Ethertap device in 2.3.40+ and 2.2.14

[Guus Sliepen]

Hello,

What is wrong with the ethertap device in the latest stable and devel
kernels? Reading/writing to /dev/tap? works fine, tcpdump -i tap0 shows
every packet that is sent and received, but the kernel does not respond to
incoming packets (example: if I ping a host that is on tap0 (virtually
ofcourse), tcpdump shows a ping request is sent AND a reply is received,
but the ping program itself tells me there are no ping replies comming
in). I'm using tinc (a VPN daemon) to handle the data that is comming from
and has to be sent to the tap device. Tinc used to work perfectly with all
previous kernels.

I've tried lots of things to make it work again, including:

* turning ARP on and off
* manually setting the ARP table
* setting tap device in promiscuous mode
* turning off rp_filter

Replacing tinc by ethertap_ping_reply from the netfilter package makes
ping work again.

This is a dump of an outgoing ping request while tinc was running:

[root en xar]~>tcpdump -n -p -i tap0 -e -x -vvv -s 1500
tcpdump: listening on tap0
23:40:58.067319 fe:fd:a:1:1:1 fe:fd:a:1:1:1 0800 98: 10.1.1.1 >
10.1.7.1: icmp: echo request (DF) (ttl 64, id 0)
                         4500 0054 0000 4000 4001 1ea6 0a01 0101
                         0a01 0701 0800 17e0 7603 0000 fabd 9438
                         ef22 0100 0809 0a0b 0c0d 0e0f 1011 1213
                         1415 1617 1819 1a1b 1c1d 1e1f 2021 2223
                         2425 2627 2829 2a2b 2c2d 2e2f 3031 3233
                         3435 3637
23:40:58.077317 fe:fd:a:1:1:1 fe:fd:a:1:7:1 0800 98: 10.1.7.1 >
10.1.1.1: icmp: echo reply (ttl 255, id 59710)
                         4500 0054 e93e 0000 ff01 b666 0a01 0701
                         0a01 0101 0000 1fe0 7603 0000 fabd 9438
                         ef22 0100 0809 0a0b 0c0d 0e0f 1011 1213
                         1415 1617 1819 1a1b 1c1d 1e1f 2021 2223
                         2425 2627 2829 2a2b 2c2d 2e2f 3031 3233
                         3435 3637

This is one with ethertap_ping_reply running:

23:43:11.271253 fe:fd:a:1:1:1 fe:fd:a:1:1:1 0800 98: 10.1.1.1 >
10.1.7.1: icmp: echo request (DF) (ttl 64, id 0)
                         4500 0054 0000 4000 4001 1ea6 0a01 0101
                         0a01 0701 0800 d9cf 7f03 0000 7fbe 9438
                         9c32 0400 0809 0a0b 0c0d 0e0f 1011 1213
                         1415 1617 1819 1a1b 1c1d 1e1f 2021 2223
                         2425 2627 2829 2a2b 2c2d 2e2f 3031 3233
                         3435 3637
23:43:11.291248 fe:fd:a:1:1:1 fe:fd:a:1:1:1 0800 98: 10.1.7.1 >
10.1.1.1: icmp: echo reply (DF) (ttl 64, id 0)
                         4500 0054 0000 4000 4001 1ea6 0a01 0701
                         0a01 0101 0000 e1cf 7f03 0000 7fbe 9438
                         9c32 0400 0809 0a0b 0c0d 0e0f 1011 1213
                         1415 1617 1819 1a1b 1c1d 1e1f 2021 2223
                         2425 2627 2829 2a2b 2c2d 2e2f 3031 3233
                         3435 3637

What is wrong? Please CC to my personal email address.

Met vriendelijke groet,
Guus Sliepen.


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