of the machines given on the command line, or everybody
it can find using the universal broadcast address if no arguments
were supplied.<a href="#FOOTNOTE-13">[13]</a> Not all
machines have a time server enabled, especially across firewall
boundaries, but those that do will send you back a 4-byte integer
packed in network byte order that represents what time that machine
thinks it is.  The time returned, however, is in the number of
seconds since 1900.  You have to subtract the number of seconds
between 1900 and 1970 to feed that time to the <tt class="literal">localtime</tt> or
<tt class="literal">gmtime</tt> conversion functions.
<blockquote>
<pre class="programlisting">#!/usr/bin/perl
# clockdrift - compare other systems' clocks with this one
#              without arguments, broadcast to anyone listening.
#              wait one-half second for an answer.

use v5.6.0;  # or better
use warnings;
use strict;
use Socket;

unshift(@ARGV, inet_ntoa(INADDR_BROADCAST))
    unless @ARGV;

socket(my $msgsock, PF_INET, SOCK_DGRAM, getprotobyname("udp"))
    or die "socket: $!";

# Some borked machines need this.  Shouldn't hurt anyone else.
setsockopt($msgsock, SOL_SOCKET, SO_BROADCAST, 1)
    or die "setsockopt: $!";

my $portno = getservbyname("time", "udp")    
    or die "no udp time port";

for my $target (@ARGV) {
    print "Sending to $target:$portno\n";
    my $destpaddr = sockaddr_in($portno, inet_aton($target));
    send($msgsock, "x", 0, $destpaddr)
        or die "send: $!";
}

# daytime service returns 32-bit time in seconds since 1900
my $FROM_1900_TO_EPOCH = 2_208_988_800;
my $time_fmt = "N";   # and it does so in this binary format
my $time_len = length(pack($time_fmt, 1));  # any number's fine

my $inmask = '';  # string to store the fileno bits for select
vec($inmask, fileno($msgsock), 1) = 1;

# wait only half a second for input to show up
while (select(my $outmask = $inmask, undef, undef, 0.5)) {
    defined(my $srcpaddr = recv($msgsock, my $bintime, $time_len, 0))
        or die "recv: $!";
    my($port, $ipaddr) = sockaddr_in($srcpaddr);
    my $sendhost = sprintf "%s [%s]",
                    gethostbyaddr($ipaddr, AF_INET) || 'UNKNOWN',
                    inet_ntoa($ipaddr);
    my $delta = unpack($time_fmt, $bintime) -
                      $FROM_1900_TO_EPOCH - time();
    print "Clock on $sendhost is $delta seconds ahead of this one.\n";
}</pre>
</blockquote>
</p>
<blockquote class="footnote">

<a name="FOOTNOTE-13"></a>
<p>[13] If that doesn't work, run <em class="emphasis">ifconfig 
-a</em>
to find the proper local broadcast address.</p>

</blockquote>

<a name="INDEX-3115"></a><a name="INDEX-3116"></a>



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