Snort mailing list archives
Few strange problems with Snort and Stream5 preprocessor
From: Arun Koshal <akoshal04 () gmail com>
Date: Fri, 27 Mar 2015 16:00:02 +0530
Hi, I have written a dynamic preprocessor for inspecting some custom application. This dynamic preprocessor depends on stream5 preprocessor from getting the TCP stream. I am using snort in PCAP mode. I am facing following very strange problems - 1. The data in the rebuilt stream given by stream5 does not match with the TCP sequence number. For example - for a given TCP packet the sequence number in packet (pkt->tcp_header->sequence) is 507351850, but the data in pkt->payload is actually same as that of some old packet, having TCP sequence number 507343162. This scenario happens in case there are lot of packets getting dropped. I confirmed this with wireshark packet capture and I have observed multiple such instances. I also noticed that I am getting all the packets in the same buffer (pkt->payload remains same for all packets). So it seems like that I am getting a new packet with new header but old data. If I configure the pcap buffer_size as 512MB, the packets do not drop and this problem does not happen. 2. The second problem that I faced was with the pcap snap_len. In my setup, I had snort running with default pcap snap_len. I noticed that snort was not receiving packets having 1448 bytes data (1500 bytes, excluding Ethernet header). On reducing the MTU of the traffic generator from 1500 to 1484, Snort started receiving those packets. As a workaround, I increased the snap_len in sfdaq.c to 1546. Is this behaviour correct? It would be really great if someone can provide some inputs on these issues. Thanks and regards. Arun
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- Few strange problems with Snort and Stream5 preprocessor Arun Koshal (Mar 27)