[dnscap-users] interpretation of VLAN 0 on command line

Wessels, Duane dwessels at verisign.com
Mon Aug 24 17:28:40 UTC 2015

Greetings dnscap users!

dnscap has the ability to filter on VLAN with the -l and -L command line

Currently "-l 0" is taken as instruction to add "vlan and" to the BPF
program so that VLAN-tagged packets will be received, but to NOT otherwise
filter by VLAN ID.  In other words, VLAN 0 is treated as a wildcard.

Although VLAN 0 is reserved (meaning you can't configure it I suppose),
it seems that it can appear on the wire.  Currently dnscap is not able to
process VLAN 0 packets because of the above and also because it assumes
VLAN 0 wouldn't ever appear on the wire.

VLAN 0xFFF is also reserved and appears to have the meaning which dnscap
gives to 0 -- namely that of a wildcard.  If my understanding is correct,
0xFFF should not be configurable on a device and should never appear on
the wire.

Therefore, I may propose that the dnscap command line interpretation be
changed so that "-l 4095" is taken to mean that dnscap should capture all

Before doing that, however, I wonder if many users have scripts and processes
built around the current meaning of "-l 0"?


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