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HTTP Reassembly issue PAF enabled
From: Parmendra Pratap <parmendra.pratap () yahoo com>
Date: Tue, 2 Apr 2013 15:38:14 -0700 (PDT)

Hi

I have been looking into snort lately for IDS and network data analysis needs at work.
Looks like have uncovered an issue with PAF enabled reassembly of HTTP data or may be perhaps in the alert plugins.
Any input is well appreciated.

Apparently with PAF enabled the HTTP specific checks fire off only on the receipt of subsequent ACK packet even if the 
HTTP PDU data fits completely in the current packet.
With PAF disabled the issue seems to correct itself.
Looking at the code in snort_httpinspect.c , it appears that HTTP specific processing is short circuited due to the 
following check
#ifdef ENABLE_PAF  if ( ScPafEnabled() &&  (p->packet_flags & PKT_STREAM_INSERT) && 
        !PacketHasFullPDU(p) ) 
PacketHasFullPDU(p) only evaluates true on receipt of subsequent ACK even if the current packet does hold the complete 
PDU.

Steps to replicate:
Below is a rule to print alert on receipt of  GET HTTP requests.
custom_log tcp any any <> any 80 (msg:"DUMP"; content:"GET"; http_method; sid:1000001;rev:1)
ruletype custom_log {                                                                 
 type  alert
 output alert_full
 output log_tcpdump: log_alert.csv.dump
} 

Try any curl/wget request now.

Test 1 -(PAF enabled)
Log from alert_full with paf_max = 16000
[Priority: 0]
04/02-22:59:33.414512 10.0.144.XXX:2228 -> 10.8.XX.XX
TCP TTL:64 TOS:0x0 ID:23556 IpLen:20 DgmLen:443 DF
***A**** Seq: 0x88CC7A97  Ack: 0x504A44DD  Win: 0x3CB8  TcpLen: 20

Test 2 -(PAF disabled)
Log from alert_full with paf_max = 0
[**] [1:1000001:1] DUMP [**]
[Priority: 0]
04/02-23:09:35.761077 10.0.XX.XX:2286 -> 10.8.XX.XX
   TCP TTL:122 TOS:0x0 ID:52317 IpLen:20 DgmLen:443 DF
    ***AP*** Seq: 0xDB1E4DB7  Ack: 0x66F537C3  Win: 0xFFFF  TcpLen: 20

Close look will reveal that the TCP headers are different - A vs AP.
TCP dump and wireshark capture confirms that Test 2 results are correct whereas Test 1 is showing incorrect TCP data in 
the alert log.

Thanks
PP
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