/*
  *  OpenVPN -- An application to securely tunnel IP networks
  *             over a single TCP/UDP port, with support for SSL/TLS-based
  *             session authentication and key exchange,
  *             packet encryption, packet authentication, and
  *             packet compression.
  *

  *  Copyright (C) 2002-2010 OpenVPN Technologies, Inc. <sales@openvpn.net>

  *
  *  This program is free software; you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License version 2
  *  as published by the Free Software Foundation.
  *
  *  This program is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  *  GNU General Public License for more details.
  *
  *  You should have received a copy of the GNU General Public License
  *  along with this program (see the file COPYING included with this
  *  distribution); if not, write to the Free Software Foundation, Inc.,
  *  59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
 #include "syshead.h"
 #include "error.h"
 #include "mss.h"
 #include "memdbg.h"
 
 /*
  * Lower MSS on TCP SYN packets to fix MTU
  * problems which arise from protocol
  * encapsulation.
  */
 void
 mss_fixup (struct buffer *buf, int maxmss)
 {
   const struct openvpn_iphdr *pip;
   int hlen;
 
   if (BLEN (buf) < (int) sizeof (struct openvpn_iphdr))
     return;
   
   verify_align_4 (buf);
   pip = (struct openvpn_iphdr *) BPTR (buf);
 
   hlen = OPENVPN_IPH_GET_LEN (pip->version_len);
 
   if (pip->protocol == OPENVPN_IPPROTO_TCP
       && ntohs (pip->tot_len) == BLEN (buf)
       && (ntohs (pip->frag_off) & OPENVPN_IP_OFFMASK) == 0
       && hlen <= BLEN (buf)
       && BLEN (buf) - hlen
          >= (int) sizeof (struct openvpn_tcphdr))
     {
       struct buffer newbuf = *buf;
       if (buf_advance (&newbuf, hlen))
 	{
 	  struct openvpn_tcphdr *tc = (struct openvpn_tcphdr *) BPTR (&newbuf);
 	  if (tc->flags & OPENVPN_TCPH_SYN_MASK)
 	    mss_fixup_dowork (&newbuf, (uint16_t) maxmss);
 	}
     }
 }
 
 void
 mss_fixup_dowork (struct buffer *buf, uint16_t maxmss)
 {
   int hlen, olen, optlen;
   uint8_t *opt;
   uint16_t *mss;
   int accumulate;
   struct openvpn_tcphdr *tc;
 
   ASSERT (BLEN (buf) >= (int) sizeof (struct openvpn_tcphdr));
 
   verify_align_4 (buf);
   tc = (struct openvpn_tcphdr *) BPTR (buf);
   hlen = OPENVPN_TCPH_GET_DOFF (tc->doff_res);
 
   /* Invalid header length or header without options. */
   if (hlen <= (int) sizeof (struct openvpn_tcphdr)
       || hlen > BLEN (buf))
     return;
 
   for (olen = hlen - sizeof (struct openvpn_tcphdr),
 	 opt = (uint8_t *)(tc + 1);
        olen > 0;
        olen -= optlen, opt += optlen) {
     if (*opt == OPENVPN_TCPOPT_EOL)
       break;
     else if (*opt == OPENVPN_TCPOPT_NOP)
       optlen = 1;
     else {
       optlen = *(opt + 1);
       if (optlen <= 0 || optlen > olen)
         break;
       if (*opt == OPENVPN_TCPOPT_MAXSEG) {
         if (optlen != OPENVPN_TCPOLEN_MAXSEG)
           continue;
         mss = (uint16_t *)(opt + 2);
         if (ntohs (*mss) > maxmss) {
           dmsg (D_MSS, "MSS: %d -> %d",
                (int) ntohs (*mss),
 	       (int) maxmss);
           accumulate = *mss;
           *mss = htons (maxmss);
           accumulate -= *mss;
           ADJUST_CHECKSUM (accumulate, tc->check);
         }
       }
     }
   }
 }