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/*

 *  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 1.1.1.1 2004-04-15 01:13:54 phoenix Exp $

 *

 *  Missing: IPv6 support.

 */

#include <linux/tcp.h>

#include <linux/slab.h>

#include <linux/random.h>

#include <net/tcp.h>

extern int sysctl_tcp_syncookies;

/*

 * 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;

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

        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)

{

        __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, sk->tp_pinfo.af_tcp.last_synq_overflow + TCP_TIMEOUT_INIT) ||

            (mss = cookie_check(skb, cookie)) == 0) {

                NET_INC_STATS_BH(SyncookiesFailed);

                goto out;

        }

        NET_INC_STATS_BH(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.

         */

        if (ip_route_output(&rt,

                            opt &&

                            opt->srr ? opt->faddr : req->af.v4_req.rmt_addr,

                            req->af.v4_req.loc_addr,

                            RT_CONN_FLAGS(sk),

                            0)) {

                tcp_openreq_free(req);

                goto out;

        }

        /* Try to redo what tcp_v4_send_synack did. */

        req->window_clamp = rt->u.dst.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;

}