Re: UDP packet handling weird behaviour of various operating systems

From: Keith Warno (keith.warnoat_private)
Date: Fri Jul 27 2001 - 15:18:01 PDT

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    "Juergen P. Meier" wrote:
    > 
    > > http://rootshell.com/archive-j457nxiqi3gq59dv/199803/biffit.c
    > >
    > > 1. Linux 2.4.7 UP (pristine source, waiting for a new shiny Alan Cox patch)
    > >       - system gets frozen after 3 seconds of flood on a gigabit link.
    > > Same result at a 100Mbps. The top utility shows (at least as long as it can)
    > > that system(kernel) gets 100% of the CPU in its march to death. Same for
    > > Linux kernel 2.2.19.
    > 
    > 2.4.6 (modular, unpatched, selfcompiled) on an old P133:
    > 
    > biffit against loopback: 99% cpu(system), no slowdown, system
    > responds normaly. (no slowdown)
    > biffit against eth0: same effect. (doh, cause linux sends it over loopback)
    
    Confirmed, with kernel 2.4.7 (unpatched, selfcompiled, modular).  ~99%
    CPU usage, but no slowdown.  Although the hardware is quite different --
    proc: Thunderbird 1000
    ram: 384 MB
    mobo: MSI MS6340 micro atx, VIA KT 133
    NIC: LinkSys LNE100TX v4.1 (using kernel-distributed tulip driver, not
    that from Scyld)
    (eth0: ADMtek Comet rev 17) -- altho this is irrelevent isn't it.
    
    I've been using 2.4.whatever for only the past couple of weeks.  Maybe
    I'm missing something.  I don't run any UDP services whatsoever and
    would never run comsat under any circumstance.  So, why does the
    sendto() in biffit.c not fail when sending to localhost?  When I boot
    back to 2.2.19 and try the same thing, I get what I expect: Connection
    refused.  Hrmm.  Unless I missed something in the kernel docs regarding
    loopback behavior, the displayed 2.4.7 behavior seems like a Bad Thing.
    
    kw
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