odhcp6c: fix handling of RFC6603 Prefix Exclude Option

Several bugs in the encoding and (more importantly) decoding of
DHCPV6_OPT_PD_EXCLUDE lead to the option being ignored in some received
messages, or the excluded subnet id being mangled to an incorrect value.

Fix both encoding and decoding, being more explicit and slightly more
verbose for clarity.

Signed-off-by: Mark H. Spatz <mark.h.spatz@gmail.com>

Author: mmmspatz

src/dhcpv6.c

@@ -822,13 +822,15 @@ static void dhcpv6_send(enum dhcpv6_msg req_msg_type, uint8_t trid[3], uint32_t
 				if (pd_entries[j].iaid != iaid)
 					continue;
 
-				uint8_t ex_len = 0;
-				if (pd_entries[j].exclusion_length > 0)
-					ex_len = ((pd_entries[j].exclusion_length - pd_entries[j].length - 1) / 8) + 6;
-
+				uint8_t excl_subnet_id_nbits, excl_subnet_id_nbytes, excl_opt_len = 0;
+				if (pd_entries[j].exclusion_length > 0) {
+					excl_subnet_id_nbits = pd_entries[j].exclusion_length - pd_entries[j].length;
+					excl_subnet_id_nbytes = ((excl_subnet_id_nbits - 1) / 8)  + 1;
+					excl_opt_len = excl_subnet_id_nbytes + DHCPV6_OPT_HDR_SIZE + 1;
+				}
 				struct dhcpv6_ia_prefix p = {
 					.type = htons(DHCPV6_OPT_IA_PREFIX),
-					.len = htons(sizeof(p) - DHCPV6_OPT_HDR_SIZE_U + ex_len),
+					.len = htons(sizeof(p) - DHCPV6_OPT_HDR_SIZE_U + excl_opt_len),
 					.prefix = pd_entries[j].length,
 					.addr = pd_entries[j].target
 				};
@@ -841,21 +843,22 @@ static void dhcpv6_send(enum dhcpv6_msg req_msg_type, uint8_t trid[3], uint32_t
 				memcpy(ia_pd + ia_pd_len, &p, sizeof(p));
 				ia_pd_len += sizeof(p);
 
-				if (ex_len) {
+				if (excl_opt_len) {
 					ia_pd[ia_pd_len++] = 0;
 					ia_pd[ia_pd_len++] = DHCPV6_OPT_PD_EXCLUDE;
 					ia_pd[ia_pd_len++] = 0;
-					ia_pd[ia_pd_len++] = ex_len - DHCPV6_OPT_HDR_SIZE;
+					ia_pd[ia_pd_len++] = excl_opt_len - DHCPV6_OPT_HDR_SIZE;
 					ia_pd[ia_pd_len++] = pd_entries[j].exclusion_length;
 
-					uint32_t excl = ntohl(pd_entries[j].router.s6_addr32[1]);
-					excl >>= (64 - pd_entries[j].exclusion_length);
-					excl <<= 8 - ((pd_entries[j].exclusion_length - pd_entries[j].length) % 8);
+					uint32_t excluded_bits = ntohl(pd_entries[j].router.s6_addr32[1]);
+					excluded_bits >>= (64 - pd_entries[j].exclusion_length); /* Right align subnet ID bits */
+					excluded_bits <<= (32 - excl_subnet_id_nbits); /* Left align subnet ID bits */
 
-					for (size_t k = ex_len - 5; k > 0; --k, excl >>= 8)
-						ia_pd[ia_pd_len + k] = excl & 0xff;
-
-					ia_pd_len += ex_len - 5;
+					/* Copy subnet ID bits into the option MSB first */
+					for (size_t k = 0; k < excl_subnet_id_nbytes; ++k) {
+						ia_pd[ia_pd_len++] = excluded_bits >> 24;
+						excluded_bits <<= 8;
+					}
 				}
 
 				hdr->len = htons(ntohs(hdr->len) + ntohs(p.len) + 4U);
@@ -1826,9 +1829,10 @@ static unsigned int dhcpv6_parse_ia(void *opt, void *end, int *ret)
 
 					dhcpv6_log_status_code(code, "IA_PREFIX", status_msg, msg_len);
 					if (ret) *ret = 0; // renewal failed
-				} else if (stype == DHCPV6_OPT_PD_EXCLUDE && slen > 2) {
+				} else if (stype == DHCPV6_OPT_PD_EXCLUDE && slen >= 2) {
 					/*	RFC 6603 §4.2 Prefix Exclude option */
 					uint8_t exclude_length = sdata[0];
+					uint8_t *excl_subnet_id = &sdata[1];
 					if (exclude_length > 64)
 						exclude_length = 64;
 
@@ -1837,21 +1841,19 @@ static unsigned int dhcpv6_parse_ia(void *opt, void *end, int *ret)
 						continue;
 					}
 
-					uint8_t bytes_needed = ((exclude_length - entry.length - 1) / 8) + 1;
-					if (slen <= bytes_needed) {
+					uint8_t excl_subnet_id_nbits = exclude_length - entry.length;
+					uint8_t excl_subnet_id_nbytes = ((excl_subnet_id_nbits - 1) / 8) + 1;
+					if ((excl_subnet_id + excl_subnet_id_nbytes) > (sdata + slen)) {
 						update_state = false;
 						continue;
 					}
 
-					// this decrements through the ipaddr bytes masking against 
-					// the address in the option until byte 0, the option length field.
 					uint32_t excluded_bits = 0;
-					do {
-						excluded_bits = excluded_bits << 8 | sdata[bytes_needed];
-					} while (--bytes_needed);
-
-					excluded_bits >>= 8 - ((exclude_length - entry.length) % 8);
-					excluded_bits <<= 64 - exclude_length;
+					/* Copy subnet ID bits out of the option MSB first */
+					for(size_t i = 0; i < excl_subnet_id_nbytes; i++)
+						excluded_bits = (excluded_bits << 8) | excl_subnet_id[i];
+					excluded_bits >>= (8 * excl_subnet_id_nbytes) - excl_subnet_id_nbits; /* Right align subnet ID bits */
+					excluded_bits <<= (64 - exclude_length); /* Shift subnet ID bits into the low-order bits of the prefix */
 
 					// Re-using router field to hold the prefix
 					entry.router = entry.target; // base prefix
@@ -1867,6 +1869,12 @@ static unsigned int dhcpv6_parse_ia(void *opt, void *end, int *ret)
 				info("%s/%d preferred %d valid %d",
 						inet_ntop(AF_INET6, &entry.target, buf, sizeof(buf)),
 						entry.length, entry.preferred , entry.valid);
+
+				if (entry.exclusion_length) {
+					info("PD_EXCLUDE %s/%d",
+						inet_ntop(AF_INET6, &entry.router, buf, sizeof(buf)),
+						entry.exclusion_length);
+				}
 			}
 
 			entry.priority = 0;