Capturing packets when no IP address is assigned to the capturing interface

[Greg Troxel]

As mlelstv@ said, it's almost certainly about UP, not address.

So, after booting.

ifconfig foo0 up
tcpdump -i foo0

and see how that goes.

[Greg Troxel]

Post by Greg Troxel
So, after booting.
ifconfig foo0 up
tcpdump -i foo0
and see how that goes.

You're both absolutely correct. Thanks! That also solves the bridging
problem for me as well.

That's good -- and note that it is not a bug that an interface that is
not UP doesn't do anything.

But Staffan had an IP address configured prior to trying to set up
bridging with the tap interface, so that doesn't explain why that
(bridging multiple physical interfaces and a tap interface with an IP
address on the tap) didn't work for him.

It is very likely that there is a bug lurking in the parts of NetBSD
that Staffan is using.

I read that some (most)
wireless interfaces don't support bridging. I wonder if that's the
problem in his case. Here are the two links about wireless not
https://www.linux-kvm.org/page/Networking#Public_Bridge
https://oldwiki.archive.openwrt.org/doc/howto/clientmode?s%5B%5D=bridging#bridged_client_mode_issues

There is no fundamental reason why bridging should work or not work
depending on the link-layer technology of an Ethernet-like interface.
There is some complexity about 802.11 interfaces in other than BSS or
HOSTAP modes, probably. But, Linux docs don't apply.

[Jason Mitchell]

Post by Greg Troxel
So, after booting.
ifconfig foo0 up
tcpdump -i foo0
and see how that goes.

You're both absolutely correct. Thanks! That also solves the bridging
problem for me as well.

But Staffan had an IP address configured prior to trying to set up
bridging with the tap interface, so that doesn't explain why that
(bridging multiple physical interfaces and a tap interface with an IP
address on the tap) didn't work for him. I read that some (most)
wireless interfaces don't support bridging. I wonder if that's the
problem in his case. Here are the two links about wireless not
supporting bridging:

https://www.linux-kvm.org/page/Networking#Public_Bridge

https://oldwiki.archive.openwrt.org/doc/howto/clientmode?s%5B%5D=bridging#bridged_client_mode_issues

Does this also apply to NetBSD?

Thanks again!

Jason M.

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[Michael van Elst]

Post by Jason Mitchell
address on the tap) didn't work for him. I read that some (most)
wireless interfaces don't support bridging. I wonder if that's the
problem in his case. Here are the two links about wireless not
https://www.linux-kvm.org/page/Networking#Public_Bridge
https://oldwiki.archive.openwrt.org/doc/howto/clientmode?s%5B%5D=bridging#bridged_client_mode_issues

For bridging to work, an interface has to send and receive packets
with arbitrary MAC adresses. That doesn't work with your regular
Wifi protocol where the MAC adress is used to identify a client.

Wifi interfaces that support hostap mode could act as multiple
clients to make bridging work. Another possibility is WDS that
is used by wireless repeaters. Either needs software support.

Post by Jason Mitchell
Does this also apply to NetBSD?

Indeed.


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