[Snort-devel] Stream5 issue

Arun Koshal akoshal04 at ...2499...
Mon Mar 30 05:53:18 EDT 2015


Hi Emiliano,

Thanks a lot for your inputs!!!

I am facing the issue # 1 even after disabling NIC segmentation features.
Please find below the output of the ethtool command:
-bash-4.1# ethtool -k eth1
Features for eth1:
rx-checksumming: off
tx-checksumming: off
scatter-gather: off
tcp-segmentation-offload: off
udp-fragmentation-offload: off
generic-segmentation-offload: off
generic-receive-offload: off
large-receive-offload: off
rx-vlan-offload: on
tx-vlan-offload: on
ntuple-filters: off
receive-hashing: off

Please find below the stream5 configuration in my setup:
preprocessor stream5_global: track_tcp yes, \
   track_udp yes, \
   track_icmp no, \
   max_tcp 262144, \
   max_udp 131072, \
   max_active_responses 2, \
   min_response_seconds 5
preprocessor stream5_tcp: policy linux, detect_anomalies, require_3whs 180,
\
   overlap_limit 10, small_segments 3 bytes 150, timeout 180,
max_queued_bytes 0, max_queued_segs 0,\
    ports client 21 22 23 25 42 53 70 79 109 110 111 113 119 135 136 137
139 143 \
        161 445 513 514 587 593 691 1433 1521 1741 2100 3306 6070 6633 6653
6665 6666 6667 6668 6669 \
        7000 8181 32770 32771 32772 32773 32774 32775 32776 32777 32778
32779, \
    ports both 36 80 81 82 83 84 85 86 87 88 89 90 110 311 383 443 465 563
555 591 593 631 636 801 808 818 901 972 989 992 993 994 995 1158 1220 1414
1533 1741 1830 1942 2231 2301 2381 2809 2980 3029 3037 3057 3128 3443 3702
4000 4343 4848 5000 5117 5250 5600 6080 6173 6988 7907 7000 7001 7071 7144
7145 7510 7802 7770 7777 7778 7779 \
        7801 7900 7901 7902 7903 7904 7905 7906 7908 7909 7910 7911 7912
7913 7914 7915 7916 \
        7917 7918 7919 7920 8000 8008 8014 8028 8080 8081 8082 8085 8088
8090 8118 8123 8180 8181 8222 8243 8280 8300 8333 8344 8500 8509 8800 8888
8899 8983 9000 9060 9080 9090 9091 9111 9290 9443 9999 10000 11371 12601
13014 15489 29991 33300 34412 34443 34444 41080 44449 50000 50002 51423
53331 55252 55555 56712
preprocessor stream5_udp: timeout 180

As mentioned earlier, I have Snort running in pcap mode, with following
options:
./snort -A fast -b -d -k none -i eth1 --daq pcap --daq-var
buffer_size=536870912 -u root -g root -c /etc/snort/snort.conf -l
/var/log/snort -A unsock

I am streaming the client side traffic only.

Please let me know how should I debug this problem. The first issue has
really become a bottleneck.

Thanks and regards,
Arun



On Sat, Mar 28, 2015 at 6:59 PM, Emiliano Fausto <emiliano.fausto at ...2499...>
wrote:

> Hello Arun,
>
> I'd suggest you to read this article:
> http://blog.securityonion.net/2011/10/when-is-full-packet-capture-not-full.html
>
> I had similar troubles too some time ago because of it.
>
> Also depending on what you are trying to capture, this option could also
> be influencing the capture:
>
> -k *checksum-mode*
>
> Controls which packet checksums Snort computes and verifies. Valid
> checksum modes include all, noip, notcp, noudp, noicmp, and none. This
> can be used to eliminate packets that fail their checksums - caused either
> by network faults or IDS evasion attempts
>
> Maybe if  your checksum is known to fail, you may  put: "-k none"
>
> Hope it helps!!
>
> Regards,
> Emiliano.
>
> On Sat, Mar 28, 2015 at 1:34 AM, Arun Koshal <akoshal04 at ...2499...> wrote:
>
>> 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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