Re: [PATCH net-next] tcp: provide SYN headers for passive connections

[John Heffner <johnwheffner-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Mon, May 4, 2015 at 12:34 AM, Eric Dumazet <eric.dumazet-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> From: Eric Dumazet <edumazet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>
> This patch allows a server application to get the TCP SYN headers for
> its passive connections.  This is useful if the server is doing
> fingerprinting of clients based on SYN packet contents.
>
> Two socket options are added: TCP_SAVE_SYN and TCP_SAVED_SYN.
>
> The first is used on a socket to enable saving the SYN headers
> for child connections. This can be set before or after the listen()
> call.
>
> The latter is used to retrieve the SYN headers for passive connections,
> if the parent listener has enabled TCP_SAVE_SYN.
>
> TCP_SAVED_SYN is read once, it frees the saved SYN headers.
>
> The data returned in TCP_SAVED_SYN are network (IPv4/IPv6) and TCP
> headers.
>
> Original patch was written by Tom Herbert, I changed it to not hold
> a full skb (and associated dst and conntracking reference).
>
> We have used such patch for about 3 years at Google.


Nice idea, seems handy.  But a couple (somewhat related) questions:

* Other than convenience, are there reasons not use an existing, more
general-purpose and portable mechanism like pcap?  (Permissions, I
guess?)
* Are there conditions where, for security purposes, you don't want an
application to have access to the raw SYNs?

Thanks,
  -John