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tshark filter options broken with NapaTech NIC

asked 2025-08-22 01:18:03 +0000

updated 2025-08-22 11:08:20 +0000

grahamb gravatar image

When running tshark on a NapaTech NIC, the host (and other) keywords do not work. We are using the libpcap that Napatech provides. NapaTech seems to think it's a bug in the non-Napatech libpcap files.

Versions

  • RHEL 8.10
  • libpcap 1.10.4
  • tshark 2.6.2

Using grep, the packets are there

[root@nspcapprd101 ~]# /usr/bin/tshark -i napa2 | grep 10.239.81.123
Running as user "root" and group "root". This could be dangerous.
Capturing on 'napa2'
34795 38185 0.088616101 10.239.81.123 → 10.41.177.39 UDP 66 2598 → 56164 Len=20
40795 46374 0.106482092 10.41.177.39 → 10.239.81.123 UDP 66 56164 → 2598 Len=20
106792 108290 0.283687032 10.239.81.123 → 10.41.177.39 UDP 77 2598 → 56164 Len=31

But using the host keyword, nothing is there for several minutes

[root@nspcapprd101 ~]# /usr/bin/tshark -i napa2 host 10.239.81.123
Running as user "root" and group "root". This could be dangerous.
Capturing on 'napa2'
0 packets captured

Then running with grep again to make sure packets still coming in

[root@nspcapprd101 ~]# /usr/bin/tshark -i napa2 | grep 10.239.81.123
Running as user "root" and group "root". This could be dangerous.
Capturing on 'napa2'
22800 28556 0.098213558 10.239.81.123 → 10.41.177.39 UDP 76 2598 → 56164 Len=30
28799 29634 0.101830401 10.41.177.39 → 10.239.81.123 UDP 86 56164 → 2598 Len=40
29703 0.102196547 10.239.81.123 → 10.41.177.39 UDP 66 2598 → 56164 Len=20

Host keyword works on other interfaces on server

[root@nspcapprd101 lib]# /usr/bin/tshark -i eno2 host 192.168.12.22
Running as user "root" and group "root". This could be dangerous.
Capturing on 'eno2'
1 0.000000000 192.168.12.22 → 192.168.12.127 SNMP 154 get-request
2 0.001103924 192.168.12.22 → 192.168.12.127 SNMP 185 getBulkRequest 1.3.6.1.2.1.17.7.1.2.2.1.2.3046.232.106.100.118.61.14
3 0.057710947 192.168.12.22 → 192.168.12.127 SNMP 110 getBulkRequest
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Comments

You may want to take note of the warning about running as root, especially when using such an old version of tshark.

grahamb gravatar imagegrahamb ( 2025-08-22 11:09:47 +0000 )edit
Chuckc gravatar imageChuckc ( 2025-08-22 11:30:16 +0000 )edit

2 Answers

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answered 2025-08-22 11:26:08 +0000

SYN-bit gravatar image

The most common reason for a L3/L4 capture filter to not work is that the packets are encapsulated at a lower level. Usually vlan tagging. What do you see without a filter when you do a tcpdump -neli <interface>?

If there are vlan tags, try the following filter -f "vlan and host 10.239.81.123".

If that does not work, it would be handy to see a very small unfiltered capture to see what might be in the way.

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answered 2025-08-22 03:19:23 +0000

cmaynard gravatar image

updated 2025-08-22 03:21:43 +0000

You need to use the -f <capture filter> option. For example:

/usr/bin/tshark -i napa2 -f "host 10.239.81.123"

For a more efficient capture filter, I'd recommend using the following instead:

/usr/bin/tshark -i napa2 -f "ip and host 10.239.81.123"

You can compare these two filters yourself using dumpcap's -d option.

Refs:

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Asked: 2025-08-22 01:18:03 +0000

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Last updated: 3 hours ago