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ankit
Asked: 2011-03-03 21:22:44 +0800 CST

Check packet vlan tag using Tap virtual interface

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I am trying to learn how to implement virtual interfaces using the Tap driver.

So far my understanding is that using the tap driver I can create a virtual interface and then have a userspace program attach to this interface to analyse the data coming into this device.

Now what if I attach a cisco switch to my LAN interface using a TRUNK link, forward all the packets coming into the LAN interface to the virtual tap interface and then in my program attached to this interface do some coding to analyze the vlan tag in the packet and only allow certain vlans to be forwarded to the WAN interface?

Does this sound plausible or is there is flaw in my basic understanding?

Update: Now that I have played more with it I have few more questions:

So i have my tun interface created (tun0) that does receive packets. With these packets I am doing some filtering (allowing/dropping) based on the "SRC MAC Address" in the ethernet frame. The question, how do I send the allowed packets to one of my WAN interfaces (eth0) now?

interface linux-networking tap

1 Answers

  1. It's certainly possible to do this, and it's actually possible to do this with existing kernel modules today:

    LAN -> (nic module) -> (dot1q module) -> (bridge module) -> (dot1q module) -> (nic module) -> WAN

    Or:

    eth0 -> eth0.10 -> br0 -> eth1.20 -> eth1

    (takes anything tagged as vlan 10 off eth0, re-tags it as vlan 20 and pushes it out eth1, and visa-versa. Additional access control can be done using ebtables.)

    If you don't need to change tags, you can simplify down to:

    eth0 -> br0 -> eth1

    And apply ebtables to br0.

    That said, if your application can do the "eth0.10 -> br0 -> eth1.20" piece internally, though it would not require tap devices to do it, since you can read frames off one interface, filter, and write to the other.

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