Re: [PATCH] Allow TCP connections to cache SYN packet for userspace inspection

[Andy Lutomirski]

On Fri, May 1, 2015 at 1:01 PM, Eric B Munson <emunson@xxxxxxxxxx> wrote:
> On Fri, 01 May 2015, Andy Lutomirski wrote:
>
>> On Fri, May 1, 2015 at 10:43 AM, Eric B Munson <emunson@xxxxxxxxxx> wrote:
>> > In order to enable policy decisions in userspace, the data contained in
>> > the SYN packet would be useful for tracking or identifying connections.
>> > Only parts of this data are available to userspace after the hand shake
>> > is completed.  This patch exposes a new setsockopt() option that will,
>> > when used with a listening socket, ask the kernel to cache the skb
>> > holding the SYN packet for retrieval later.  The SYN skbs will not be
>> > saved while the kernel is in syn cookie mode.
>> >
>> > The same option will ask the kernel for the packet headers when used
>> > with getsockopt() with the socket returned from accept().  The cached
>> > packet will only be available for the first getsockopt() call, the skb
>> > is consumed after the requested data is copied to userspace.  Subsequent
>> > calls will return -ENOENT.  Because of this behavior, getsockopt() will
>> > return -E2BIG if the caller supplied a buffer that is too small to hold
>> > the skb header.
>>
>> What's the purpose and what headers are you returning?
>
> Currently the ethernet, IP, and TCP headers are being returned.  The IP
> and TCP headers will be used by userspace to make decisions on how to
> handle incoming connections.  The ethernet headers are being returned
> for completeness, I would be fine not including them in what is copied
> if that is a concern, however the team requesting this change here
> requires the IP and TCP headers.
>
>>
>> There was a bit of a mixup with tx timestamps where the set of headers
>> returned was possibly excessive and incompletely thought out the first
>> time around.
>
> With this in mind, we could drop copying the ethernet headers and simply
> hold onto the IP and TCP headers.

That's probably better.  If nothing else, you avoid breaking userspace
when you're not using Ethernet.

--Andy
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