[Syncookies]: Add support for TCP options via timestamps.

Allow the use of SACK and window scaling when syncookies are used
and the client supports tcp timestamps. Options are encoded into
the timestamp sent in the syn-ack and restored from the timestamp
echo when the ack is received.

Based on earlier work by Glenn Griffin.
This patch avoids increasing the size of structs by encoding TCP
options into the least significant bits of the timestamp and
by not using any 'timestamp offset'.

The downside is that the timestamp sent in the packet after the synack
will increase by several seconds.

changes since v1:
 don't duplicate timestamp echo decoding function, put it into ipv4/syncookie.c
 and have ipv6/syncookies.c use it.
 Feedback from Glenn Griffin: fix line indented with spaces, kill redundant if ()

Reviewed-by: Hagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 723b36851dde..7b41bb962b9c 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -442,6 +442,9 @@ extern struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 extern __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, 
                                      __u16 *mss);
 
+extern __u32 cookie_init_timestamp(struct request_sock *req);
+extern void cookie_check_timestamp(struct tcp_options_received *tcp_opt);
+
 /* From net/ipv6/syncookies.c */
 extern struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb);
 extern __u32 cookie_v6_init_sequence(struct sock *sk, struct sk_buff *skb,
@@ -956,6 +959,7 @@ static inline void tcp_openreq_init(struct request_sock *req,
         struct inet_request_sock *ireq = inet_rsk(req);
 
         req->rcv_wnd = 0;               /* So that tcp_send_synack() knows! */
+        req->cookie_ts = 0;
         tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
         req->mss = rx_opt->mss_clamp;
         req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;