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VE4KLM > AX25 11.12.20 04:07l 36 Lines 1337 Bytes #999 (0) @ WW
BID : A08ZJ_VE4KLM
Read: GAST
Subj: SABM gets double UA response ? why ?
Path: DB0FFL<OE2XZR<DB0FHN<DB0RBS<DB0ERF<DB0RES<ON0AR<OZ5BBS<CX2SA<F4DUR<
LU4ECL<GB7CIP<VE4KLM
Sent: 201211/0555z @:VE4KLM.#WPG.MB.CAN.NOAM [Winnipeg] $:A08ZJ_VE4KLM
I have stumbled across a problem where if I connect to a node that
depends on
the AX25 Linux stack it is now sending me 2 (TWO) UA frames in response to
my opening SABM frame. I've never seen this happen until the past week ...
The first UA frame comes in and I move to a LAPB_CONNECTED state, but
then almost instantly a second UA frame comes in. From what I can see, JNOS
will see this as a reason to try and re-establish the link, and suddenly
a firestorm
is unleashed, effectively creating a DOS attack. It's quite unreal to
watch on the
logs (trace).
My instant fix for this (and I confirm it works) is to ignore UA FRMR if
we are
in a LAPB_CONNECTED state, which probably should have been the case all
along. But honestly in the 16+ years of xNOS, this is a first. I've
never seen this
before, ever ! I can fix it on my end no problem, but I want to find the
cause.
I can't find in the AX25 specs what a system should do it it receives a
UA while
in LAPB_CONNECTED, but I never saw this situation ever happen until now, so
it was never a problem.
The system I am connecting to is dependent on AX25 Linux Stack if I am not
mistaken, so is anyone aware of an bug or issue in the AX25 utils or
tools or
whatever they are called (sorry, but I don't use the AX25 Linux Stack) that
might be causing this ?
Maiko / VE4KLM
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