Queries regarding TAP interface

Queries regarding TAP interface

[Kashyap, Saurav]

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

Re: Queries regarding TAP interface

[Stephen Hemminger]

On Fri, 12 May 2017 05:02:37 +0000
"Kashyap, Saurav" <Saurav.Kashyap@cavium.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?
The carrier state of tap device is up/down based on whether the corresponding tap device is open

Re: Queries regarding TAP interface

[Kashyap, Saurav]

Hi Stephen,
Comments inline.



>On Fri, 12 May 2017 05:02:37 +0000
>"Kashyap, Saurav" <Saurav.Kashyap@cavium.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