/*
 *  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 1.7 1999/03/17 02:34:57 davem Exp $
 *
 *  Missing: IPv6 support. 
 */

#include <linux/config.h>
#if defined(CONFIG_SYN_COOKIES) 
#include <linux/tcp.h>
#include <linux/malloc.h>
#include <linux/random.h>
#include <net/tcp.h>

extern int sysctl_tcp_syncookies;

static unsigned long tcp_lastsynq_overflow;

/* 
 * 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 (sizeof(msstab)/sizeof(msstab[0]) - 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)
{
        int mssind;
        const __u16 mss = *mssp;

        tcp_lastsynq_overflow = jiffies;
        /* XXX sort msstab[] by probability?  Binary search? */
        for (mssind = 0; mss > msstab[mssind+1]; mssind++)
                ;
        *mssp = msstab[mssind]+1;

        net_statistics.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;

        if ((jiffies - tcp_lastsynq_overflow) > TCP_TIMEOUT_INIT)
                return 0; 

        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_opt *tp = &sk->tp_pinfo.af_tcp;

        sk = tp->af_specific->syn_recv_sock(sk, skb, req, dst);
        req->sk = sk; 
        
        /* Queue up for accept() */
        tcp_synq_queue(tp, req);
        
        return sk; 
}

struct sock *
cookie_v4_check(struct sock *sk, struct sk_buff *skb, struct ip_options *opt)
{
        __u32 cookie = ntohl(skb->h.th->ack_seq)-1; 
        struct open_request *req; 
        int mss; 
        struct rtable *rt; 
        __u8 rcv_wscale;

        if (!sysctl_tcp_syncookies)
                return sk;
        if (!skb->h.th->ack)
                return sk; 

        mss = cookie_check(skb, cookie);
        if (mss == 0) {
                net_statistics.SyncookiesFailed++;
                return sk;
        }

        net_statistics.SyncookiesRecv++;

        req = tcp_openreq_alloc();
        if (req == NULL)
                return NULL;    

        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_s(req->af.v4_req.opt, opt_size);
                                req->af.v4_req.opt = NULL;
                        }
                }
        }
        
        req->snd_wscale = req->rcv_wscale = req->tstamp_ok = 0;
        req->wscale_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. 
         */
        if (ip_route_output(&rt,
                            opt && 
                            opt->srr ? opt->faddr : req->af.v4_req.rmt_addr,
                            req->af.v4_req.loc_addr,
                            sk->ip_tos | RTO_CONN,
                            0)) { 
            tcp_openreq_free(req);
            return NULL; 
        }

        /* Try to redo what tcp_v4_send_synack did. */
        req->window_clamp = rt->u.dst.window;  
        tcp_select_initial_window(sock_rspace(sk)/2,req->mss,
                                  &req->rcv_wnd, &req->window_clamp, 
                                  0, &rcv_wscale);
        req->rcv_wscale = rcv_wscale; 

        return get_cookie_sock(sk, skb, req, &rt->u.dst);
}

#endif