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  * $FreeBSD$
  27  */
  28
  29 #include <sys/param.h>
  30
  31 #include <sys/socket.h>
  32 #include <net/route.h>
  33 #include <netinet/in_systm.h>
  34 #include <netinet/in.h>
  35 #include <netinet/ip.h>
  36 #include <netinet/tcp.h>
  37 #include <sys/un.h>
  38
  39 #include <termios.h>
  40
  41 #include "layer.h"
  42 #include "defs.h"
  43 #include "log.h"
  44 #include "timer.h"
  45 #include "fsm.h"
  46 #include "mbuf.h"
  47 #include "throughput.h"
  48 #include "lqr.h"
  49 #include "hdlc.h"
  50 #include "lcp.h"
  51 #include "ccp.h"
  52 #include "link.h"
  53 #include "iplist.h"
  54 #include "slcompress.h"
  55 #include "ncpaddr.h"
  56 #include "ipcp.h"
  57 #include "filter.h"
  58 #include "descriptor.h"
  59 #include "mp.h"
  60 #include "iface.h"
  61 #ifndef NORADIUS
  62 #include "radius.h"
  63 #endif
  64 #include "ipv6cp.h"
  65 #include "ncp.h"
  66 #include "bundle.h"
  67
  68
  69 /*-
  70  * We are in a liberal position about MSS
  71  * (RFC 879, section 7).
  72  */
  73 #define MAXMSS(mtu) ((mtu) - sizeof(struct ip) - sizeof(struct tcphdr) - 12)
  74
  75
  76 /*-
  77  * The following macro is used to update an
  78  * internet checksum.  "acc" is a 32-bit
  79  * accumulation of all the changes to the
  80  * checksum (adding in old 16-bit words and
  81  * subtracting out new words), and "cksum"
  82  * is the checksum value to be updated.
  83  */
  84 #define ADJUST_CHECKSUM(acc, cksum) { \
  85   acc += cksum; \
  86   if (acc < 0) { \
  87     acc = -acc; \
  88     acc = (acc >> 16) + (acc & 0xffff); \
  89     acc += acc >> 16; \
  90     cksum = (u_short) ~acc; \
  91   } else { \
  92     acc = (acc >> 16) + (acc & 0xffff); \
  93     acc += acc >> 16; \
  94     cksum = (u_short) acc; \
  95   } \
  96 }
  97
  98 static void
  99 MSSFixup(struct tcphdr *tc, size_t pktlen, u_int16_t maxmss)
 100 {
 101   size_t hlen, olen, optlen;
 102   u_char *opt;
 103   u_int16_t *mss;
 104   int accumulate;
 105
 106   hlen = tc->th_off << 2;
 107
 108   /* Invalid header length or header without options. */
 109   if (hlen <= sizeof(struct tcphdr) || hlen > pktlen)
 110     return;
 111
 112   /* MSS option only allowed within SYN packets. */
 113   if (!(tc->th_flags & TH_SYN))
 114     return;
 115
 116   for (olen = hlen - sizeof(struct tcphdr), opt = (u_char *)(tc + 1);
 117        olen > 0; olen -= optlen, opt += optlen) {
 118     if (*opt == TCPOPT_EOL)
 119       break;
 120     else if (*opt == TCPOPT_NOP)
 121       optlen = 1;
 122     else {
 123       optlen = *(opt + 1);
 124       if (optlen <= 0 || optlen > olen)
 125         break;
 126       if (*opt == TCPOPT_MAXSEG) {
 127         if (optlen != TCPOLEN_MAXSEG)
 128           continue;
 129         mss = (u_int16_t *)(opt + 2);
 130         if (ntohs(*mss) > maxmss) {
 131           log_Printf(LogDEBUG, "MSS: %u -> %u\n",
 132                ntohs(*mss), maxmss);
 133           accumulate = *mss;
 134           *mss = htons(maxmss);
 135           accumulate -= *mss;
 136           ADJUST_CHECKSUM(accumulate, tc->th_sum);
 137         }
 138       }
 139     }
 140   }
 141 }
 142
 143 static struct mbuf *
 144 tcpmss_Check(struct bundle *bundle, struct mbuf *bp)
 145 {
 146   struct ip *pip;
 147   size_t hlen, plen;
 148
 149   if (!Enabled(bundle, OPT_TCPMSSFIXUP))
 150     return bp;
 151
 152   bp = m_pullup(bp);
 153   plen = m_length(bp);
 154   pip = (struct ip *)MBUF_CTOP(bp);
 155   hlen = pip->ip_hl << 2;
 156
 157   /*
 158    * Check for MSS option only for TCP packets with zero fragment offsets
 159    * and correct total and header lengths.
 160    */
 161   if (pip->ip_p == IPPROTO_TCP && (ntohs(pip->ip_off) & IP_OFFMASK) == 0 &&
 162       ntohs(pip->ip_len) == plen && hlen <= plen &&
 163       plen >= sizeof(struct tcphdr) + hlen)
 164     MSSFixup((struct tcphdr *)(MBUF_CTOP(bp) + hlen), plen - hlen,
 165              MAXMSS(bundle->iface->mtu));
 166
 167   return bp;
 168 }
 169
 170 static struct mbuf *
 171 tcpmss_LayerPush(struct bundle *bundle, struct link *l __unused,
 172                  struct mbuf *bp, int pri __unused, u_short *proto __unused)
 173 {
 174         return tcpmss_Check(bundle, bp);
 175 }
 176
 177 static struct mbuf *
 178 tcpmss_LayerPull(struct bundle *bundle, struct link *l __unused,
 179                  struct mbuf *bp, u_short *proto __unused)
 180 {
 181         return tcpmss_Check(bundle, bp);
 182 }
 183
 184 struct layer tcpmsslayer =
 185   { LAYER_PROTO, "tcpmss", tcpmss_LayerPush, tcpmss_LayerPull };