xref: /freebsd/sbin/dhclient/packet.c (revision 181ab08d9eb9b1da3e830b41bb24040d5dd7323f)
1 /*	$OpenBSD: packet.c,v 1.9 2004/05/04 18:58:50 deraadt Exp $	*/
2 
3 /* Packet assembly code, originally contributed by Archie Cobbs. */
4 
5 /*
6  * Copyright (c) 1995, 1996, 1999 The Internet Software Consortium.
7  * All rights reserved.
8  *
9  * Redistribution and use in source and binary forms, with or without
10  * modification, are permitted provided that the following conditions
11  * are met:
12  *
13  * 1. Redistributions of source code must retain the above copyright
14  *    notice, this list of conditions and the following disclaimer.
15  * 2. Redistributions in binary form must reproduce the above copyright
16  *    notice, this list of conditions and the following disclaimer in the
17  *    documentation and/or other materials provided with the distribution.
18  * 3. Neither the name of The Internet Software Consortium nor the names
19  *    of its contributors may be used to endorse or promote products derived
20  *    from this software without specific prior written permission.
21  *
22  * THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND
23  * CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
24  * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
25  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
26  * DISCLAIMED.  IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR
27  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
28  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
29  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
30  * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
31  * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
32  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
33  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34  * SUCH DAMAGE.
35  *
36  * This software has been written for the Internet Software Consortium
37  * by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
38  * Enterprises.  To learn more about the Internet Software Consortium,
39  * see ``http://www.vix.com/isc''.  To learn more about Vixie
40  * Enterprises, see ``http://www.vix.com''.
41  */
42 
43 #include <sys/cdefs.h>
44 __FBSDID("$FreeBSD$");
45 
46 #include "dhcpd.h"
47 
48 #include <netinet/in_systm.h>
49 #include <netinet/ip.h>
50 #include <netinet/udp.h>
51 #include <netinet/if_ether.h>
52 
53 #define ETHER_HEADER_SIZE (ETHER_ADDR_LEN * 2 + sizeof(u_int16_t))
54 
55 u_int32_t	checksum(unsigned char *, unsigned, u_int32_t);
56 u_int32_t	wrapsum(u_int32_t);
57 
58 u_int32_t
59 checksum(unsigned char *buf, unsigned nbytes, u_int32_t sum)
60 {
61 	int i;
62 
63 	/* Checksum all the pairs of bytes first... */
64 	for (i = 0; i < (nbytes & ~1U); i += 2) {
65 		sum += (u_int16_t)ntohs(*((u_int16_t *)(buf + i)));
66 		if (sum > 0xFFFF)
67 			sum -= 0xFFFF;
68 	}
69 
70 	/*
71 	 * If there's a single byte left over, checksum it, too.
72 	 * Network byte order is big-endian, so the remaining byte is
73 	 * the high byte.
74 	 */
75 	if (i < nbytes) {
76 		sum += buf[i] << 8;
77 		if (sum > 0xFFFF)
78 			sum -= 0xFFFF;
79 	}
80 
81 	return (sum);
82 }
83 
84 u_int32_t
85 wrapsum(u_int32_t sum)
86 {
87 	sum = ~sum & 0xFFFF;
88 	return (htons(sum));
89 }
90 
91 void
92 assemble_hw_header(struct interface_info *interface, unsigned char *buf,
93     int *bufix, struct hardware *to)
94 {
95 	struct ether_header eh;
96 
97 	if (to != NULL && to->hlen == 6) /* XXX */
98 		memcpy(eh.ether_dhost, to->haddr, sizeof(eh.ether_dhost));
99 	else
100 		memset(eh.ether_dhost, 0xff, sizeof(eh.ether_dhost));
101 	if (interface->hw_address.hlen == sizeof(eh.ether_shost))
102 		memcpy(eh.ether_shost, interface->hw_address.haddr,
103 		    sizeof(eh.ether_shost));
104 	else
105 		memset(eh.ether_shost, 0x00, sizeof(eh.ether_shost));
106 
107 	eh.ether_type = htons(ETHERTYPE_IP);
108 
109 	memcpy(&buf[*bufix], &eh, ETHER_HEADER_SIZE);
110 	*bufix += ETHER_HEADER_SIZE;
111 }
112 
113 void
114 assemble_udp_ip_header(unsigned char *buf, int *bufix, u_int32_t from,
115     u_int32_t to, unsigned int port, unsigned char *data, int len)
116 {
117 	struct ip ip;
118 	struct udphdr udp;
119 
120 	ip.ip_v = 4;
121 	ip.ip_hl = 5;
122 	ip.ip_tos = IPTOS_LOWDELAY;
123 	ip.ip_len = htons(sizeof(ip) + sizeof(udp) + len);
124 	ip.ip_id = 0;
125 	ip.ip_off = 0;
126 	ip.ip_ttl = 128;
127 	ip.ip_p = IPPROTO_UDP;
128 	ip.ip_sum = 0;
129 	ip.ip_src.s_addr = from;
130 	ip.ip_dst.s_addr = to;
131 
132 	ip.ip_sum = wrapsum(checksum((unsigned char *)&ip, sizeof(ip), 0));
133 
134 	/*
135 	 * While the BPF -- used for broadcasts -- expects a "true" IP header
136 	 * with all the bytes in network byte order, the raw socket interface
137 	 * which is used for unicasts expects the ip_len field to be in host
138 	 * byte order.  In both cases, the checksum has to be correct, so this
139 	 * is as good a place as any to turn the bytes around again.
