tcp: Generalized TTL Security Mechanism

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 <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/in.h b/include/linux/in.h
index b615649db129..583c76f9c30f 100644
--- a/include/linux/in.h
+++ b/include/linux/in.h
@@ -84,6 +84,8 @@ struct in_addr {
 #define IP_ORIGDSTADDR       20
 #define IP_RECVORIGDSTADDR   IP_ORIGDSTADDR
 
+#define IP_MINTTL       21
+
 /* IP_MTU_DISCOVER values */
 #define IP_PMTUDISC_DONT                0       /* Never send DF frames */
 #define IP_PMTUDISC_WANT                1       /* Use per route hints  */