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-rw-r--r--src/openvpn/mss.c120
1 files changed, 120 insertions, 0 deletions
diff --git a/src/openvpn/mss.c b/src/openvpn/mss.c
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+++ b/src/openvpn/mss.c
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+/*
+ * 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);
+ }
+ }
+ }
+ }
+}