Re: sending MAC packets --- again, and again

From: Charles Swiger (cswiger_at_mac.com)
Date: 05/21/05

  • Next message: Daniel Valencia: "Re: sending MAC packets --- again, and again" 
    Date: Sat, 21 May 2005 12:56:49 -0400
To: Daniel Valencia <fetrovsky@yahoo.com>

    On May 21, 2005, at 12:27 PM, Daniel Valencia wrote:
    > [ ...pcap reading code stalling... ]
    >
    > Is this normal pcap behaviour, or is it some
    > FreeBSD-specific behaviour, or is it just me?

    It's unfortunately normal. You're running into this (from "man pcap"):

            NOTE: when reading a live capture, pcap_dispatch() will not
    necessarily
            return when the read times out; on some platforms, the read
    timeout
            isn't supported, and, on other platforms, the timer doesn't
    start until
            at least one packet arrives. This means that the read timeout
    should
            NOT be used in, for example, an interactive application, to
    allow the
            packet capture loop to ``poll'' for user input periodically, as
    there's
            no guarantee that pcap_dispatch() will return after the
    timeout
            expires.

    You can help prompt the pcap timeout to work better by *sending* a
    packet just before you try to sniff packets. However, in the end
    you'll probably find yourself either going multithreaded or using
    alarm() and a signal handler to periodicly wake the program up to pay
    attention to other things if the pcap sniffing goes into long
    hibernation. Or maybe both. :-)

    But then, the standard calls for getting hostnames (getaddrinfo,
    gethostbyname) block, too, so you maybe you'll throw DNS lookup code
    into another thread anyway, or else end up crafting your own DNS
    resolver code since you're already sniffing the packets.... 

    -- 
-Chuck
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  • Next message: Daniel Valencia: "Re: sending MAC packets --- again, and again"