detect being blocked by a firewall

This is a discussion on detect being blocked by a firewall within the Networking/Device Communication forums, part of the General Programming Boards category; Hi all! as a maker of (a legitimate) application that need a connection to the server, how can I precisely ...

  1. #1
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    Question detect being blocked by a firewall

    Hi all!
    as a maker of (a legitimate) application that need a connection to the server, how can I precisely detect that connection was blocked by a firewall (the problem is that user may accidetially block the application when firewall's pop-up window appeared, or forget to configure firewall, etc. - I want to tell him that application is blocked and suggest to make appropriate actions)

    Thanks!

  2. #2
    and the hat of wrongness Salem's Avatar
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    I'm thinking its going to be pretty tricky to tell the difference between being blocked by a firewall, and having the connection refused by a reachable server which is say too busy, or a server which is simply off-line at that moment.

    Not to mention telling the difference between say an application level firewall running on the local machine, and a perimeter firewall separating the local LAN from the internet.

    Mostly I would prefer a simple popup dialog suggesting courses of action.
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    Basicly, you can't. Their are 2 common ways a firewall will "block" an incomming connection, either completely drop the packets, and no respond at all (act like a black hole as they say), or send a reset icmp packet. The reset packet will normally cause your socket to close with an error I think. Check your API docs (man pages or MSDN, etc, depends on OS) for more.
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  4. #4
    * Death to Visual Basic * Devil Panther's Avatar
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    Quote Originally Posted by Xipher
    Basicly, you can't. Their are 2 common ways a firewall will "block" an incomming connection, either completely drop the packets, and no respond at all (act like a black hole as they say), or send a reset icmp packet. The reset packet will normally cause your socket to close with an error I think. Check your API docs (man pages or MSDN, etc, depends on OS) for more.

    i think he's talking about the firewall and the application on the same system, a local firewall (like zonealarm, for example). in that case the icmp reset it pretty much useless, you will only recieve the icmp if your packets are trying to pass through the firewall, and not from within it to the outside world.
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