Snort mailing list archives
stream5 preprocessor: 'STREAM5_NO_TIMESTAMP' alert in combination with TCP Keep-Alives from BSD/Darwin
From: Bram <bram-fabeg () mail wizbit be>
Date: Thu, 01 Aug 2013 16:42:27 +0200
Hi,The TCP implementation on *BSD (and by extension on Darwin) appears to contain a bug: When the TCP session is idle then it sends a 'TCP Keep-Alive' packet to determine if the connection still exists.
This is expected. However: the 'TCP Keep-Alive' packet does not have the timestamp options set.. This causes snort to generate the alert 'STREAM5_NO_TIMESTAMP'.While the event is correct it is a bit undesirable since this makes it difficult to see unexplained anomalies/actual 'problems.
Attached is a patch which detects the 'TCP KeepAlive' packets send by BSD/Darwin and prevents the alert from being generated. I'm not sure if the 'TCP KeepAlive' packet should be ignored by default.. perhaps it's better to add a config options for it?
Also: when *BSD/Darwin sends an ack on a 'TCP Keep-Alive' packet then it does appear to include the timestamp.
(This was detected due to a PPTP client being connected from a Mac - tcp idle -> keep alives send)
Attached are four dumps:* keepalive.pcap: connection between NetBSD and Linux (NetBSD sending Keep-Alive)
* keepalive2.pcap: connection between NetBSD and NetBSD* keepalive4.pcap: connection between Linux and NetBSD host (Linux sending Keep-Alive)
* no_timestamp.pcap: tcp session created using raw sockets Configuration file used: config checksum_mode: all dynamicpreprocessor directory /usr/lib/snort_dynamicpreprocessor/ preprocessor stream5_global: track_tcp yes, \ track_udp no, \ track_icmp no, \ max_tcp 262144, \ max_udp 131072 preprocessor stream5_tcp: policy windows, detect_anomaliesalert ( msg: "STREAM5_NO_TIMESTAMP"; sid: 14; gid: 129; rev: 1; metadata: rule-type preproc ; )
output alert_fast: stdout Output:$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/keepalive.pcap 2>&1 | grep '129:' 07/22-14:16:03.787282 [**] [129:14:1] TCP Timestamp is missing [**] [Priority: 0] {TCP} 192.168.173.51:52185 -> 192.168.173.50:6666 07/22-14:16:13.787173 [**] [129:14:1] TCP Timestamp is missing [**] [Priority: 0] {TCP} 192.168.173.51:52185 -> 192.168.173.50:6666
$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/keepalive2.pcap 2>&1 | grep '129:' 07/22-14:18:45.965624 [**] [129:14:1] TCP Timestamp is missing [**] [Priority: 0] {TCP} 192.168.173.51:52179 -> 192.168.173.51:6666 07/22-14:18:55.965523 [**] [129:14:1] TCP Timestamp is missing [**] [Priority: 0] {TCP} 192.168.173.51:52179 -> 192.168.173.51:6666
$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/keepalive3.pcap 2>&1 | grep '129:'
$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/no_timestamp.pcap 2>&1 | grep '129:' 08/01-16:33:02.253871 [**] [129:14:1] TCP Timestamp is missing [**] [Priority: 0] {TCP} 192.168.173.1:6000 -> 192.168.173.153:33705
Output with patched version:$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/keepalive.pcap 2>&1 | grep '129:'
$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/keepalive2.pcap 2>&1 | grep '129:'
$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/keepalive3.pcap 2>&1 | grep '129:'
$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/no_timestamp.pcap 2>&1 | grep '129:' 08/01-16:33:02.253871 [**] [129:14:1] TCP Timestamp is missing [**] [Priority: 0] {TCP} 192.168.173.1:6000 -> 192.168.173.153:33705
=> No alert on TCP Keep-Alive from BSD/Darwin. Best regards, Bram ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.
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keepalive.pcap
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keepalive2.pcap
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keepalive3.pcap
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no_timestamp.pcap
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Attachment:
snort-2.9.5-tcp-keepalive.patch
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Current thread:
- stream5 preprocessor: 'STREAM5_NO_TIMESTAMP' alert in combination with TCP Keep-Alives from BSD/Darwin Bram (Aug 01)
- Re: stream5 preprocessor: 'STREAM5_NO_TIMESTAMP' alert in combination with TCP Keep-Alives from BSD/Darwin Bram (Aug 20)