[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, 2 insertions, 3 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index df89a566a5a..52e3ae603ca 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1299,10 +1299,8 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 
         tcp_parse_options(skb, &tmp_opt, 0);
 
-        if (want_cookie) {
+        if (want_cookie && !tmp_opt.saw_tstamp)
                 tcp_clear_options(&tmp_opt);
-                tmp_opt.saw_tstamp = 0;
-        }
 
         if (tmp_opt.saw_tstamp && !tmp_opt.rcv_tsval) {
                 /* Some OSes (unknown ones, but I see them on web server, which
@@ -1330,6 +1328,7 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
         if (want_cookie) {
 #ifdef CONFIG_SYN_COOKIES
                 syn_flood_warning(skb);
+                req->cookie_ts = tmp_opt.tstamp_ok;
 #endif
                 isn = cookie_v4_init_sequence(sk, skb, &req->mss);
         } else if (!isn) {