net/ipv4/syncookies.c

   1 /*
   2  *  Syncookies implementation for the Linux kernel
   3  *
   4  *  Copyright (C) 1997 Andi Kleen
   5  *  Based on ideas by D.J.Bernstein and Eric Schenk.
   6  *
   7  *      This program is free software; you can redistribute it and/or
   8  *      modify it under the terms of the GNU General Public License
   9  *      as published by the Free Software Foundation; either version
  10  *      2 of the License, or (at your option) any later version.
  11  *
  12  *  $Id: syncookies.c,v 1.11 2000/01/16 05:11:27 davem Exp $
  13  *
  14  *  Missing: IPv6 support.
  15  */
  16
  17 #include <linux/config.h>
  18 #if defined(CONFIG_SYN_COOKIES)
  19 #include <linux/tcp.h>
  20 #include <linux/malloc.h>
  21 #include <linux/random.h>
  22 #include <net/tcp.h>
  23
  24 extern int sysctl_tcp_syncookies;
  25
  26 static unsigned long tcp_lastsynq_overflow;
  27
  28 /*
  29  * This table has to be sorted and terminated with (__u16)-1.
  30  * XXX generate a better table.
  31  * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
  32  */
  33 static __u16 const msstab[] = {
  34         64-1,
  35         256-1,
  36         512-1,
  37         536-1,
  38         1024-1,
  39         1440-1,
  40         1460-1,
  41         4312-1,
  42         (__u16)-1
  43 };
  44 /* The number doesn't include the -1 terminator */
  45 #define NUM_MSS (sizeof(msstab)/sizeof(msstab[0]) - 1)
  46
  47 /*
  48  * Generate a syncookie.  mssp points to the mss, which is returned
  49  * rounded down to the value encoded in the cookie.
  50  */
  51 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb,
  52                               __u16 *mssp)
  53 {
  54         int mssind;
  55         const __u16 mss = *mssp;
  56
  57         tcp_lastsynq_overflow = jiffies;
  58         /* XXX sort msstab[] by probability?  Binary search? */
  59         for (mssind = 0; mss > msstab[mssind+1]; mssind++)
  60                 ;
  61         *mssp = msstab[mssind]+1;
  62
  63         NET_INC_STATS_BH(SyncookiesSent);
  64
  65         return secure_tcp_syn_cookie(skb->nh.iph->saddr, skb->nh.iph->daddr,
  66                                      skb->h.th->source, skb->h.th->dest,
  67                                      ntohl(skb->h.th->seq),
  68                                      jiffies / (HZ*60), mssind);
  69 }
  70
  71 /*
  72  * This (misnamed) value is the age of syncookie which is permitted.
  73  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
  74  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
  75  * backoff) to compute at runtime so it's currently hardcoded here.
  76  */
  77 #define COUNTER_TRIES 4
  78 /*
  79  * Check if a ack sequence number is a valid syncookie.
  80  * Return the decoded mss if it is, or 0 if not.
  81  */
  82 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
  83 {
  84         __u32 seq;
  85         __u32 mssind;
  86
  87         if ((jiffies - tcp_lastsynq_overflow) > TCP_TIMEOUT_INIT)
  88                 return 0;
  89
  90         seq = ntohl(skb->h.th->seq)-1;
  91         mssind = check_tcp_syn_cookie(cookie,
  92                                       skb->nh.iph->saddr, skb->nh.iph->daddr,
  93                                       skb->h.th->source, skb->h.th->dest,
  94                                       seq, jiffies/(HZ*60), COUNTER_TRIES);
  95
  96         return mssind < NUM_MSS ? msstab[mssind]+1 : 0;
  97 }
  98
  99 extern struct or_calltable or_ipv4;
 100
 101 static inline struct sock *
 102 get_cookie_sock(struct sock *sk, struct sk_buff *skb, struct open_request *req,
 103                 struct dst_entry *dst)
 104 {
 105         struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
 106         struct sock *child;
 107
 108         child = tp->af_specific->syn_recv_sock(sk, skb, req, dst);
 109         if (child)
 110                 tcp_acceptq_queue(sk, req, child);
 111         else
 112                 tcp_openreq_free(req);
 113
 114         return child;
 115 }
 116
 117 struct sock *
 118 cookie_v4_check(struct sock *sk, struct sk_buff *skb, struct ip_options *opt)
 119 {
 120         __u32 cookie = ntohl(skb->h.th->ack_seq)-1;
 121         struct open_request *req;
 122         int mss;
 123         struct rtable *rt;
 124         __u8 rcv_wscale;
 125
 126         if (!sysctl_tcp_syncookies)
 127                 return sk;
 128         if (!skb->h.th->ack)
 129                 return sk;
 130
 131         mss = cookie_check(skb, cookie);
 132         if (mss == 0) {
 133                 NET_INC_STATS_BH(SyncookiesFailed);
 134                 return sk;
 135         }
 136
 137         NET_INC_STATS_BH(SyncookiesRecv);
 138
 139         req = tcp_openreq_alloc();
 140         if (req == NULL)
 141                 return NULL;
 142
 143         req->rcv_isn = htonl(skb->h.th->seq)-1;
 144         req->snt_isn = cookie;
 145         req->mss = mss;
 146         req->rmt_port = skb->h.th->source;
 147         req->af.v4_req.loc_addr = skb->nh.iph->daddr;
 148         req->af.v4_req.rmt_addr = skb->nh.iph->saddr;
 149         req->class = &or_ipv4; /* for savety */
 150
 151         req->af.v4_req.opt = NULL;
 152
 153         /* We throwed the options of the initial SYN away, so we hope
 154          * the ACK carries the same options again (see RFC1122 4.2.3.8)
 155          */
 156         if (opt && opt->optlen) {
 157                 int opt_size = sizeof(struct ip_options) + opt->optlen;
 158
 159                 req->af.v4_req.opt = kmalloc(opt_size, GFP_ATOMIC);
 160                 if (req->af.v4_req.opt) {
 161                         if (ip_options_echo(req->af.v4_req.opt, skb)) {
 162                                 kfree(req->af.v4_req.opt);
 163                                 req->af.v4_req.opt = NULL;
 164                         }
 165                 }
 166         }
 167
 168         req->snd_wscale = req->rcv_wscale = req->tstamp_ok = 0;
 169         req->wscale_ok = req->sack_ok = 0;
 170         req->expires = 0UL;
 171         req->retrans = 0;
 172
 173         /*
 174          * We need to lookup the route here to get at the correct
 175          * window size. We should better make sure that the window size
 176          * hasn't changed since we received the original syn, but I see
 177          * no easy way to do this.
 178          */
 179         if (ip_route_output(&rt,
 180                             opt &&
 181                             opt->srr ? opt->faddr : req->af.v4_req.rmt_addr,
 182                             req->af.v4_req.loc_addr,
 183                             sk->protinfo.af_inet.tos | RTO_CONN,
 184                             0)) {
 185                 tcp_openreq_free(req);
 186                 return NULL;
 187         }
 188
 189         /* Try to redo what tcp_v4_send_synack did. */
 190         req->window_clamp = rt->u.dst.window;
 191         tcp_select_initial_window(tcp_full_space(sk),req->mss,
 192                                   &req->rcv_wnd, &req->window_clamp,
 193                                   0, &rcv_wscale);
 194         /* BTW win scale with syncookies is 0 by definition */
 195         req->rcv_wscale = rcv_wscale;
 196
 197         return get_cookie_sock(sk, skb, req, &rt->u.dst);
 198 }
 199
 200 #endif