Re: Data in SYN Packets

[scott hollatz <shollatz () D UMN EDU>]

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> > In our IPS log I see the following entry *TCP C2S Ambiguity: Data in
> > SYN Packet* daily directed towards our DNS server. These packets are
> > coming from four or so different addresses in China.  I did a brief
> > Google search with results being a few or more years old. A couple of
> > the posts reported the same *Data in SYN Packet* with the
> > originating addresses also from China.
> >
> > Can anybody shed light on this?
>
> Well, it may be somebody is actually deploying this RFC:
>
> 1644 T/TCP -- TCP Extensions for Transactions Functional
>     Specification. R. Braden. July 1994. (Format: TXT=87362 bytes)
>     (Updates RFC1379) (Status: EXPERIMENTAL)
>
> (Executive summary from the RFC: follows)
>
>       TCP A  (Client)                                    TCP B (Server)
>       CLOSED                                                     LISTEN
>   #1  SYN-SENT*        --> <SYN,data1,FIN,CC=x> -->         CLOSE-WAIT*
>                                                           (TAO test OK)
>                                                         (data1->user_B)
>                                                           <-- LAST-ACK*
>   #2  TIME-WAIT   <-- <SYN,ACK(FIN),data2,FIN,CC=y,CC.ECHO=x>
>     (data2->user_A)
>   #3  TIME-WAIT          --> <ACK(FIN),CC=x> -->                 CLOSED
>       (timeout)
>         CLOSED
>
> Most of the deployment I've seen has been broken spamware that did it
> basically in a fire-n-forget mode, targeting the fact that at least one
> high-market-share vendor's TCP stack was buggy and would queue up the
> whole SMTP transaction without bothering to actually check everything, so
> spamware could send a SYN EHLO MAIL FROM RCPT TO DATA <text> . in one long
> fragmented packet and it would actually work.  Gaak.

The original observation was to TCP 53 not TCP 25.

The above scenario is interesting, though, but the SMTP transaction
would fail on MTAs which implement a pre-greet strategy similar to what's
in sendmail.

- --
scott hollatz                                        net shollatz () d UMn eDu
information technology systems and services          tel +1 218 726 8851
university of minnesota duluth mn usa                fax +1 218 726 7674
                                                                         --
                                              "Asn aD ta zlAp em uT zt33rg"
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