1: /* packet.c
2:
3: Packet assembly code, originally contributed by Archie Cobbs. */
4:
5: /*
6: * Copyright (c) 2004,2005,2007,2009 by Internet Systems Consortium, Inc. ("ISC")
7: * Copyright (c) 1996-2003 by Internet Software Consortium
8: *
9: * Permission to use, copy, modify, and distribute this software for any
10: * purpose with or without fee is hereby granted, provided that the above
11: * copyright notice and this permission notice appear in all copies.
12: *
13: * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
14: * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
15: * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR
16: * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
17: * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
18: * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
19: * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
20: *
21: * Internet Systems Consortium, Inc.
22: * 950 Charter Street
23: * Redwood City, CA 94063
24: * <info@isc.org>
25: * https://www.isc.org/
26: *
27: * This code was originally contributed by Archie Cobbs, and is still
28: * very similar to that contribution, although the packet checksum code
29: * has been hacked significantly with the help of quite a few ISC DHCP
30: * users, without whose gracious and thorough help the checksum code would
31: * still be disabled.
32: */
33:
34: #include "dhcpd.h"
35:
36: #if defined (PACKET_ASSEMBLY) || defined (PACKET_DECODING)
37: #include "includes/netinet/ip.h"
38: #include "includes/netinet/udp.h"
39: #include "includes/netinet/if_ether.h"
40: #endif /* PACKET_ASSEMBLY || PACKET_DECODING */
41:
42: /* Compute the easy part of the checksum on a range of bytes. */
43:
44: u_int32_t checksum (buf, nbytes, sum)
45: unsigned char *buf;
46: unsigned nbytes;
47: u_int32_t sum;
48: {
49: unsigned i;
50:
51: #ifdef DEBUG_CHECKSUM
52: log_debug ("checksum (%x %d %x)", buf, nbytes, sum);
53: #endif
54:
55: /* Checksum all the pairs of bytes first... */
56: for (i = 0; i < (nbytes & ~1U); i += 2) {
57: #ifdef DEBUG_CHECKSUM_VERBOSE
58: log_debug ("sum = %x", sum);
59: #endif
60: sum += (u_int16_t) ntohs(*((u_int16_t *)(buf + i)));
61: /* Add carry. */
62: if (sum > 0xFFFF)
63: sum -= 0xFFFF;
64: }
65:
66: /* If there's a single byte left over, checksum it, too. Network
67: byte order is big-endian, so the remaining byte is the high byte. */
68: if (i < nbytes) {
69: #ifdef DEBUG_CHECKSUM_VERBOSE
70: log_debug ("sum = %x", sum);
71: #endif
72: sum += buf [i] << 8;
73: /* Add carry. */
74: if (sum > 0xFFFF)
75: sum -= 0xFFFF;
76: }
77:
78: return sum;
79: }
80:
81: /* Finish computing the checksum, and then put it into network byte order. */
82:
83: u_int32_t wrapsum (sum)
84: u_int32_t sum;
85: {
86: #ifdef DEBUG_CHECKSUM
87: log_debug ("wrapsum (%x)", sum);
88: #endif
89:
90: sum = ~sum & 0xFFFF;
91: #ifdef DEBUG_CHECKSUM_VERBOSE
92: log_debug ("sum = %x", sum);
93: #endif
94:
95: #ifdef DEBUG_CHECKSUM
96: log_debug ("wrapsum returns %x", htons (sum));
97: #endif
98: return htons(sum);
99: }
100:
101: #ifdef PACKET_ASSEMBLY
102: void assemble_hw_header (interface, buf, bufix, to)
103: struct interface_info *interface;
104: unsigned char *buf;
105: unsigned *bufix;
106: struct hardware *to;
107: {
108: #if defined (HAVE_TR_SUPPORT)
109: if (interface -> hw_address.hbuf [0] == HTYPE_IEEE802)
110: assemble_tr_header (interface, buf, bufix, to);
111: else
112: #endif
113: #if defined (DEC_FDDI)
114: if (interface -> hw_address.hbuf [0] == HTYPE_FDDI)
115: assemble_fddi_header (interface, buf, bufix, to);
116: else
117: #endif
