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

On Mon, May 4, 2015 at 12:34 AM, Eric Dumazet [off-list ref] wrote:

From: Eric Dumazet <redacted>

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