xref: /freebsd/usr.sbin/ppp/tcpmss.c (revision 94d7be5294679c1ee2d2fb15b501744ca42c19ca)
1 /*-
2  * Copyright (c) 2000 Ruslan Ermilov and Brian Somers <brian@Awfulhak.org>
3  * All rights reserved.
4  *
5  * Redistribution and use in source and binary forms, with or without
6  * modification, are permitted provided that the following conditions
7  * are met:
8  * 1. Redistributions of source code must retain the above copyright
9  *    notice, this list of conditions and the following disclaimer.
10  * 2. Redistributions in binary form must reproduce the above copyright
11  *    notice, this list of conditions and the following disclaimer in the
12  *    documentation and/or other materials provided with the distribution.
13  *
14  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24  * SUCH DAMAGE.
25  *
26  * $FreeBSD$
27  */
28 
29 #include <sys/param.h>
30 
31 #include <netinet/in_systm.h>
32 #include <netinet/in.h>
33 #include <netinet/ip.h>
34 #include <netinet/tcp.h>
35 #include <sys/un.h>
36 
37 #include <termios.h>
38 
39 #include "layer.h"
40 #include "defs.h"
41 #include "log.h"
42 #include "timer.h"
43 #include "fsm.h"
44 #include "mbuf.h"
45 #include "throughput.h"
46 #include "lqr.h"
47 #include "hdlc.h"
48 #include "lcp.h"
49 #include "ccp.h"
50 #include "link.h"
51 #include "iplist.h"
52 #include "slcompress.h"
53 #include "ipcp.h"
54 #include "filter.h"
55 #include "descriptor.h"
56 #include "mp.h"
57 #ifndef NORADIUS
58 #include "radius.h"
59 #endif
60 #include "bundle.h"
61 
62 
63 /*-
64  * We are in a liberal position about MSS
65  * (RFC 879, section 7).
66  */
67 #define MAXMSS(mtu) (mtu - sizeof(struct ip) - sizeof(struct tcphdr))
68 
69 
70 static void MSSFixup(struct tcphdr *, ssize_t, u_int16_t);
71 
72 
73 static struct mbuf *
74 tcpmss_LayerPush(struct bundle *bundle, struct link *l, struct mbuf *bp,
75                  int pri, u_short *proto)
76 {
77   struct ip *pip;
78   int hlen, plen;
79 
80   if (!Enabled(bundle, OPT_TCPMSSFIXUP))
81     return bp;
82 
83   bp = m_pullup(bp);
84   plen = m_length(bp);
85   pip = (struct ip *)MBUF_CTOP(bp);
86   hlen = pip->ip_hl << 2;
87 
88   /*
89    * Check for MSS option only for TCP packets with zero fragment offsets
90    * and correct total and header lengths.
91    */
92   if (pip->ip_p == IPPROTO_TCP && (ntohs(pip->ip_off) & IP_OFFMASK) == 0 &&
93       ntohs(pip->ip_len) == plen && hlen <= plen &&
94       plen - hlen >= sizeof(struct tcphdr))
95     MSSFixup((struct tcphdr *)(MBUF_CTOP(bp) + hlen), plen - hlen,
96              MAXMSS(bundle->mtu));
97 
98   return bp;
99 }
100 
101 /*-
102  * The following macro is used to update an
103  * internet checksum.  "acc" is a 32-bit
104  * accumulation of all the changes to the
105  * checksum (adding in old 16-bit words and
106  * subtracting out new words), and "cksum"
107  * is the checksum value to be updated.
108  */
109 #define ADJUST_CHECKSUM(acc, cksum) { \
110   acc += cksum; \
111   if (acc < 0) { \
112     acc = -acc; \
113     acc = (acc >> 16) + (acc & 0xffff); \
114     acc += acc >> 16; \
115     cksum = (u_short) ~acc; \
116   } else { \
117     acc = (acc >> 16) + (acc & 0xffff); \
118     acc += acc >> 16; \
119     cksum = (u_short) acc; \
120   } \
121 }
122 
123 static void
124 MSSFixup(struct tcphdr *tc, ssize_t pktlen, u_int16_t maxmss)
125 {
126   int hlen, olen, optlen;
127   u_char *opt;
128   u_int16_t *mss;
129   int accumulate;
130 
131   hlen = tc->th_off << 2;
132 
133   /* Invalid header length or header without options. */
134   if (hlen <= sizeof(struct tcphdr) || hlen > pktlen)
135     return;
136 
137   /* MSS option only allowed within SYN packets. */
138   if (!(tc->th_flags & TH_SYN))
139     return;
140 
141   for (olen = hlen - sizeof(struct tcphdr), opt = (u_char *)(tc + 1);
142        olen > 0; olen -= optlen, opt += optlen) {
143     if (*opt == TCPOPT_EOL)
144       break;
145     else if (*opt == TCPOPT_NOP)
146       optlen = 1;
147     else {
148       optlen = *(opt + 1);
149       if (optlen <= 0 || optlen > olen)
150         break;
151       if (*opt == TCPOPT_MAXSEG) {
152         if (optlen != TCPOLEN_MAXSEG)
153           continue;
154         mss = (u_int16_t *)(opt + 2);
155         if (ntohs(*mss) > maxmss) {
156           log_Printf(LogDEBUG, "MSS: %u -> %u\n",
157                ntohs(*mss), maxmss);
158           accumulate = *mss;
159           *mss = htons(maxmss);
160           accumulate -= *mss;
161           ADJUST_CHECKSUM(accumulate, tc->th_sum);
162         }
163       }
164     }
165   }
166 }
167 
168 struct layer tcpmsslayer = { LAYER_PROTO, "tcpmss", tcpmss_LayerPush, NULL };
169