118: assemble_ethernet_header (interface, buf, bufix, to);
119:
120: }
121:
122: /* UDP header and IP header assembled together for convenience. */
123:
124: void assemble_udp_ip_header (interface, buf, bufix,
125: from, to, port, data, len)
126: struct interface_info *interface;
127: unsigned char *buf;
128: unsigned *bufix;
129: u_int32_t from;
130: u_int32_t to;
131: u_int32_t port;
132: unsigned char *data;
133: unsigned len;
134: {
135: struct ip ip;
136: struct udphdr udp;
137:
138: memset (&ip, 0, sizeof ip);
139:
140: /* Fill out the IP header */
141: IP_V_SET (&ip, 4);
142: IP_HL_SET (&ip, 20);
143: ip.ip_tos = IPTOS_LOWDELAY;
144: ip.ip_len = htons(sizeof(ip) + sizeof(udp) + len);
145: ip.ip_id = 0;
146: ip.ip_off = 0;
147: ip.ip_ttl = 128;
148: ip.ip_p = IPPROTO_UDP;
149: ip.ip_sum = 0;
150: ip.ip_src.s_addr = from;
151: ip.ip_dst.s_addr = to;
152:
153: /* Checksum the IP header... */
154: ip.ip_sum = wrapsum (checksum ((unsigned char *)&ip, sizeof ip, 0));
155:
156: /* Copy the ip header into the buffer... */
157: memcpy (&buf [*bufix], &ip, sizeof ip);
158: *bufix += sizeof ip;
159:
160: /* Fill out the UDP header */
161: udp.uh_sport = local_port; /* XXX */
162: udp.uh_dport = port; /* XXX */
163: udp.uh_ulen = htons(sizeof(udp) + len);
164: memset (&udp.uh_sum, 0, sizeof udp.uh_sum);
165:
166: /* Compute UDP checksums, including the ``pseudo-header'', the UDP
167: header and the data. */
168:
169: udp.uh_sum =
170: wrapsum (checksum ((unsigned char *)&udp, sizeof udp,
171: checksum (data, len,
172: checksum ((unsigned char *)
173: &ip.ip_src,
174: 2 * sizeof ip.ip_src,
175: IPPROTO_UDP +
176: (u_int32_t)
177: ntohs (udp.uh_ulen)))));
178:
179: /* Copy the udp header into the buffer... */
180: memcpy (&buf [*bufix], &udp, sizeof udp);
181: *bufix += sizeof udp;
182: }
183: #endif /* PACKET_ASSEMBLY */
184:
185: #ifdef PACKET_DECODING
186: /* Decode a hardware header... */
187: /* XXX currently only supports ethernet; doesn't check for other types. */
188:
189: ssize_t decode_hw_header (interface, buf, bufix, from)
190: struct interface_info *interface;
191: unsigned char *buf;
192: unsigned bufix;
193: struct hardware *from;
194: {
195: #if defined (HAVE_TR_SUPPORT)
196: if (interface -> hw_address.hbuf [0] == HTYPE_IEEE802)
197: return decode_tr_header (interface, buf, bufix, from);
198: else
199: #endif
200: #if defined (DEC_FDDI)
201: if (interface -> hw_address.hbuf [0] == HTYPE_FDDI)
202: return decode_fddi_header (interface, buf, bufix, from);
203: else
204: #endif
205: return decode_ethernet_header (interface, buf, bufix, from);
206: }
207:
208: /* UDP header and IP header decoded together for convenience. */
209:
210: ssize_t
211: decode_udp_ip_header(struct interface_info *interface,
212: unsigned char *buf, unsigned bufix,
213: struct sockaddr_in *from, unsigned buflen,
214: unsigned *rbuflen)
215: {
216: unsigned char *data;
217: struct ip ip;
218: struct udphdr udp;
219: unsigned char *upp, *endbuf;
220: u_int32_t ip_len, ulen, pkt_len;
221: u_int32_t sum, usum;
222: static int ip_packets_seen;
223: static int ip_packets_bad_checksum;
224: static int udp_packets_seen;
225: static int udp_packets_bad_checksum;
226: static int udp_packets_length_checked;
227: static int udp_packets_length_overflow;
228: unsigned len;
229:
230: /* Designate the end of the input buffer for bounds checks. */
231: endbuf = buf + bufix + buflen;
232:
233: /* Assure there is at least an IP header there. */
234: if ((buf + bufix + sizeof(ip)) > endbuf)
235: return -1;
236:
237: /* Copy the IP header into a stack aligned structure for inspection.
