[net-next,v2] nlmon: use standard rtnetlink link api for add/del devices

On Tuesday, July 16, 2013 04:49:53 PM Daniel Borkmann wrote:
> On 07/16/2013 04:08 PM, Vladimir Kondratiev wrote:
> > Jiri Pirko <jiri <at> resnulli.us> writes:
> >
> >> It is not nice when netdev is created right after module load and with
> >> some implicit name. So rather change nlmon to use standard rtnl link API.
> >
> > Could you please elaborate a bit - how to capture netlink skb's after your
> > patch? Before, it was netdev that may be used with tcpdump. Now, there is
> > no such netdev. How to create it?
> 
> modprobe nlmon
> ip link add type nlmon
> ip link set nlmon0 up
> 
> tcpdump -i nlmon0 ....
> 
> ip link set nlmon0 down
> ip link del dev nlmon0
> rmmod nlmon

Thanks a lot! I guess it is worth to have this mentioned somewhere.
It will save lots of questions. For example, in Kconfig:

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On 07/16/2013 05:59 PM, Vladimir Kondratiev wrote:
> On Tuesday, July 16, 2013 04:49:53 PM Daniel Borkmann wrote:
>> On 07/16/2013 04:08 PM, Vladimir Kondratiev wrote:
>>> Jiri Pirko <jiri <at> resnulli.us> writes:
>>>
>>>> It is not nice when netdev is created right after module load and with
>>>> some implicit name. So rather change nlmon to use standard rtnl link API.
>>>
>>> Could you please elaborate a bit - how to capture netlink skb's after your
>>> patch? Before, it was netdev that may be used with tcpdump. Now, there is
>>> no such netdev. How to create it?
>>
>> modprobe nlmon
>> ip link add type nlmon
>> ip link set nlmon0 up
>>
>> tcpdump -i nlmon0 ....
>>
>> ip link set nlmon0 down
>> ip link del dev nlmon0
>> rmmod nlmon
>
> Thanks a lot! I guess it is worth to have this mentioned somewhere.
> It will save lots of questions. For example, in Kconfig:

Ok, I don't have a strong opinion on the below patch, I'm fine either way.

But, given that for such devices it is *common* to use the standard rtnl
link API (Jiri is right that this is a better/cleaner approach), I'm not
sure if this needs to be documented then ...

> diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
> index b45b240..13acea2 100644
> --- a/drivers/net/Kconfig
> +++ b/drivers/net/Kconfig
> @@ -247,8 +247,18 @@ config NLMON
>            purpose of this is to analyze netlink messages with packet sockets.
>            Thus applications like tcpdump will be able to see local netlink
>            messages if they tap into the netlink device, record pcaps for further
> -         diagnostics, etc. This is mostly intended for developers or support
> -         to debug netlink issues. If unsure, say N.
> +         diagnostics, etc. Typical flow is:
> +
> +           modprobe nlmon
> +           ip link add type nlmon
> +           ip link set nlmon0 up
> +           tcpdump -i nlmon0 ....
> +           ip link set nlmon0 down
> +           ip link del dev nlmon0
> +           rmmod nlmon
> +
> +         This is mostly intended for developers or support to debug netlink
> +         issues. If unsure, say N.
>
>   endif # NET_CORE
>
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Tue, Jul 16, 2013 at 06:10:03PM CEST, dborkman@redhat.com wrote:
>On 07/16/2013 05:59 PM, Vladimir Kondratiev wrote:
>>On Tuesday, July 16, 2013 04:49:53 PM Daniel Borkmann wrote:
>>>On 07/16/2013 04:08 PM, Vladimir Kondratiev wrote:
>>>>Jiri Pirko <jiri <at> resnulli.us> writes:
>>>>
>>>>>It is not nice when netdev is created right after module load and with
>>>>>some implicit name. So rather change nlmon to use standard rtnl link API.
>>>>
>>>>Could you please elaborate a bit - how to capture netlink skb's after your
>>>>patch? Before, it was netdev that may be used with tcpdump. Now, there is
>>>>no such netdev. How to create it?
>>>
>>>modprobe nlmon
>>>ip link add type nlmon
>>>ip link set nlmon0 up
>>>
>>>tcpdump -i nlmon0 ....
>>>
>>>ip link set nlmon0 down
>>>ip link del dev nlmon0
>>>rmmod nlmon
>>
>>Thanks a lot! I guess it is worth to have this mentioned somewhere.
>>It will save lots of questions. For example, in Kconfig:
>
>Ok, I don't have a strong opinion on the below patch, I'm fine either way.
>
>But, given that for such devices it is *common* to use the standard rtnl
>link API (Jiri is right that this is a better/cleaner approach), I'm not
>sure if this needs to be documented then ...

I believe that this does not need any further documentation. RTNL (ip
link add ..) is a standard api to add devices...


>
>>diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
>>index b45b240..13acea2 100644
>>--- a/drivers/net/Kconfig
>>+++ b/drivers/net/Kconfig
>>@@ -247,8 +247,18 @@ config NLMON
>>           purpose of this is to analyze netlink messages with packet sockets.
>>           Thus applications like tcpdump will be able to see local netlink
>>           messages if they tap into the netlink device, record pcaps for further
>>-         diagnostics, etc. This is mostly intended for developers or support
>>-         to debug netlink issues. If unsure, say N.
>>+         diagnostics, etc. Typical flow is:
>>+
>>+           modprobe nlmon
>>+           ip link add type nlmon
>>+           ip link set nlmon0 up
>>+           tcpdump -i nlmon0 ....
>>+           ip link set nlmon0 down
>>+           ip link del dev nlmon0
>>+           rmmod nlmon
>>+
>>+         This is mostly intended for developers or support to debug netlink
>>+         issues. If unsure, say N.
>>
>>  endif # NET_CORE
>>
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