The following is a description of a protocol I designed for an job candidate examination:

   Receives lines from a client:
      iam <id>
      need-done <args>
   In response to a need-done, sends to a client that has given
   an "iam" message:
      dothis <args>
   Server ignores any other lines

It's basically a trivial job dispatch protocol where clients register to do work with a central server and are also able to request jobs. Designwise, it's pretty much all that the described chat server would do.

The exam question asked them to describe the protocol given the following code and then identify at least three security holes (with proposed attacks) in the protocol as well as explain how you'd fix the security problems. Which is why it was written so badly. Sorry...

use Socket;

socket S, PF_INET, SOCK_STREAM, getprotobyname('tcp') or die "$!";
setsockopt S, &SOL_SOCKET, &SO_REUSEADDR, 1 or die "$!";
bind S, sockaddr_in( 23456, INADDR_ANY ) or die "$!";
listen S, 5 or die "$!";

$SIG{__DIE__} = sub { close S; };

$rin = "";
vec( $rin, fileno(S), 1 ) = 1;
while( 1 ) {
   next unless select( $rout = $rin, undef, undef, undef ) > 0;
   if( vec( $rout, fileno(S), 1 ) ) {
      local *H;
      $p = accept( H, S );
      select( (select(H), $| = 1)[0]);

      push @known, *H;
      vec( $rin, fileno(H), 1 ) = 1;
   }
   foreach my $fd (@known) {
      if( vec( $rout, fileno($fd), 1 ) ) {
         $s = readline $fd;
         push @able, $fd if $s =~ /^iam\s+(.+)\s*$/io;
         if( @able and $s =~ /^need-done\s+(.+)\s*$/io ) {
            print { $able[(unpack "%32C*",$1) % @able] } "dothis ", $1
+, "\n";
         }
      }
   }
}
[download]

For some reason, only one candidate even attempted the question.

That aside, it's certainly possible to write a chat server in perl. It's not threaded, but I don't see why you'd really want a threaded server when so much state has to be shared between all the processes and the overhead of serving requests is so low.

c.

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