1 files changed, 120 insertions, 0 deletions
diff --git a/mss.c b/mss.c
new file mode 100644
index 0000000..25e77b0
--- /dev/null
+++ b/mss.c
@@ -0,0 +1,120 @@
+/*
+ *  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) 2002-2005 OpenVPN Solutions LLC <info@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
+ */
+
+#ifdef WIN32
+#include "config-win32.h"
+#else
+#include "config.h"
+#endif
+
+#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);
+        }
+      }
+    }
+  }
+}