Network game

Basics

Since release 5.4.0, Liquid War includes network support, that's to say that people can play over a LAN (Local Area Network). However, due to limitations in Liquid War's legacy code, and also because of the lack of time I have, it might be a little tricky to set up a network game at first. So please read this section carefully.

You should keep in mind that:

  • DOS only releases of Liquid War do not include network support, only Windows and GNU/Linux versions will allow you to set up a network game.
  • The game should run fine on any LAN, but there's no garantee the game will be playable on the Internet. Indeed if your "ping delay" is not good enough, the game will be awfully slow. Bandwidth is not an issue, since Liquid War rarely needs more than 2 Kb/sec.
  • You'll need to know what an IP address is.
  • You don't need to set up a network game to run a multiplayer game. Liquid War was originally a multiplayer game without network support. Network support is here only for people who don't feel comfortable when playing at 6 on the same keyboard 8-)

Getting started

What do you need?

You'll basically need 2 computers connected on the same LAN. We'll call them computer A and B. You might be able to play over the Internet too, but the game can be harder to set up and - which is worse - very slow.

You'll also need to know the IP address of computer A. Type "ipconfig" under Windows or "ifconfig" as root under GNU/Linux to get this information if you don't have it.

Starting the server

Liquid War uses a very traditionnal client/server approach. Basically, the server gets informations from all the clients and then dispatches the collected information to everybody.

So you'll need to start a server on computer A by running "liquidwar-server" on GNU/Linux or "lwwinsrv.exe" on windows. This is a console application, ie it does not set up any graphic mode.

Here's a small example of a server start on GNU/Linux:

  $ liquidwar-server
  How many teams will connect to this server?

At this point you must enter a number between 2 and 6, and then press "ENTER". In this example we will answer 2. The server really needs to know how many teams will be in the game: when enough teams are connected, the game starts. It can also be a good idea to answer 6 systematically and then force game start by clicking the "Start now" button within the client. It's possible to skip this question by typing "liquidwar-server -2" instead of a plain "liquidwar-server".

  Use "-2" to get rid of this question.
  Register on "www.ufoot.org/metaserver/" (y/n)?

Now if we answer "y", then the server will automatically contact the "meta-server" and it will be listed on http://www.ufoot.org/liquidwar/v5/metaserver.php3

This can be convenient for people who want to find other gamers to play with on the Net. For now, let's answer "n", we'll test this meta-server stuff later 8-)

  Use "-private" to get rid of this question.
  2002-06-03 16:43:00: Listening on port 8035...
  2002-06-03 16:43:00: Waiting for 2 teams...

Now the server is ready to accept clients. By default it listens to clients on port 8035. You could change this behavior setting by calling "liquidwar-server -port 8061" for instance, but let's use the default port to make things easier.

Starting the clients

Start the client on computer A normally by typing "liquidwar" on GNU/Linux or double-click "lwwin.exe" on Windows.

Go to the "Teams" menu and select 2 teams, a red human and a blue CPU for instance. If you don't know how to do this, then try and play Liquid War on a single computer first. It's important to notice that by default the "blue CPU" won't connect on the network game. It's possible to allow bots to connect on network games, but they are disabled by default.

Now come back to the main menu, and a "Net Game" button should be available. Click it. Now you should be able to:

  • Start the game.
  • Change the IP address of the server.
  • Change the communication port.
  • Set a password.
  • Search for internet games automatically.

Since the server is also running on the same machine (A), you can leave the default IP address as is (127.0.0.1).

Now you are ready to start the second client on computer B. Like with computer A, you'll have to:

  • Select 2 teams, green an yellow this time.
  • Select "Net Game" in the main menu.

But this time you'll also need to change the server address, since the client is not running on the same computer than the server.

Now click on "Start game" on computer A. The server should play a "system beep", which is usefull if you run a server and want to be notified of client connections without watching the console all the time, and it should display messages like this:

  2002-06-03 16:44:48: Connection from "127.0.0.1:34677"
  2002-06-03 16:44:48: Team "Napoleon" on client "127.0.0.1:34677" accepted
  2002-06-03 16:44:49: Client "127.0.0.1:34677" accepted
  2002-06-03 16:44:49: Waiting for 1 team...

And on the client you should see a screen which says "Waiting for 1 team(s)" with the list of connected players below (Napoleon). You do not need to click on the "Start now" button.

