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

[Andy Lutomirski]

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?

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.

--Andy
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