netdev - Re: Queries regarding TAP interface

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Date:   Mon, 15 May 2017 05:00:36 +0000
From:   "Kashyap, Saurav" <Saurav.Kashyap@...ium.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Queries regarding TAP interface

Hi Stephen,
Comments inline.



>On Fri, 12 May 2017 05:02:37 +0000
>"Kashyap, Saurav" <Saurav.Kashyap@...ium.com> wrote:
>
>> Hi,
>> 
>> I am using upstream kernel 4.11.0 and base is RHEL 7.2. I am playing around with TAP devices and created it using following commands
>> =======
>> ip tuntap add tap10 mode tap
>> ip addr add 172.28.12.1/24 dev tap10
>> ip link set tap10 up
>> ethtool -s tap10 msglvl 1
>> =======
>> 
>> I enabled debug in driver and also added some prints in tun_net_xmit function. I tried to ping using tap10 but I don’t see request reaching driver. I also tried to create a bridge using brctl and link with physical ethernet interface but it didn’t work.
>> 
>> Second experiment I did was to create two tap device on same system and try pinging one from another, that also didn’t worked. 
>> I have following queries
>> 1) Is I am missing something? 
>> 2) Is is possible to hook my own tx/rx to that tap device, so that data packets actually goes on wire?
>> 3) Does DHCP works with tap devices?
>> 
>> 
>> Thanks,
>> ~Saurav
>> 
>
>Don't you need an application reading/writing the tap device?
<SK> So ping is not intelligent enough to work with TAP? I had wrote a simple tap program that open a tun device (/dev/net/tun) and does read/write. With this program I can see that calls are going to the driver. I ran this program and from other terminal I tried ping but ping failed. Does ping/arping/dhcp utilities work with TAP interface?

>The carrier state of tap device is up/down based on whether the corresponding tap device is open

<SK> Yes, thank for an info.

Thanks,
~Saurav