238: * There may be bits in the IP header that we're not decoding, so we
239: * copy out the bits we grok and skip ahead by ip.ip_hl * 4.
240: */
241: upp = buf + bufix;
242: memcpy(&ip, upp, sizeof(ip));
243: ip_len = (*upp & 0x0f) << 2;
244: upp += ip_len;
245:
246: /* Check the IP packet length. */
247: pkt_len = ntohs(ip.ip_len);
248: if (pkt_len > buflen)
249: return -1;
250:
251: /* Assure after ip_len bytes that there is enough room for a UDP header. */
252: if ((upp + sizeof(udp)) > endbuf)
253: return -1;
254:
255: /* Copy the UDP header into a stack aligned structure for inspection. */
256: memcpy(&udp, upp, sizeof(udp));
257:
258: #ifdef USERLAND_FILTER
259: /* Is it a UDP packet? */
260: if (ip.ip_p != IPPROTO_UDP)
261: return -1;
262:
263: /* Is it to the port we're serving? */
264: if (udp.uh_dport != local_port)
265: return -1;
266: #endif /* USERLAND_FILTER */
267:
268: ulen = ntohs(udp.uh_ulen);
269: if (ulen < sizeof(udp))
270: return -1;
271:
272: udp_packets_length_checked++;
273: if ((upp + ulen) > endbuf) {
274: udp_packets_length_overflow++;
275: if ((udp_packets_length_checked > 4) &&
276: ((udp_packets_length_checked /
277: udp_packets_length_overflow) < 2)) {
278: log_info("%d udp packets in %d too long - dropped",
279: udp_packets_length_overflow,
280: udp_packets_length_checked);
281: udp_packets_length_overflow = 0;
282: udp_packets_length_checked = 0;
283: }
284: return -1;
285: }
286:
287: if ((ulen < sizeof(udp)) || ((upp + ulen) > endbuf))
288: return -1;
289:
290: /* Check the IP header checksum - it should be zero. */
291: ++ip_packets_seen;
292: if (wrapsum (checksum (buf + bufix, ip_len, 0))) {
293: ++ip_packets_bad_checksum;
294: if (ip_packets_seen > 4 &&
295: (ip_packets_seen / ip_packets_bad_checksum) < 2) {
296: log_info ("%d bad IP checksums seen in %d packets",
297: ip_packets_bad_checksum, ip_packets_seen);
298: ip_packets_seen = ip_packets_bad_checksum = 0;
299: }
300: return -1;
301: }
302:
303: /* Copy out the IP source address... */
304: memcpy(&from->sin_addr, &ip.ip_src, 4);
305:
306: /* Compute UDP checksums, including the ``pseudo-header'', the UDP
307: header and the data. If the UDP checksum field is zero, we're
308: not supposed to do a checksum. */
309:
310: data = upp + sizeof(udp);
311: len = ulen - sizeof(udp);
312:
313: usum = udp.uh_sum;
314: udp.uh_sum = 0;
315:
316: /* XXX: We have to pass &udp, because we have to zero the checksum
317: * field before calculating the sum...'upp' isn't zeroed.
318: */
319: sum = wrapsum(checksum((unsigned char *)&udp, sizeof(udp),
320: checksum(data, len,
321: checksum((unsigned char *)&ip.ip_src,
322: 8, IPPROTO_UDP + ulen))));
323:
324: udp_packets_seen++;
325: if (usum && usum != sum) {
326: udp_packets_bad_checksum++;
327: if (udp_packets_seen > 4 &&
328: (udp_packets_seen / udp_packets_bad_checksum) < 2) {
329: log_info ("%d bad udp checksums in %d packets",
330: udp_packets_bad_checksum, udp_packets_seen);
331: udp_packets_seen = udp_packets_bad_checksum = 0;
332: }
333: return -1;
334: }
335:
336: /* Copy out the port... */
337: memcpy (&from -> sin_port, &udp.uh_sport, sizeof udp.uh_sport);
338:
339: /* Save the length of the UDP payload. */
340: if (rbuflen != NULL)
341: *rbuflen = len;
342:
343: /* Return the index to the UDP payload. */
344: return ip_len + sizeof udp;
345: }
346: #endif /* PACKET_DECODING */
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