Subject: Re: [PATCH] tcp: Generalized TTL Security Mechanism

From: Stephen Hemminger <[email-masked]>
Date: Mon, 11 Jan 2010 08:25:29 -0800

On Mon, 11 Jan 2010 12:25:23 +0100
Eric Dumazet <[email-masked]> wrote:

> Le 11/01/2010 07:00, Stephen Hemminger a écrit :
> > This patch adds the kernel portions needed to implement
> > RFC 5082 Generalized TTL Security Mechanism (GTSM).
> > It is a lightweight security measure against forged
> > packets causing DoS attacks (for BGP). 
> > 
> > This is already implemented the same way in BSD kernels.
> > For the necessary Quagga patch 
> >   http://www.gossamer-threads.com/lists/quagga/dev/17389
> > 
> > Description from Cisco
> >   http://www.cisco.com/en/US/docs/ios/12_3t/12_3t7/feature/guide/gt_btsh.html
> > 
> > It does add one byte to each socket structure, but I did
> > a little rearrangement to reuse a hole (on 64 bit), but it
> > does grow the structure on 32 bit
> > 
> > This should be documented on ip(4) man page and the Glibc in.h
> > file also needs update.  IPV6_MINHOPLIMIT should also be added
> > (although BSD doesn't support that).  
> > 
> > Only TCP is supported, but could also be added to UDP, DCCP, SCTP
> > if desired.
> > 
> > Signed-off-by: Stephen Hemminger <[email-masked]>
> > 
> 
> > --- a/net/ipv4/tcp_ipv4.c	2010-01-10 21:06:42.931093698 -0800
> > +++ b/net/ipv4/tcp_ipv4.c	2010-01-10 21:08:21.537513427 -0800
> > @@ -1649,6 +1649,9 @@ int tcp_v4_rcv(struct sk_buff *skb)
> >  	if (!sk)
> >  		goto no_tcp_socket;
> >  
> > +	if (iph->ttl < inet_sk(sk)->min_ttl)
> > +		goto discard_and_relse;
> > +
> >  process:
> >  	if (sk->sk_state == TCP_TIME_WAIT)
> >  		goto do_time_wait;
> 
> Just wondering if perfoming the check at connection establishment time
> (SYN or SYN-ACK packet) instead of every received packet would be enough ?

We could but:
  1. GTSM is trying to protect against Man in the Middle attacks to existing
     BGP connections
  2. That is not what BSD (or other vendors) do.

> Of course, for listeners waiting for connexions from different peers (and different
> ttl values), it would be tricky.
> 
> Check should be done at user level, if we store ttl value of SYN packet and let
> user application read its value by a getsockopt()

I think IP_RECVTTL would work for that idea.