But if you want concurrency, look at POE. Pretty easy to "appear" to do things simultaneously there, sharing the data structures needed. In fact, there's already an IRC server framework there, so you could use standard IRC clients with your "server".

If you insist on coding from scratch, you can have your child processes communicate via some lightweight database, like DBM::Deep or DBD::SQLite. It'd be pretty simple to write a classic "fork on accept" server that then uses the database to keep in touch with other children to share the chat info. Probably have to use a timed-out read loop to go check the database from time to time.

++merlyn for poor man's web chat

One bug: there's a ->epath where it should be ->path

The net server set of modules are very nice. We use Net::Server::Multiplex it to handle a thousand or so active chat clients. The interface is quite nice. Another nice package, is Event::Lib. The use of Kqueues/Epoll is nice to have when large numbers of clients are connected.

Besides the CookBook, mentioned above, I would get a copy of Network Programming with Perl. It provides many examples of writting a chat client. Even if you don't use one of them, you will get a good understanding of different methods for writting one. i.e. using Threads, IO::Poll, Pre-Forking sockets.... etc.

I would second the above comment. A single process, multiplexing server is going to be the quickest and simplest road to your solution. You can use Net::Server::Multiplex if you want to reuse someone else's solution to get up and running quickly.

However, if you want to learn how to do this yourself, you should get the Cookbook and read Chapter 17, and particularly recipe 17.13.

   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

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]

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.

It looks like there are many opportunities for denial of service attacks:
• Because accepted connections are never closed, one attack would be to connect and disconnect repeatedly, until your server has used up all of its file descriptors.
• Another attack would be to connect and send a packet not terminated by an end-of-line marker and then hold the connection open, causing readline in the server to block, thus preventing the server and its horde of worker bees from doing anything else.
• Another would be to connect with a client that never reads from its end of the connection. Eventually, the server would block while trying to send the misbehaving client a message. The client could speed the process by sending repeated requests with incrementing need-done payloads to cause the checksum-based dispatcher to sweep across all of the "able" file descriptors, guaranteeing that the server would send a request to the misbehaving client. (The client could grab more "able" slots for itself by issuing lots of "iam" messages, but this isn't really necessary.)
Were these the kinds of things you had in mind?

Tom Moertel : Blog / Talks / CPAN / LectroTest / PXSL / Coffee / Movie Rating Decoder

It looks like there are many opportunities for denial of service attacks

Heaping piles of 'em, yeah, and your attacks are correct but you lose marks on 1 and 2 for not describing any fixes.

Here's some of the "guideline" answers I wrote up at the time (I work for the Canadian government... they want a fairly comprehensive list of expected answers submitted with the questions so there's no "dirty pool" during marking).

• blocking I/O. don't block waiting for complete requests from clients or you have a DoS. Fix with either handler threads with timeouts, just timeouts, or select/read into multiple buffers.
• no verification of clients. Need something like a host list. Shared secrets, crypto, etc won't work because of the backwards compatibility constraint.
• no checking of scripts. Need to ensure that the things like "unlink /etc/passwd" or embedded perl command aren't sent. Server sandboxing (chroot), some kind of safe/taint mode, etc would be needed. In this case, sanitizing the scripts is critical but might not actually be feasible. Some might naively think this is a client problem, but we specifically asked for fixes that would be backwards compatible with older clients so it has to be fixed in the server. In this case, we never actually identified what these scripts do or how they work other than they have to be on one line. 1 of 4 points is for mentioning this as a problem in providing a complete answer.
• no checking the length of the input. This provides the opportunity for a resource-based DoS. The answer needs to explain that there has to be a limit to request length. 1 of the 4 points require mentioning that there's no way to determine what that limit actually is without a concrete client implementation.
• lack of error checking in the implementation means that when a client goes away, we don't know and continue to send "dothis" messages to it. Enough of those clients will cause a resource-based DoS (file handles) as well as polluting the list of hosts enough that we couldn't process requests. Server needs to see when things go away and remove hosts from the list.

Hmmm... forgot to mention the backwards compatibility constraint. Any proposed "fix" was supposed to be backwards compatible with the original protocol. This does mean that some problems could never really be fixed, but... Well, the question was supposed to reflect some work we do regularly... take a really old and badly (un)documented piece of code, understand (and document) how it works, and fix/improve it without breaking the entire infrastructure.

c.

The client could grab more "able" slots for itself by issuing lots of "iam" messages, but this isn't really necessary.

Neat, I never noticed that attack. It's subtly different from a file descriptor DoS but probably nastier because there's no external limits on how much resources can be used.

c.

makes perl threads more similar to a toy than a tool: the impossibility to share objects, especially if they are of the filehandle kind...

With caveats--mostly surmountable--it is possible to share globs between threads. But it is a bit messy and awkward, which is why I have (mostly) avoided discussing the method publicly.

I think that this is an unecessary restriction of threads::shared that could and should be lifted. At the basic level filehandles are a per process resource and are accessible from every thread. The current restriction is artificially imposed and can be bypassed. What is required is a documented and tested mechanism for doing shared access--rather than the current situation whereby it can only be done using obscure coding tricks.

There are two things that prevent me from attempting to get the retriction lifted. The first is my lack of internals skills. The second is that whatever I develop will only be tried and tested on Win32. Without addressing these two areas, the best I could hope to do is offer suggestions for change which is a lot less likely to get anywhere than a "tested on XX and YY" patch.

All of that said, it is totally possible, and even quite easy to write a simple, complete and efficient chat server using threads. The first thing you have to do is throw away the "fork and exec" approach to the problem, and view it from the perspective of "How do I use threads to listen for new connections whilst maintaining responsiveness to existing ones".

Once you shed the shackles of preconditioned thinking, it becomes relatively easy :)

How do I use threads to listen for new connections whilst maintaining responsiveness to existing ones

You may not need threads to do that. Although perlipc and perlfunc don't mention it, you can run select on a generic socket, at least on Unix. In more detail:

You open perlipc, find the section labelled 'Internet TCP Clients and Servers', and use the recipe to create a generic server socket: socket(), setsockopt(), bind(), listen(). You now build a bitmask and go into a select loop. (If you've not used select before, perlfunc explains how. select is a way of sleeping until there's some activity on one or more of any number of connections.) When select says there's something ready for your generic socket to read, it means there's an incoming connection attempt: so you accept the connection, flip a bit in your bitmask to represent the new connection, and go back to the top of your select loop. You're now in an interesting position: select will return when you're ready either to accept a new connection or to read data from an existing connection. So, if you can service an incoming packet quickly enough, a single fibre is all you need.

That said, I'd be fascinated to know how to share globs between Perl threads. I threw away a day's work earlier this week because Perl told me it couldn't be done and (as this was work time) I decided not to spend time trying to hack it. At the very least, this restriction should be documented in perlthrtut and/or threads::shared.

Markus

I'd be fascinated to know how to share globs between Perl threads.

Supersearch for a thread by me with a title including "threads" and "globs" for my initial experiments in doing this. Basically, you need to pass a "handle" to a glob through a shared variable in such a way that threads::shared doesn't stick it's nose in and reject you. Be warned: The technique I used there has problems.

I've a couple of other ways of doing it that I am experimenting with, but I would rather keep them quiet till I've proven to myself that they can be used reliably.

Perl threads have an undeservedly bad rep as it is, without me causing more problems sharing speculative ideas without checking them out first. (Which is why I'm not linking to the post in question--If you want to try it, your gonna have to do a little work:)

---

Examine what is said, not who speaks.

Silence betokens consent.

Love the truth but pardon error.