Now click on "Start game" on computer B. The server should display messages like this:

  2002-06-03 16:49:14: Connection from "192.168.1.1:1098"
  2002-06-03 16:49:14: Team "Henri IV" on client "192.168.1.1:1098" accepted
  2002-06-03 16:49:15: Client "192.168.1.1:1098" accepted
  2002-06-03 16:49:15: Client "192.168.1.1:1098" ready
  2002-06-03 16:49:15: Client "127.0.0.1:34677" ready
  2002-06-03 16:49:15: Sending info to "127.0.0.1:34677"
  2002-06-03 16:49:15: Sending info to "192.168.1.1:1098"
  2002-06-03 16:49:16: Game start

And at that point, the game should start 8-)

Restart a new game

Once the game is over, you can start another network game on the clients without touching the server, because the server automatically restarts and waits for players to connect.

To stop the server - if you want to change its settings for instance - just go to the console where it's running and press CTRL-C.

Using the meta-server

Basics

The meta-server is a piece of software which is running on my web site, and allows servers to register themselves so that client can get a list of available servers.

It's written in PHP and is _very_ basic but I believe it's enough for what has to be done: maintain a list of running servers.

The source code for the meta-server is included in the source package of Liquid War, so you might run such a server yourself if you want to. However, by default, servers will register themselves on my web site, and will be listed on http://www.ufoot.org/liquidwar/v5/metaserver.php3

How to register a server

Launch the server, and when you get the question:

  Register on "www.ufoot.org/metaserver/" (y/n)?

answer "y".

Note that if you're behind a proxy or a firewall, the server might be unable to register itself. Clients might also have problems to connect themselves on your server if there's a machine which does NAT (Network Address Translation) between you and the meta-server.

How to find a server

In the main menu, click on "Net Game" and then "Search for internet games".

Now you should see a list of available servers. You can click on the items in the list to get more informations about a given server. Once you have chosen a server, click on "Join now".

Now you get on a "Waiting for teams" screen. You might be interested in using the "Start now" button. Indeed, if you are 4 players connected on a server that accepts up to 6 players, maybe you'll want to start the game right away without waiting for 2 more players. In this case, every player must click "Start now". A "*" character will replace the "-" in the players list when a player clicks on "Start now". When all the players are displayed with a "*a, the game starts.

You can also chat with other players by entering text in the area above the "Send message" button, and then click on this button. Keep in mind that this is a very primitive chat and that the best way to chat efficiently is IMHO to play in windowed mode and have an IRC client at hand.

Note that you can also get the list of available servers from http://www.ufoot.org/liquidwar/v5/metaserver.php3 There you'll also find a little chat-box which will allow you to send exchange messages with other players.

Options

Server options

You can pass options to the server using the command line. The following parameters are accepted:

  • "-n" where "n" is a number between 2 and 6 : with this option you can tell the server how many teams will connect to the game. Beware, there can be several teams on the same computer, so if you want to have a computer with 2 players on it and 2 other computers with a single player, then you need to use the "-4" option.
  • "-lag n" where "n" is an integer : with this option, you can control the lag used at startup. Normally, Liquid War handles this parameter automatically, but you might want to force it to a given value.
  • "-port n" where "n" is an integer : allows you to change the IP port used by the server to listen to the clients. if you omit this parameter, the default port is (8035) is used.
  • "-log file.log" : dumps all informations in "file.log" instead of using the standard output.
  • "-public" : skips the "Register on ..." question, and registers the server automatically on the meta-server, so that clients can find it easily.
  • "-private" : skips the "Register on ..." question, and does not register the server at all.
  • "-comment This_is_a_comment" : associates a comment to the server, which will be displayed by the meta-server. Note that the character "_" will be replaced by spaces. This makes command line parsing easier. I'm lazy 8-)
  • "-password xxx" : associates a password to the server. With this option, clients will need to give the right password to be able to connect on the server.
  • "-callback cmd" : with this option, the command "cmd" will be launched whenever someone connects on an empty server. For instance, if you want to run a permanent server and want to know when someone connects but do not wish to permanently keep an eye on the server's log, you might use this option and put in "cmd" a command that automatically sends you a mail. You might also use a command that fires a popup window. It's up to you. 2 sample scripts are available in the "misc" directory of the Liquid War source distribution. One is "misc/liquidwar_notify.sh" and works on UNIX platforms, and the other one is "misc/lwpopup.js", which is automatically used by "misc/lwserver.bat", and works on Windows.

Common options

These options work on both client and server, even if they are rather "server-oriented".

  • "-metaserver url" : redefines the URL of the meta-server. Usefull if you want to use your own meta-server.
  • "-netlog" : if you use this option, the server will dump all the network traffic on the standard output. This is usefull for debugging.
  • "-nobeep" : Disables the system beeps that the application might fire. These beeps are mostly used on the server to notify clients connections.