140 	 */
141 	if (to != INADDR_BROADCAST)
142 		ip.ip_len = ntohs(ip.ip_len);
143 
144 	memcpy(&buf[*bufix], &ip, sizeof(ip));
145 	*bufix += sizeof(ip);
146 
147 	udp.uh_sport = htons(LOCAL_PORT);	/* XXX */
148 	udp.uh_dport = port;			/* XXX */
149 	udp.uh_ulen = htons(sizeof(udp) + len);
150 	memset(&udp.uh_sum, 0, sizeof(udp.uh_sum));
151 
152 	udp.uh_sum = wrapsum(checksum((unsigned char *)&udp, sizeof(udp),
153 	    checksum(data, len, checksum((unsigned char *)&ip.ip_src,
154 	    2 * sizeof(ip.ip_src),
155 	    IPPROTO_UDP + (u_int32_t)ntohs(udp.uh_ulen)))));
156 
157 	memcpy(&buf[*bufix], &udp, sizeof(udp));
158 	*bufix += sizeof(udp);
159 }
160 
161 ssize_t
162 decode_hw_header(unsigned char *buf, int bufix, struct hardware *from)
163 {
164 	struct ether_header eh;
165 
166 	memcpy(&eh, buf + bufix, ETHER_HEADER_SIZE);
167 
168 	memcpy(from->haddr, eh.ether_shost, sizeof(eh.ether_shost));
169 	from->htype = ARPHRD_ETHER;
170 	from->hlen = sizeof(eh.ether_shost);
171 
172 	return (sizeof(eh));
173 }
174 
175 ssize_t
176 decode_udp_ip_header(unsigned char *buf, int bufix, struct sockaddr_in *from,
177     unsigned char *data, int buflen)
178 {
179 	struct ip *ip;
180 	struct udphdr *udp;
181 	u_int32_t ip_len = (buf[bufix] & 0xf) << 2;
182 	u_int32_t sum, usum;
183 	static int ip_packets_seen;
184 	static int ip_packets_bad_checksum;
185 	static int udp_packets_seen;
186 	static int udp_packets_bad_checksum;
187 	static int udp_packets_length_checked;
188 	static int udp_packets_length_overflow;
189 	int len = 0;
190 
191 	ip = (struct ip *)(buf + bufix);
192 	udp = (struct udphdr *)(buf + bufix + ip_len);
193 
194 	/* Check the IP header checksum - it should be zero. */
195 	ip_packets_seen++;
196 	if (wrapsum(checksum(buf + bufix, ip_len, 0)) != 0) {
197 		ip_packets_bad_checksum++;
198 		if (ip_packets_seen > 4 &&
199 		    (ip_packets_seen / ip_packets_bad_checksum) < 2) {
200 			note("%d bad IP checksums seen in %d packets",
201 			    ip_packets_bad_checksum, ip_packets_seen);
202 			ip_packets_seen = ip_packets_bad_checksum = 0;
203 		}
204 		return (-1);
205 	}
206 
207 	if (ntohs(ip->ip_len) != buflen)
208 		debug("ip length %d disagrees with bytes received %d.",
209 		    ntohs(ip->ip_len), buflen);
210 
211 	memcpy(&from->sin_addr, &ip->ip_src, 4);
212 
213 	/*
214 	 * Compute UDP checksums, including the ``pseudo-header'', the
215 	 * UDP header and the data.   If the UDP checksum field is zero,
216 	 * we're not supposed to do a checksum.
217 	 */
218 	if (!data) {
219 		data = buf + bufix + ip_len + sizeof(*udp);
220 		len = ntohs(udp->uh_ulen) - sizeof(*udp);
221 		udp_packets_length_checked++;
222 		if (len + data > buf + bufix + buflen) {
223 			udp_packets_length_overflow++;
224 			if (udp_packets_length_checked > 4 &&
225 			    (udp_packets_length_checked /
226 			    udp_packets_length_overflow) < 2) {
227 				note("%d udp packets in %d too long - dropped",
228 				    udp_packets_length_overflow,
229 				    udp_packets_length_checked);
230 				udp_packets_length_overflow =
231 				    udp_packets_length_checked = 0;
232 			}
233 			return (-1);
234 		}
235 		if (len + data != buf + bufix + buflen)
236 			debug("accepting packet with data after udp payload.");
237 	}
238 
239 	usum = udp->uh_sum;
240 	udp->uh_sum = 0;
241 
242 	sum = wrapsum(checksum((unsigned char *)udp, sizeof(*udp),
243 	    checksum(data, len, checksum((unsigned char *)&ip->ip_src,
244 	    2 * sizeof(ip->ip_src),
245 	    IPPROTO_UDP + (u_int32_t)ntohs(udp->uh_ulen)))));
246 
247 	udp_packets_seen++;
248 	if (usum && usum != sum) {
249 		udp_packets_bad_checksum++;
250 		if (udp_packets_seen > 4 &&
251 		    (udp_packets_seen / udp_packets_bad_checksum) < 2) {
252 			note("%d bad udp checksums in %d packets",
253 			    udp_packets_bad_checksum, udp_packets_seen);
254 			udp_packets_seen = udp_packets_bad_checksum = 0;
255 		}
256 		return (-1);
257 	}
258 
259 	memcpy(&from->sin_port, &udp->uh_sport, sizeof(udp->uh_sport));
260 
261 	return (ip_len + sizeof(*udp));
262 }
263