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[aprssig] Someone's Server Mangling Packets

Bob Bruninga bruninga at usna.edu
Wed Jan 25 13:44:34 UTC 2012


Concatenated packets:

I have seen these rarely since the beginning of APRS.  I assumed they were
my own DOS processor taking a few milliseconds break and as a result
concatenating packets.  Since the TNC would not make such a mistake due to
the CRC check, I assumed it was in the  APRS client.

However, if we realize that almost every packet heard these days is a
concatenation of one weaker packet overlayed with the most recent strongest
packet, and since the ending flag is the same as a beginning flag, there is
a finite probability that the sum of a few bit errors could result in a
concatenated packet with a valid checksum at the end...

I think I theorized this once before, and maybe someone shot it down, but
I'd like to see the probabiltites again...

Bob, Wb4APR


-----Original Message-----
From: aprssig-bounces at tapr.org [mailto:aprssig-bounces at tapr.org] On Behalf
Of Lynn W. Deffenbaugh (Mr)
Sent: Wednesday, January 25, 2012 8:25 AM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] Someone's Server Mangling Packets

I've been trying to home in the source of these, what I call
"concatenated" packets for some time now.  There's various kinds of
corruption, but here's another recent classic example:

2012-01-25T13:14:35.707 server->client,qAS,sq3fyk-3:
F1ZZF-2>APNW01,qAR,LX1CU-3:=4922.53N100607.89E#PHG2200 W1 Thionville
(57) 324m ASL

And another pair of the *'d q-servers:

2012-01-25T13:17:35.864
KA0GFC-3>APN391,KA0GFC-2,WIDE2,qAR,KV0S-3*:!3856.83NS09244.74W#PHG9280
W3,MOn KA0GFC BOONVILLE
2012-01-25T13:18:31.216
KM0R-3>APN391,qAR,KV0S-3*:;IRLP-3121*111111z3859.51NI09222.35W0442.325Mhz
T127
KM0R

KV0S-3 itself uses an APN382 identifier
(http://aprs.fi/?c=raw&call=KV0S-3&limit=50&view=normal) which is
documented at http://www.aprs.org/aprs11/tocalls.txt as "APN3xx
Kantronics KPC-3 rom versions"

I'm sure you have your own tools, but if you bring up an APRSIS32
instance connected to a full feed, just open up Enables / View Logs /
Concat and enable it.  You'll see various "suspect" packets that I'm
ignoring and possibly some of my ideas about what I might do with such
packets in the future.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

On 1/25/2012 7:23 AM, Pete Loveall AE5PL Lists wrote:
> I noticed some packets yesterday such as:
>
> FWDSVR>APRS,qAO,AE5PL-WX*::NWS_WARN
:251100z,SVR_STORM,TXC331{PAJAAFWDSVR>APRS,qAO,AE5PL-WX:;FWDSVRAJA*251100z30
34.20N\09713.80WT014/035 SVR_STORM }d0]PNPOQNSJSJSJRIRIRIQIQINJMMPN{PAJAA
>
> Note the asterisk after AE5PL-WX which is an invalid modification of the
path (never alter the "last digi'ed" indicator) and the concatenation of the
object with the message.  Authors, please check your software and ensure you
are not mangling packets like this.  Server sysops, monitor the output of
your server to ensure you are not generating these mangled packets (monitor
the input and output).  We have had an increase of software mangling packets
like this as well as adding trailing white spaces.  Unfortunately, for the
volume of packets handled by core servers, there is really no way to
effectively prevent passage of these mangled packets without blocking some
valid packets as well.  These are only examples.  I will be issuing an
update to javAPRSSrvr that blocks any packets with an asterisk following the
q construct as that is invalid at all times.  I would hope the software
author of the offending software would correct their software as well since
it is doing other ma!
ngling of packets.
>
> 73,
>
> Pete Loveall AE5PL
> www.ae5pl.net
>
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