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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) 2002-2010 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
*/
#ifdef HAVE_CONFIG_H
#include "config.h"
#elif defined(_MSC_VER)
#include "config-msvc.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.
*/
/*
* IPv4 packet: find TCP header, check flags for "SYN"
* if yes, hand to mss_fixup_dowork()
*/
void
mss_fixup_ipv4 (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);
}
}
}
/*
* IPv6 packet: find TCP header, check flags for "SYN"
* if yes, hand to mss_fixup_dowork()
* (IPv6 header structure is sufficiently different from IPv4...)
*/
void
mss_fixup_ipv6 (struct buffer *buf, int maxmss)
{
const struct openvpn_ipv6hdr *pip6;
struct buffer newbuf;
if (BLEN (buf) < (int) sizeof (struct openvpn_ipv6hdr))
return;
verify_align_4 (buf);
pip6 = (struct openvpn_ipv6hdr *) BPTR (buf);
/* do we have the full IPv6 packet?
* "payload_len" does not include IPv6 header (+40 bytes)
*/
if (BLEN (buf) != (int) ntohs(pip6->payload_len)+40 )
return;
/* follow header chain until we reach final header, then check for TCP
*
* An IPv6 packet could, theoretically, have a chain of multiple headers
* before the final header (TCP, UDP, ...), so we'd need to walk that
* chain (see RFC 2460 and RFC 6564 for details).
*
* In practice, "most typically used" extention headers (AH, routing,
* fragment, mobility) are very unlikely to be seen inside an OpenVPN
* tun, so for now, we only handle the case of "single next header = TCP"
*/
if ( pip6->nexthdr != OPENVPN_IPPROTO_TCP )
return;
newbuf = *buf;
if ( buf_advance( &newbuf, 40 ) )
{
struct openvpn_tcphdr *tc = (struct openvpn_tcphdr *) BPTR (&newbuf);
if (tc->flags & OPENVPN_TCPH_SYN_MASK)
mss_fixup_dowork (&newbuf, (uint16_t) maxmss-20);
}
}
/*
* change TCP MSS option in SYN/SYN-ACK packets, if present
* this is generic for IPv4 and IPv6, as the TCP header is the same
*/
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);
}
}
}
}
}
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