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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) 2002-2021 OpenVPN 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; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 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 "crypto.h"
#include "ssl_common.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" extension 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;
}
/* skip IPv6 header (40 bytes),
* verify remainder is large enough to contain a full TCP header
*/
newbuf = *buf;
if (buf_advance( &newbuf, 40 )
&& BLEN(&newbuf) >= (int) sizeof(struct openvpn_tcphdr))
{
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 mssval;
int accumulate;
struct openvpn_tcphdr *tc;
if (BLEN(buf) < (int) sizeof(struct openvpn_tcphdr))
{
return;
}
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 > 1;
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;
}
mssval = (opt[2]<<8)+opt[3];
if (mssval > maxmss)
{
dmsg(D_MSS, "MSS: %d -> %d", (int) mssval, (int) maxmss);
accumulate = htons(mssval);
opt[2] = (maxmss>>8)&0xff;
opt[3] = maxmss&0xff;
accumulate -= htons(maxmss);
ADJUST_CHECKSUM(accumulate, tc->check);
}
}
}
}
}
void
frame_calculate_mssfix(struct frame *frame, struct key_type *kt,
const struct options *options)
{
if (options->ce.mssfix == 0)
{
return;
}
unsigned int payload_size;
unsigned int overhead;
payload_size = frame_calculate_payload_size(frame, options);
overhead = frame_calculate_protocol_header_size(kt, options,
payload_size, false);
/* Calculate the number of bytes that the payload differs from the payload
* MTU. This are fragment/compression/ethernet headers */
unsigned payload_overhead = frame_calculate_payload_overhead(frame, options, true);
/* We are in a "liberal" position with respect to MSS,
* i.e. we assume that MSS can be calculated from MTU
* by subtracting out only the IP and TCP header sizes
* without options.
*
* (RFC 879, section 7). */
/* Add 20 bytes for the IPv4 header and 20 byte for the TCP header of the
* payload, the mssfix method will add 20 extra if payload is IPv6 */
overhead += 20 + 20;
/* Calculate the maximum MSS value from the max link layer size specified
* by ce.mssfix */
frame->mss_fix = options->ce.mssfix - overhead - payload_overhead;
}
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