1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3 *
4 * Copyright (c) 2000 Ruslan Ermilov and Brian Somers <brian@Awfulhak.org>
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 * notice, this list of conditions and the following disclaimer in the
14 * documentation and/or other materials provided with the distribution.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 *
28 * $FreeBSD: stable/12/usr.sbin/ppp/tcpmss.c 326276 2017-11-27 15:37:16Z pfg $
29 */
30
31 #include <sys/param.h>
32
33 #include <sys/socket.h>
34 #include <net/route.h>
35 #include <netinet/in_systm.h>
36 #include <netinet/in.h>
37 #include <netinet/ip.h>
38 #include <netinet/tcp.h>
39 #include <sys/un.h>
40
41 #include <termios.h>
42
43 #include "layer.h"
44 #include "defs.h"
45 #include "log.h"
46 #include "timer.h"
47 #include "fsm.h"
48 #include "mbuf.h"
49 #include "throughput.h"
50 #include "lqr.h"
51 #include "hdlc.h"
52 #include "lcp.h"
53 #include "ccp.h"
54 #include "link.h"
55 #include "iplist.h"
56 #include "slcompress.h"
57 #include "ncpaddr.h"
58 #include "ipcp.h"
59 #include "filter.h"
60 #include "descriptor.h"
61 #include "mp.h"
62 #include "iface.h"
63 #ifndef NORADIUS
64 #include "radius.h"
65 #endif
66 #include "ipv6cp.h"
67 #include "ncp.h"
68 #include "bundle.h"
69
70
71 /*-
72 * We are in a liberal position about MSS
73 * (RFC 879, section 7).
74 */
75 #define MAXMSS(mtu) ((mtu) - sizeof(struct ip) - sizeof(struct tcphdr) - 12)
76
77
78 /*-
79 * The following macro is used to update an
80 * internet checksum. "acc" is a 32-bit
81 * accumulation of all the changes to the
82 * checksum (adding in old 16-bit words and
83 * subtracting out new words), and "cksum"
84 * is the checksum value to be updated.
85 */
86 #define ADJUST_CHECKSUM(acc, cksum) { \
87 acc += cksum; \
88 if (acc < 0) { \
89 acc = -acc; \
90 acc = (acc >> 16) + (acc & 0xffff); \
91 acc += acc >> 16; \
92 cksum = (u_short) ~acc; \
93 } else { \
94 acc = (acc >> 16) + (acc & 0xffff); \
95 acc += acc >> 16; \
96 cksum = (u_short) acc; \
97 } \
98 }
99
100 static void
MSSFixup(struct tcphdr * tc,size_t pktlen,u_int16_t maxmss)101 MSSFixup(struct tcphdr *tc, size_t pktlen, u_int16_t maxmss)
102 {
103 size_t hlen, olen, optlen;
104 u_char *opt;
105 u_int16_t *mss;
106 int accumulate;
107
108 hlen = tc->th_off << 2;
109
110 /* Invalid header length or header without options. */
111 if (hlen <= sizeof(struct tcphdr) || hlen > pktlen)
112 return;
113
114 /* MSS option only allowed within SYN packets. */
115 if (!(tc->th_flags & TH_SYN))
116 return;
117
118 for (olen = hlen - sizeof(struct tcphdr), opt = (u_char *)(tc + 1);
119 olen > 0; olen -= optlen, opt += optlen) {
120 if (*opt == TCPOPT_EOL)
121 break;
122 else if (*opt == TCPOPT_NOP)
123 optlen = 1;
124 else {
125 optlen = *(opt + 1);
126 if (optlen <= 0 || optlen > olen)
127 break;
128 if (*opt == TCPOPT_MAXSEG) {
129 if (optlen != TCPOLEN_MAXSEG)
130 continue;
131 mss = (u_int16_t *)(opt + 2);
132 if (ntohs(*mss) > maxmss) {
133 log_Printf(LogDEBUG, "MSS: %u -> %u\n",
134 ntohs(*mss), maxmss);
135 accumulate = *mss;
136 *mss = htons(maxmss);
137 accumulate -= *mss;
138 ADJUST_CHECKSUM(accumulate, tc->th_sum);
139 }
140 }
141 }
142 }
143 }
144
145 static struct mbuf *
tcpmss_Check(struct bundle * bundle,struct mbuf * bp)146 tcpmss_Check(struct bundle *bundle, struct mbuf *bp)
147 {
148 struct ip *pip;
149 size_t hlen, plen;
150
151 if (!Enabled(bundle, OPT_TCPMSSFIXUP))
152 return bp;
153
154 bp = m_pullup(bp);
155 plen = m_length(bp);
156 pip = (struct ip *)MBUF_CTOP(bp);
157 hlen = pip->ip_hl << 2;
158
159 /*
160 * Check for MSS option only for TCP packets with zero fragment offsets
161 * and correct total and header lengths.
162 */
163 if (pip->ip_p == IPPROTO_TCP && (ntohs(pip->ip_off) & IP_OFFMASK) == 0 &&
164 ntohs(pip->ip_len) == plen && hlen <= plen &&
165 plen >= sizeof(struct tcphdr) + hlen)
166 MSSFixup((struct tcphdr *)(MBUF_CTOP(bp) + hlen), plen - hlen,
167 MAXMSS(bundle->iface->mtu));
168
169 return bp;
170 }
171
172 static struct mbuf *
tcpmss_LayerPush(struct bundle * bundle,struct link * l __unused,struct mbuf * bp,int pri __unused,u_short * proto __unused)173 tcpmss_LayerPush(struct bundle *bundle, struct link *l __unused,
174 struct mbuf *bp, int pri __unused, u_short *proto __unused)
175 {
176 return tcpmss_Check(bundle, bp);
177 }
178
179 static struct mbuf *
tcpmss_LayerPull(struct bundle * bundle,struct link * l __unused,struct mbuf * bp,u_short * proto __unused)180 tcpmss_LayerPull(struct bundle *bundle, struct link *l __unused,
181 struct mbuf *bp, u_short *proto __unused)
182 {
183 return tcpmss_Check(bundle, bp);
184 }
185
186 struct layer tcpmsslayer =
187 { LAYER_PROTO, "tcpmss", tcpmss_LayerPush, tcpmss_LayerPull };
188