[Snort-users] flexresp3: Reset with TTL of 0

Jim Hranicky jfh at ...5250...
Tue Oct 26 17:27:09 EDT 2010

On Tue, 26 Oct 2010 14:32:01 -0400
Jim Hranicky <jfh at ...5250...> wrote:

> Has anyone else seen this behavior? I've likely missed a step somewhere. 
> I'll be glad to supply more info if needed. 

Sorry for the double post. I forgot to mention the OS: 

  2.6.32-24-server #39-Ubuntu SMP Wed Jul 28 06:21:40 UTC 2010 x86_64 GNU/Linux

The sniff interface is an Intel X520 and I've compiled PF_RING support into

I went ahead and attached a gdb to one of the running snorts. I set it to break
in active.c:Active_SendReset() and then triggered the reset: 

  #0  Active_SendReset (p=0x7ffffaf9f980, ef=2147483648) at active.c:214
  #1  0x000000000046ab51 in Resp3_Send (p=0x7ffffaf9f980, pv=0x362bf30) at sp_respond3.c:306
  #2  0x000000000040d971 in Active_SendResponses (p=0x7ffffaf9f980) at active.c:121
  #3  0x000000000042a6f8 in ProcessPacket (user=0x0, pkthdr=0x7ffffafa07b0, pkt=0x3aac070 "", ft=0x0) at snort.c:1484
  #4  0x000000000042a3cd in PacketCallback (user=0x0, pkthdr=0x7ffffafa07b0, pkt=0x3aac070 "") at snort.c:1382
  #5  0x00000000004d6f6a in pcap_process_loop ()
  #6  0x00007f904e5cb514 in pcap_read_linux () from /opt/local/lib/libpcap.so.1
  #7  0x00000000004d7026 in pcap_daq_acquire ()
  #8  0x00000000004d4342 in daq_acquire ()
  #9  0x00000000004494a8 in DAQ_Acquire (max=-1, callback=0x42a307 <PacketCallback>, user=0x0) at sfdaq.c:453
  #10 0x000000000042cb29 in PacketLoop () at snort.c:2757
  #11 0x000000000042957a in SnortMain (argc=9, argv=0x7ffffafa0b68) at snort.c:717
  #12 0x000000000042948d in main (argc=9, argv=0x7ffffafa0b68) at snort.c:660

Then stepped into this function

  rej = Encode_Reject(ENC_TCP_RST, flags|value, p, &len);

which took me here: 

  #0  Eth_Encode (enc=0x7ffffaf9f860, in=0x7ffffaf9f7f0, out=0x7ffffaf9f7d0) at encode.c:502
  #1  0x000000000040c357 in Encode_Packet (enc=0x7ffffaf9f860, p=0x7ffffaf9f980, len=0x7ffffaf9f8d0) at encode.c:355
  #2  0x000000000040bdd5 in Encode_Reject (type=ENC_TCP_RST, flags=2684354560, p=0x7ffffaf9f980, len=0x7ffffaf9f8d0) at encode.c:175

after coming out I printed the value of rej in hex: 

  (gdb) x/64x rej
  0x739b20 <s_pkt>:       0x00    0x0e    0x83    0xc6    0xa3    0x40    0x00    0xd0
  0x739b28 <s_pkt+8>:     0x02    0x1c    0xf0    0x00    0x08    0x00    0x45    0x00
  0x739b30 <s_pkt+16>:    0x00    0x28    0x33    0xfd    0x00    0x00    0x00    0x06
  0x739b38 <s_pkt+24>:    0x24    0x4a    [ ... ]

If I'm not mistaken, the ttl should be at byte 22 and is 0 in this case. 

At one point I appeared to trip a Heisenbug and actually got a ttl of 64 shown in tcpdump,
but that only happened once or twice in the call of Active_SendReset() . 

Also, stepping through the code it looks like rej is actually a pointer to a static 
variable, so the next line

      if ( !rej ) return;

will never trigger a return unless I'm mistaken. I don't know if that's a factor here
or not.

Is this enough to file a bug report? I can send pcaps & whatnot. OTOH, it's entirely
possible I'm wildly wrong :-)

Jim Hranicky
IT Security Engineer
Office of Information Security and Compliance
University of Florida

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