About Liquid War's network implementation

Basics

Liquid War uses TCP sockets, and a single-threaded server. This implies that:

  • The game can sometimes get blocked if you play on Internet.
  • The server can't talk simultaneously with several clients.

I needed to use TCP sockets, since LW's algorithm can not cope with any data loss and it's not a reasonnable to try and anticipate what the map would be like if the player did not move etc...

I did not implement any complex multithreaded stuff since I'm lazy and however, clients need to have informations about all the other before something can be done. However, implementing a mutltithreaded server could have advantages over the current solution.

What is this lag stuff anyway?

In Liquid War, all the clients send their key presses to the server, and then the server dispatches this information to everyone. This has to be done for every round.

You can easily imagine that if a player has a poor connection, with a very long "ping delay", it can take quite a long time to send the information to the server, and then get it back.

So what Liquid War does is that at the beginning of the game, the server sends a couple of "blank" key strokes to the clients. This way, clients receive data from the server before thay have sent any. The number of key strokes sent at the beginning of the game is called the "lag".

So if it takes 200 msec to send and then receive data from the server (approx the time returned by the "ping" command) then with a lag of 6, you can theorically play at a rate of (1/0.2)*6=30 rounds/sec.

On one hand, setting the lag parameter to a high value will avoid many network errors and allow you to play at a very fast pace, but the big drawback is that there will be quite a long time between the instant you send a key stroke to the server and the moment it comes back to you. On the other hand, setting the lag to a low value will limit drastically he number of rounds per second, but make the game more "responsive".

However, since release 5.4.1, the "lag" is modified automatically and should adapt itself to the situation. I've not been able to test it in real conditions yet, but it should work 8-)

Still, setting the lag to a sensible default value can save you some trouble. Indeed, by default, Liquid War will choose a value (6), but it can not guess if you are playing on Internet or on a 100 Mbit LAN, and it can take quite a long time before Liquid War automatically finds the right value. To know the right value which should be used with the "-lag" option, simply play a few games and watch the average lag (which is displayed on the server console every minute) at the end of the game.

Performance issues

Liquid War uses a "light" server, and one of the advantages of this solution is that it allows you to run the server on low-end computers. I personnally run a permanent server on a 486 DX2, and it runs like a charm.

The only thing you have to take care of when running a server is bandwidth. Don't worry, you won't need a 10Mbit connection, basically, each clients sends and receives 12 bytes of data at each round. If you add TCP/IP headers and the facts that stuff will probably be bundled in bigger packets, a client must deliver about 15 Kbit/sec (up and down) for a game that runs at 100 frames/sec. A 56K V90 modem is enough for this.

So if you run a server with 2 clients connected, the server will need to deliver 30 Kbit/sec in both ways. A 56K V90 modem _can_ do that, but your provider needs to be a good one 8-)

And if you run a server with 6 clients, you simply won't be able to reach the 100 frames/sec with a 56K V90 modem. It will necessarly drop to something less than 30 frames/sec, and is likely to drop to about 15 frames/sec. OK this is not a big deal, since few Internet games run at more than 30 frames/sec, but well, if the server has troubles receiving/sending data, everyone will wait, and the fun will go away.

As a conclusion: if you have the choice, choose the friend who has the best bandwidth to run the server, without even considering the power of his computer.

Troubleshooting

General information

Network support in 5.4 and 5.5 is still experimental in many ways, so you might get weird behaviors. Basically, if you have a problem, just do the following:

  • Stop and restart the server when something goes wrong. To stop it, use CTRL-C.
  • Check out that you have entered the correct IP addresses.
  • Try and start the client and the server using the "-netlog" option to have an idea about what's happening.

Bugs in 5.4.x corrected in 5.4.2

Liquid War 5.4.0 and 5.4.1 were very hard to play over the Internet. The reason is that the network routines did not do enough error checking, and therefore there were very often errors when sending and/or receiving the map to the server. Hopefully, this bug should not appear anymore in 5.4.2 or any other recent release.

About security

Network games passwords

As you might have noticed, under the box where you can enter the password, a little notice explains that you must choose a "weak" password. Now you'll tell me -> people keep on explaining me that passwords must be something complex like "aS\r!Y9p" and now I'm told to use "hello", what's up?

OK, keep in mind Liquid War is a game. This password stuff is just a way to be able to play with your friends only and keep on using the meta-server's services. Liquid War does not encrypt data and I can see no good reason to do it for, so the password is stored and sent to the server in clear, as plain text.

