Liquid War (v5.6.4) - 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...

