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>

4 files changed, 21 insertions, 2 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  */
diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index bd4c53f75ac0..83fd34437cf1 100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h
@@ -122,10 +122,12 @@ struct inet_sock {
         __be32                  inet_saddr;
         __s16                   uc_ttl;
         __u16                   cmsg_flags;
-        struct ip_options       *opt;
         __be16                  inet_sport;
         __u16                   inet_id;
+
+        struct ip_options       *opt;
         __u8                    tos;
+        __u8                    min_ttl;
         __u8                    mc_ttl;
         __u8                    pmtudisc;
         __u8                    recverr:1,
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index cafad9baff03..644dc43a55de 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -451,7 +451,8 @@ static int do_ip_setsockopt(struct sock *sk, int level,
                              (1<<IP_TTL) | (1<<IP_HDRINCL) |
                              (1<<IP_MTU_DISCOVER) | (1<<IP_RECVERR) |
                              (1<<IP_ROUTER_ALERT) | (1<<IP_FREEBIND) |
-                             (1<<IP_PASSSEC) | (1<<IP_TRANSPARENT))) ||
+                             (1<<IP_PASSSEC) | (1<<IP_TRANSPARENT) |
+                             (1<<IP_MINTTL))) ||
             optname == IP_MULTICAST_TTL ||
             optname == IP_MULTICAST_ALL ||
             optname == IP_MULTICAST_LOOP ||
@@ -936,6 +937,14 @@ mc_msf_out:
                 inet->transparent = !!val;
                 break;
 
+        case IP_MINTTL:
+                if (optlen < 1)
+                        goto e_inval;
+                if (val < 0 || val > 255)
+                        goto e_inval;
+                inet->min_ttl = val;
+                break;
+
         default:
                 err = -ENOPROTOOPT;
                 break;
@@ -1198,6 +1207,9 @@ static int do_ip_getsockopt(struct sock *sk, int level, int optname,
         case IP_TRANSPARENT:
                 val = inet->transparent;
                 break;
+        case IP_MINTTL:
+                val = inet->min_ttl;
+                break;
         default:
                 release_sock(sk);
                 return -ENOPROTOOPT;
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 65b8ebfd078a..382f667238ec 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -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;