diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
new file mode 100644
index 0000000..e923d2f
--- /dev/null
+++ b/net/ipv4/syncookies.c
@@ -0,0 +1,279 @@
+/*
+ *  Syncookies implementation for the Linux kernel
+ *
+ *  Copyright (C) 1997 Andi Kleen
+ *  Based on ideas by D.J.Bernstein and Eric Schenk. 
+ *
+ *	This program is free software; you can redistribute it and/or
+ *      modify it under the terms of the GNU General Public License
+ *      as published by the Free Software Foundation; either version
+ *      2 of the License, or (at your option) any later version.
+ * 
+ *  $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $
+ *
+ *  Missing: IPv6 support. 
+ */
+
+#include <linux/tcp.h>
+#include <linux/slab.h>
+#include <linux/random.h>
+#include <linux/cryptohash.h>
+#include <linux/kernel.h>
+#include <net/tcp.h>
+
+extern int sysctl_tcp_syncookies;
+
+static __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];
+
+static __init int init_syncookies(void)
+{
+	get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
+	return 0;
+}
+module_init(init_syncookies);
+
+#define COOKIEBITS 24	/* Upper bits store count */
+#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
+
+static u32 cookie_hash(u32 saddr, u32 daddr, u32 sport, u32 dport,
+		       u32 count, int c)
+{
+	__u32 tmp[16 + 5 + SHA_WORKSPACE_WORDS];
+
+	memcpy(tmp + 3, syncookie_secret[c], sizeof(syncookie_secret[c]));
+	tmp[0] = saddr;
+	tmp[1] = daddr;
+	tmp[2] = (sport << 16) + dport;
+	tmp[3] = count;
+	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
+
+	return tmp[17];
+}
+
+static __u32 secure_tcp_syn_cookie(__u32 saddr, __u32 daddr, __u16 sport,
+				   __u16 dport, __u32 sseq, __u32 count,
+				   __u32 data)
+{
+	/*
+	 * Compute the secure sequence number.
+	 * The output should be:
+   	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
+	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
+	 * Where sseq is their sequence number and count increases every
+	 * minute by 1.
+	 * As an extra hack, we add a small "data" value that encodes the
+	 * MSS into the second hash value.
+	 */
+
+	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
+		sseq + (count << COOKIEBITS) +
+		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
+		 & COOKIEMASK));
+}
+
+/*
+ * This retrieves the small "data" value from the syncookie.
+ * If the syncookie is bad, the data returned will be out of
+ * range.  This must be checked by the caller.
+ *
+ * The count value used to generate the cookie must be within
+ * "maxdiff" if the current (passed-in) "count".  The return value
+ * is (__u32)-1 if this test fails.
+ */
+static __u32 check_tcp_syn_cookie(__u32 cookie, __u32 saddr, __u32 daddr,
+				  __u16 sport, __u16 dport, __u32 sseq,
+				  __u32 count, __u32 maxdiff)
+{
+	__u32 diff;
+
+	/* Strip away the layers from the cookie */
+	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
+
+	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
+	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
+	if (diff >= maxdiff)
+		return (__u32)-1;
+
+	return (cookie -
+		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
+		& COOKIEMASK;	/* Leaving the data behind */
+}
+
+/* 
+ * This table has to be sorted and terminated with (__u16)-1.
+ * XXX generate a better table.
+ * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
+ */
+static __u16 const msstab[] = {
+	64 - 1,
+	256 - 1,	
+	512 - 1,
+	536 - 1,
+	1024 - 1,	
+	1440 - 1,
+	1460 - 1,
+	4312 - 1,
+	(__u16)-1
+};
+/* The number doesn't include the -1 terminator */
+#define NUM_MSS (ARRAY_SIZE(msstab) - 1)
+
+/*
+ * Generate a syncookie.  mssp points to the mss, which is returned
+ * rounded down to the value encoded in the cookie.
+ */
+__u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int mssind;
+	const __u16 mss = *mssp;
+
+	
+	tp->last_synq_overflow = jiffies;
+
+	/* XXX sort msstab[] by probability?  Binary search? */
+	for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
+		;
+	*mssp = msstab[mssind] + 1;
+
+	NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESSENT);
+
+	return secure_tcp_syn_cookie(skb->nh.iph->saddr, skb->nh.iph->daddr,
+				     skb->h.th->source, skb->h.th->dest,
+				     ntohl(skb->h.th->seq),
+				     jiffies / (HZ * 60), mssind);
+}
+
+/* 
+ * This (misnamed) value is the age of syncookie which is permitted.
+ * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
+ * sysctl_tcp_retries1. It's a rather complicated formula (exponential
+ * backoff) to compute at runtime so it's currently hardcoded here.
+ */
+#define COUNTER_TRIES 4
+/*  
+ * Check if a ack sequence number is a valid syncookie. 
+ * Return the decoded mss if it is, or 0 if not.
+ */
+static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
+{
+	__u32 seq; 
+	__u32 mssind;
+
+	seq = ntohl(skb->h.th->seq)-1; 
+	mssind = check_tcp_syn_cookie(cookie,
+				      skb->nh.iph->saddr, skb->nh.iph->daddr,
+				      skb->h.th->source, skb->h.th->dest,
+				      seq, jiffies / (HZ * 60), COUNTER_TRIES);
+
+	return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
+}
+
+extern struct or_calltable or_ipv4;
+
+static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
+					   struct open_request *req,
+					   struct dst_entry *dst)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sock *child;
+
+	child = tp->af_specific->syn_recv_sock(sk, skb, req, dst);
+	if (child)
+		tcp_acceptq_queue(sk, req, child);
+	else
+		tcp_openreq_free(req);
+
+	return child;
+}
+
+struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
+			     struct ip_options *opt)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	__u32 cookie = ntohl(skb->h.th->ack_seq) - 1; 
+	struct sock *ret = sk;
+	struct open_request *req; 
+	int mss; 
+	struct rtable *rt; 
+	__u8 rcv_wscale;
+
+	if (!sysctl_tcp_syncookies || !skb->h.th->ack)
+		goto out;
+
+  	if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
+	    (mss = cookie_check(skb, cookie)) == 0) {
+	 	NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED);
+		goto out;
+	}
+
+	NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV);
+
+	req = tcp_openreq_alloc();
+	ret = NULL;
+	if (!req)
+		goto out;
+
+	req->rcv_isn		= htonl(skb->h.th->seq) - 1;
+	req->snt_isn		= cookie; 
+	req->mss		= mss;
+ 	req->rmt_port		= skb->h.th->source;
+	req->af.v4_req.loc_addr = skb->nh.iph->daddr;
+	req->af.v4_req.rmt_addr = skb->nh.iph->saddr;
+	req->class		= &or_ipv4; /* for savety */
+	req->af.v4_req.opt	= NULL;
+
+	/* We throwed the options of the initial SYN away, so we hope
+	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
+	 */
+	if (opt && opt->optlen) {
+		int opt_size = sizeof(struct ip_options) + opt->optlen;
+
+		req->af.v4_req.opt = kmalloc(opt_size, GFP_ATOMIC);
+		if (req->af.v4_req.opt) {
+			if (ip_options_echo(req->af.v4_req.opt, skb)) {
+				kfree(req->af.v4_req.opt);
+				req->af.v4_req.opt = NULL;
+			}
+		}
+	}
+
+	req->snd_wscale = req->rcv_wscale = req->tstamp_ok = 0;
+	req->wscale_ok	= req->sack_ok = 0; 
+	req->expires	= 0UL; 
+	req->retrans	= 0; 
+	
+	/*
+	 * We need to lookup the route here to get at the correct
+	 * window size. We should better make sure that the window size
+	 * hasn't changed since we received the original syn, but I see
+	 * no easy way to do this. 
+	 */
+	{
+		struct flowi fl = { .nl_u = { .ip4_u =
+					      { .daddr = ((opt && opt->srr) ?
+							  opt->faddr :
+							  req->af.v4_req.rmt_addr),
+						.saddr = req->af.v4_req.loc_addr,
+						.tos = RT_CONN_FLAGS(sk) } },
+				    .proto = IPPROTO_TCP,
+				    .uli_u = { .ports =
+					       { .sport = skb->h.th->dest,
+						 .dport = skb->h.th->source } } };
+		if (ip_route_output_key(&rt, &fl)) {
+			tcp_openreq_free(req);
+			goto out; 
+		}
+	}
+
+	/* Try to redo what tcp_v4_send_synack did. */
+	req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW);
+	tcp_select_initial_window(tcp_full_space(sk), req->mss,
+				  &req->rcv_wnd, &req->window_clamp, 
+				  0, &rcv_wscale);
+	/* BTW win scale with syncookies is 0 by definition */
+	req->rcv_wscale	  = rcv_wscale; 
+
+	ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
+out:	return ret;
+}