The consequence is that if you use a valuable password - for instance the one you use to log in on your computer - the guy who runs the server will see your password in the log file if he wishes to. Therefore, use something weak, something that if someones finds out what it is, you won't really care. So "hello" is a wise choice.

Is Liquid War likely to have security holes?

Yes.

Any program is likely to have security holes, especially when it's networked. However, I have good reasons to think that Liquid War is safe enough for a game. At least I find it safe enough to run a permanent public server on my personnal computer 8-)

FYI, here are some things which I think make Liquid War rather safe to run:

  • Liquid War does not store anything on your hard drive that would have been received from the network. The maps are kept in RAM. So you won't download any virus playing Liquid War on Internet.
  • Liquid War does not transmit any sort of code on the network. All the transmitted bytes represent plain data. So you're not likely to execute any arbitrary code - virus, worm - when playing on the Net.
  • Liquid War receives network packets in static buffers, and if the received data is too big, it is truncated. One consequence is that Liquid War has a bunch of "limits". You can't send hudge maps over the network, you can't have long nicknames, and so on. But another consequence is that if you try to send garbage in the buffer, it will be truncated. Liquid War will protest with a "network error" message and the connection will be closed, but there will be no easy exploit possible here.
  • Liquid War does not use the sprintf, strcpy and strcat functions, which are known as being rather unsecure since they can lead to buffer overflows. Instead, it uses the equivalent functions snprintf, strncpy and strncat. On platforms where these functions are supported natively, the game will use the default system functions, otherwise it will use a free implementation by Mark Martinec. FYI, Windows does not support snprintf-like functions natively, that is, it's a piece of crap.
  • Liquid War is Free Software, so I'm not likely to have put backdoors in it myself, since anyone can look at the source code 8-)

However, I have not - and I know nobody who has - audited Liquid War for security holes. So there might be some. Therefore you should respect a few things while running Liquid War:

  • Never run Liquid War as root or administrator. This is obvious but I still mention it. If you want to run a Liquid War daemon on UNIX, run it as user "nobody" or something approaching. If "root" or "administrator" does not make sense on your system (DOS, Win98...) then I assume you're not _really_ concerned about security anyway 8-P
  • If you run a server 7/7 24/24, use the "-log" option to log everything in a file. This way you'll keep a trace of network activity, and if something goes wrong, you might get a chance to see it.
  • If you use passwords in network games, *never* choose a valuable password. Use something simple like "hello" or "goodbye".
  • Keep in mind that Liquid War is a game, and not a bullet proof professionnal server.

Last point: you should be aware that version 5.4.5 of Liquid War has been proved to be vulnerable to a local buffer overflow, and one should run at least 5.5.9 to get rid of this problem. FYI by the time the exploit was found on 5.4.5, 5.5.9 was already out 8-) See http://www.securityfocus.com/bid/8629 and http://www.securityfocus.com/bid/9453 for more informations.

Can people cheat when playing on the Net?

No.

Or at least, not really. In fact, you can still find the following types of lamers:

  • A guy who lets the CPU play at his place. He'll loose anyway because the CPU is definitely not a great Liquid War Master 8-)
  • A guy who tweaks the game and gets all his bots fight anyone he wishes. That's mean.
  • A guy who manages to let you have a 500msec lag while he does not have any lag at all.

Apart from this, I can hardly see any way to cheat. Why? Because the Liquid War server does not store any information about the game. It's not aware of who wins, who looses, it knows nothing. The only thing it does is to transmit key presses between client computers.

This way, if someone plays with a tweaked release of Liquid War, thinking he will fool you, then he will fool you on his computer only... On your computer, everything will be fine. After some time, your screen and his screen will have nothing in common, and both players are likely to think they have won. Except the lamer will stay a lamer.

This also explains why it's required to play with the very same versions of the game during network games. If you plug a 5.5.2 with a 5.5.1, after a minute the screens will be completely different on each client, since there are subtle differences between the 5.5.1 and the 5.5.2 engine. However, you shouldn't be able to do this, since a network error will stop you before you can start to play.

Additionnally, versions 5.5.5 and higher have a checksum system. Every 100 rounds, each client calculates a checksum with its local map, and sends it to the server. If the checksum is incorrect, the server will log a message like:

  Checksum error on client "192.168.1.1:1098"

If you see this, then you're in one of the following situations:

  • There's a bug in the game
  • A lamer tries to cheat

FYI, all releases from 5.4.0 to 5.5.4 have a bug which causes clients to desynchronize after a while...

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Copyright © 2005 Christian Mauduit. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
Updated on Sat May 07 2005.