1 /*- 2 * Copyright (c) 2000 Ruslan Ermilov and Brian Somers <brian (at) 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: src/usr.sbin/ppp/tcpmss.c,v 1.8.26.1 2010/12/21 17:10:29 kensmith Exp $ 27 */ 28 29 #include <sys/param.h> 30 31 #include <sys/socket.h> 32 #include <net/route.h> 33 #include <netinet/in_systm.h> 34 #include <netinet/in.h> 35 #include <netinet/ip.h> 36 #include <netinet/tcp.h> 37 #include <sys/un.h> 38 39 #include <termios.h> 40 41 #include "layer.h" 42 #include "defs.h" 43 #include "log.h" 44 #include "timer.h" 45 #include "fsm.h" 46 #include "mbuf.h" 47 #include "throughput.h" 48 #include "lqr.h" 49 #include "hdlc.h" 50 #include "lcp.h" 51 #include "ccp.h" 52 #include "link.h" 53 #include "iplist.h" 54 #include "slcompress.h" 55 #include "ncpaddr.h" 56 #include "ipcp.h" 57 #include "filter.h" 58 #include "descriptor.h" 59 #include "mp.h" 60 #include "iface.h" 61 #ifndef NORADIUS 62 #include "radius.h" 63 #endif 64 #include "ipv6cp.h" 65 #include "ncp.h" 66 #include "bundle.h" 67 68 69 /*- 70 * We are in a liberal position about MSS 71 * (RFC 879, section 7). 72 */ 73 #define MAXMSS(mtu) ((mtu) - sizeof(struct ip) - sizeof(struct tcphdr) - 12) 74 75 76 /*- 77 * The following macro is used to update an 78 * internet checksum. "acc" is a 32-bit 79 * accumulation of all the changes to the 80 * checksum (adding in old 16-bit words and 81 * subtracting out new words), and "cksum" 82 * is the checksum value to be updated. 83 */ 84 #define ADJUST_CHECKSUM(acc, cksum) { \ 85 acc += cksum; \ 86 if (acc < 0) { \ 87 acc = -acc; \ 88 acc = (acc >> 16) + (acc & 0xffff); \ 89 acc += acc >> 16; \ 90 cksum = (u_short) ~acc; \ 91 } else { \ 92 acc = (acc >> 16) + (acc & 0xffff); \ 93 acc += acc >> 16; \ 94 cksum = (u_short) acc; \ 95 } \ 96 } 97 98 static void 99 MSSFixup(struct tcphdr *tc, size_t pktlen, u_int16_t maxmss) 100 { 101 size_t hlen, olen, optlen; 102 u_char *opt; 103 u_int16_t *mss; 104 int accumulate; 105 106 hlen = tc->th_off << 2; 107 108 /* Invalid header length or header without options. */ 109 if (hlen <= sizeof(struct tcphdr) || hlen > pktlen) 110 return; 111 112 /* MSS option only allowed within SYN packets. */ 113 if (!(tc->th_flags & TH_SYN)) 114 return; 115 116 for (olen = hlen - sizeof(struct tcphdr), opt = (u_char *)(tc + 1); 117 olen > 0; olen -= optlen, opt += optlen) { 118 if (*opt == TCPOPT_EOL) 119 break; 120 else if (*opt == TCPOPT_NOP) 121 optlen = 1; 122 else { 123 optlen = *(opt + 1); 124 if (optlen <= 0 || optlen > olen) 125 break; 126 if (*opt == TCPOPT_MAXSEG) { 127 if (optlen != TCPOLEN_MAXSEG) 128 continue; 129 mss = (u_int16_t *)(opt + 2); 130 if (ntohs(*mss) > maxmss) { 131 log_Printf(LogDEBUG, "MSS: %u -> %u\n", 132 ntohs(*mss), maxmss); 133 accumulate = *mss; 134 *mss = htons(maxmss); 135 accumulate -= *mss; 136 ADJUST_CHECKSUM(accumulate, tc->th_sum); 137 } 138 } 139 } 140 } 141 } 142 143 static struct mbuf * 144 tcpmss_Check(struct bundle *bundle, struct mbuf *bp) 145 { 146 struct ip *pip; 147 size_t hlen, plen; 148 149 if (!Enabled(bundle, OPT_TCPMSSFIXUP)) 150 return bp; 151 152 bp = m_pullup(bp); 153 plen = m_length(bp); 154 pip = (struct ip *)MBUF_CTOP(bp); 155 hlen = pip->ip_hl << 2; 156 157 /* 158 * Check for MSS option only for TCP packets with zero fragment offsets 159 * and correct total and header lengths. 160 */ 161 if (pip->ip_p == IPPROTO_TCP && (ntohs(pip->ip_off) & IP_OFFMASK) == 0 && 162 ntohs(pip->ip_len) == plen && hlen <= plen && 163 plen >= sizeof(struct tcphdr) + hlen) 164 MSSFixup((struct tcphdr *)(MBUF_CTOP(bp) + hlen), plen - hlen, 165 MAXMSS(bundle->iface->mtu)); 166 167 return bp; 168 } 169 170 static struct mbuf * 171 tcpmss_LayerPush(struct bundle *bundle, struct link *l __unused, 172 struct mbuf *bp, int pri __unused, u_short *proto __unused) 173 { 174 return tcpmss_Check(bundle, bp); 175 } 176 177 static struct mbuf * 178 tcpmss_LayerPull(struct bundle *bundle, struct link *l __unused, 179 struct mbuf *bp, u_short *proto __unused) 180 { 181 return tcpmss_Check(bundle, bp); 182 } 183 184 struct layer tcpmsslayer = 185 { LAYER_PROTO, "tcpmss", tcpmss_LayerPush, tcpmss_LayerPull }; 186
