Fix handling of -A and -a flags in dhcrelay; it was failing to expand

packet size as needed to add relay agent options. [rt18296]

RELNOTES

@@ -96,6 +96,9 @@ work on other platforms. Please report any problems and suggested fixes to
 - Merge DHCPv6-only "dhcrelay6" into general-purpose "dhcrelay" (use
   "-6" option to select DHCPv6 mode).
 
+- Fix handling of -A and -a flags in dhcrelay; it was failing to expand
+  packet size as needed to add relay agent options.
+
 			Changes since 4.0.0 (new features)
 
 - Added DHCPv6 rapid commit support.

relay/dhcrelay.c

@@ -62,6 +62,9 @@ int server_packets_relayed = 0;	/* Packets relayed from server to client. */
 int client_packet_errors = 0;	/* Errors sending packets to clients. */
 
 int add_agent_options = 0;	/* If nonzero, add relay agent options. */
+
+int agent_option_errors = 0;    /* Number of packets forwarded without
+				   agent options because there was no room. */
 int drop_agent_mismatches = 0;	/* If nonzero, drop server replies that
 				   don't have matching circuit-id's. */
 int corrupt_agent_options = 0;	/* Number of packets dropped because
@@ -80,7 +83,7 @@ isc_boolean_t use_if_id = ISC_FALSE;
 #endif
 
 	/* Maximum size of a packet with agent options added. */
-int dhcp_max_agent_option_packet_length = 576;
+int dhcp_max_agent_option_packet_length = DHCP_MTU_MIN;
 
 	/* What to do about packets we're asked to relay that
 	   already have a relay option: */
@@ -269,7 +272,13 @@ main(int argc, char **argv) {
 #endif
 			if (++i == argc)
 				usage();
+
 			dhcp_max_agent_option_packet_length = atoi(argv[i]);
+
+			if (dhcp_max_agent_option_packet_length > DHCP_MTU_MAX)
+				log_fatal("%s: packet length exceeds "
+					  "longest possible MTU\n",
+					  argv[i]);
 		} else if (!strcmp(argv[i], "-m")) {
 #ifdef DHCPv6
 			if (local_family_set && (local_family == AF_INET6)) {
@@ -873,7 +882,7 @@ find_interface_by_agent_option(struct dhcp_packet *packet,
 static int
 add_relay_agent_options(struct interface_info *ip, struct dhcp_packet *packet,
 			unsigned length, struct in_addr giaddr) {
-	int is_dhcp = 0;
+	int is_dhcp = 0, mms;
 	unsigned optlen;
 	u_int8_t *op, *nextop, *sp, *max, *end_pad = NULL;
 
@@ -887,7 +896,7 @@ add_relay_agent_options(struct interface_info *ip, struct dhcp_packet *packet,
 	if (memcmp(packet->options, DHCP_OPTIONS_COOKIE, 4))
 		return (length);
 
-	max = ((u_int8_t *)packet) + length;
+	max = ((u_int8_t *)packet) + dhcp_max_agent_option_packet_length;
 
 	/* Commence processing after the cookie. */
 	sp = op = &packet->options[4];
@@ -919,7 +928,17 @@ add_relay_agent_options(struct interface_info *ip, struct dhcp_packet *packet,
 		      case DHO_DHCP_MESSAGE_TYPE:
 			is_dhcp = 1;
 			goto skip;
-			break;
+
+			/*
+			 * If there's a maximum message size option, we
+			 * should pay attention to it
+			 */
+		      case DHO_DHCP_MAX_MESSAGE_SIZE:
+			mms = ntohs(*(op + 2));
+			if (mms < dhcp_max_agent_option_packet_length &&
+			    mms >= DHCP_MTU_MIN)
+				max = ((u_int8_t *)packet) + mms;
+			goto skip;
 
 			/* Quit immediately if we hit an End option. */
 		      case DHO_END:
@@ -1013,9 +1032,12 @@ add_relay_agent_options(struct interface_info *ip, struct dhcp_packet *packet,
 		log_fatal("Total agent option length(%u) out of range "
 			   "[3 - 255] on %s\n", optlen, ip->name);
 
-	/* Is there room for the option, its code+len, and DHO_END? */
+	/*
-	if ((sp > max) ||(max - sp < optlen + 3))
+	 * Is there room for the option, its code+len, and DHO_END?
-		return (0);
+	 * If not, forward without adding the option.
+	 */
+	if (max - sp >= optlen + 3) {
+		log_debug("Adding %d-byte relay agent option", optlen + 3);
 
 		/* Okay, cons up *our* Relay Agent Information option. */
 		*sp++ = DHO_DHCP_AGENT_OPTIONS;
@@ -1034,8 +1056,20 @@ add_relay_agent_options(struct interface_info *ip, struct dhcp_packet *packet,
 			memcpy(sp, ip->remote_id, ip->remote_id_len);
 			sp += ip->remote_id_len;
 		}
+	} else {
+		++agent_option_errors;
+		log_error("No room in packet (used %d of %d) "
+			  "for %d-byte relay agent option: omitted",
+			   (int) (sp - ((u_int8_t *) packet)),
+			   (int) (max - ((u_int8_t *) packet)),
+			   optlen + 3);
+	}
 
-	/* Deposit an END option. */
+	/*
+	 * Deposit an END option unless the packet is full (shouldn't
+	 * be possible).
+	 */
+	if (sp < max)
 		*sp++ = DHO_END;
 
 	/* Recalculate total packet length. */