usr.sbin/ppp/tcpmss.c

   1 /*-
   2  * Copyright (c) 2000 Ruslan Ermilov and Brian Somers <brian@Awfulhak.org>
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer.
  10  * 2. Redistributions in binary form must reproduce the above copyright
  11  *    notice, this list of conditions and the following disclaimer in the
  12  *    documentation and/or other materials provided with the distribution.
  13  *
  14  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  15  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  16  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  17  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  18  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  19  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  20  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  21  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  22  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  23  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  24  * SUCH DAMAGE.
  25  *
  26  * $FreeBSD: src/usr.sbin/ppp/tcpmss.c,v 1.1.4.4 2002/09/01 02:12:32 brian Exp $
  27  * $DragonFly: src/usr.sbin/ppp/tcpmss.c,v 1.2 2003/06/17 04:30:01 dillon Exp $
  28  */
  29
  30 #include <sys/param.h>
  31
  32 #include <sys/socket.h>
  33 #include <net/route.h>
  34 #include <netinet/in_systm.h>
  35 #include <netinet/in.h>
  36 #include <netinet/ip.h>
  37 #include <netinet/tcp.h>
  38 #include <sys/un.h>
  39
  40 #include <termios.h>
  41
  42 #include "layer.h"
  43 #include "defs.h"
  44 #include "log.h"
  45 #include "timer.h"
  46 #include "fsm.h"
  47 #include "mbuf.h"
  48 #include "throughput.h"
  49 #include "lqr.h"
  50 #include "hdlc.h"
  51 #include "lcp.h"
  52 #include "ccp.h"
  53 #include "link.h"
  54 #include "iplist.h"
  55 #include "slcompress.h"
  56 #include "ncpaddr.h"
  57 #include "ipcp.h"
  58 #include "filter.h"
  59 #include "descriptor.h"
  60 #include "mp.h"
  61 #include "iface.h"
  62 #ifndef NORADIUS
  63 #include "radius.h"
  64 #endif
  65 #include "ipv6cp.h"
  66 #include "ncp.h"
  67 #include "bundle.h"
  68
  69
  70 /*-
  71  * We are in a liberal position about MSS
  72  * (RFC 879, section 7).
  73  */
  74 #define MAXMSS(mtu) (mtu - sizeof(struct ip) - sizeof(struct tcphdr))
  75
  76
  77 /*-
  78  * The following macro is used to update an
  79  * internet checksum.  "acc" is a 32-bit
  80  * accumulation of all the changes to the
  81  * checksum (adding in old 16-bit words and
  82  * subtracting out new words), and "cksum"
  83  * is the checksum value to be updated.
  84  */
  85 #define ADJUST_CHECKSUM(acc, cksum) { \
  86   acc += cksum; \
  87   if (acc < 0) { \
  88     acc = -acc; \
  89     acc = (acc >> 16) + (acc & 0xffff); \
  90     acc += acc >> 16; \
  91     cksum = (u_short) ~acc; \
  92   } else { \
  93     acc = (acc >> 16) + (acc & 0xffff); \
  94     acc += acc >> 16; \
  95     cksum = (u_short) acc; \
  96   } \
  97 }
  98
  99 static void
 100 MSSFixup(struct tcphdr *tc, size_t pktlen, u_int16_t maxmss)
 101 {
 102   size_t hlen, olen, optlen;
 103   u_char *opt;
 104   u_int16_t *mss;
 105   int accumulate;
 106
 107   hlen = tc->th_off << 2;
 108
 109   /* Invalid header length or header without options. */
 110   if (hlen <= sizeof(struct tcphdr) || hlen > pktlen)
 111     return;
 112
 113   /* MSS option only allowed within SYN packets. */
 114   if (!(tc->th_flags & TH_SYN))
 115     return;
 116
 117   for (olen = hlen - sizeof(struct tcphdr), opt = (u_char *)(tc + 1);
 118        olen > 0; olen -= optlen, opt += optlen) {
 119     if (*opt == TCPOPT_EOL)
 120       break;
 121     else if (*opt == TCPOPT_NOP)
 122       optlen = 1;
 123     else {
 124       optlen = *(opt + 1);
 125       if (optlen <= 0 || optlen > olen)
 126         break;
 127       if (*opt == TCPOPT_MAXSEG) {
 128         if (optlen != TCPOLEN_MAXSEG)
 129           continue;
 130         mss = (u_int16_t *)(opt + 2);
 131         if (ntohs(*mss) > maxmss) {
 132           log_Printf(LogDEBUG, "MSS: %u -> %u\n",
 133                ntohs(*mss), maxmss);
 134           accumulate = *mss;
 135           *mss = htons(maxmss);
 136           accumulate -= *mss;
 137           ADJUST_CHECKSUM(accumulate, tc->th_sum);
 138         }
 139       }
 140     }
 141   }
 142 }
 143
 144 static struct mbuf *
 145 tcpmss_Check(struct bundle *bundle, struct mbuf *bp)
 146 {
 147   struct ip *pip;
 148   size_t hlen, plen;
 149
 150   if (!Enabled(bundle, OPT_TCPMSSFIXUP))
 151     return bp;
 152
 153   bp = m_pullup(bp);
 154   plen = m_length(bp);
 155   pip = (struct ip *)MBUF_CTOP(bp);
 156   hlen = pip->ip_hl << 2;
 157
 158   /*
 159    * Check for MSS option only for TCP packets with zero fragment offsets
 160    * and correct total and header lengths.
 161    */
 162   if (pip->ip_p == IPPROTO_TCP && (ntohs(pip->ip_off) & IP_OFFMASK) == 0 &&
 163       ntohs(pip->ip_len) == plen && hlen <= plen &&
 164       plen - hlen >= sizeof(struct tcphdr))
 165     MSSFixup((struct tcphdr *)(MBUF_CTOP(bp) + hlen), plen - hlen,
 166              MAXMSS(bundle->iface->mtu));
 167
 168   return bp;
 169 }
 170
 171 static struct mbuf *
 172 tcpmss_LayerPush(struct bundle *bundle, struct link *l __unused, struct mbuf *bp,
 173                  int pri __unused, u_short *proto __unused)
 174 {
 175         return tcpmss_Check(bundle, bp);
 176 }
 177
 178 static struct mbuf *
 179 tcpmss_LayerPull(struct bundle *bundle, struct link *l __unused, struct mbuf *bp,
 180                  u_short *proto __unused)
 181 {
 182         return tcpmss_Check(bundle, bp);
 183 }
 184
 185 struct layer tcpmsslayer =
 186   { LAYER_PROTO, "tcpmss", tcpmss_LayerPush, tcpmss_LayerPull };