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

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Message-ID: <1430752330.3711.180.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 04 May 2015 08:12:10 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	John Heffner <johnwheffner@...il.com>
Cc:	Eric B Munson <emunson@...mai.com>,
	Tom Herbert <tom@...bertland.com>,
	"David S. Miller" <davem@...emloft.net>, linux-api@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] tcp: provide SYN headers for passive
 connections

On Mon, 2015-05-04 at 10:41 -0400, John Heffner wrote:

> 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?)

Very hard to synchronize when say you have 32 listeners sharing a single
port (SO_REUSEPORT), and receive one million SYN per second (when my TCP
listener scaling work is finished).

libpcap here would be a serious bottleneck, even with a clever FANIN
support on the af_packet sockets, considering use of multiqueue NIC.

> * Are there conditions where, for security purposes, you don't want an
> application to have access to the raw SYNs?

Not that we are aware of : We restrict the access to IP + TCP headers,
for the passive part. All information that is available there was
provided by the remote peer on a 'open way' anyway.

Thanks.

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