check_dhcp: Don't misinterpret the "siaddr" field

RFC 2131 (2.) says: "DHCP clarifies the interpretation of the 'siaddr'
field as the address of the server to use in the next step of the
client's bootstrap process."  So, we shouldn't interpret this field as
the DHCP server's own address.  (#3503921 - Jason Ellison)

THANKS.in

@@ -273,3 +273,4 @@ Jason Lunn
 Alex Griffin
 Marc Remy
 Matej Vela
+Jason Ellison

plugins-root/check_dhcp.c

@@ -141,7 +141,7 @@ typedef struct dhcp_packet_struct{
         u_int16_t flags;                /* flags */
         struct in_addr ciaddr;          /* IP address of this machine (if we already have one) */
         struct in_addr yiaddr;          /* IP address of this machine (offered by the DHCP server) */
-        struct in_addr siaddr;          /* IP address of DHCP server */
+        struct in_addr siaddr;          /* IP address of next server */
         struct in_addr giaddr;          /* IP address of DHCP relay */
         unsigned char chaddr [MAX_DHCP_CHADDR_LENGTH];      /* hardware address of this machine */
         char sname [MAX_DHCP_SNAME_LENGTH];    /* name of DHCP server */
@@ -587,11 +587,6 @@ int get_dhcp_offer(int sock){
 		/* Save a copy of "source" into "via" even if it's via itself */
 		memcpy(&via,&source,sizeof(source)) ;
 
-		/* If siaddr is non-zero, set "source" to siaddr */
-		if(offer_packet.siaddr.s_addr != 0L){
-			source.sin_addr.s_addr = offer_packet.siaddr.s_addr ;
-			}
-
 		if(verbose){
 			printf(_("DHCPOFFER from IP address %s"),inet_ntoa(source.sin_addr));
 			printf(_(" via %s\n"),inet_ntoa(via.sin_addr));
@@ -904,9 +899,9 @@ int add_dhcp_offer(struct in_addr source,dhcp_packet *offer_packet){
 	 * the next bootstrap service (e.g., delivery of an operating system
 	 * executable image).  A DHCP server always returns its own address in
 	 * the 'server identifier' option."  'serv_ident' is the 'server
-	 * identifier' option, 'source' is the 'siaddr' field or (if 'siaddr'
-	 * wasn't available) the IP address we received the DHCPOFFER from.  If
-	 * 'serv_ident' isn't available for some reason, we use 'source'.
+	 * identifier' option, 'source' is the IP address we received the
+	 * DHCPOFFER from.  If 'serv_ident' isn't available for some reason, we
+	 * use 'source'.
 	 */
 	new_offer->server_address=serv_ident.s_addr?serv_ident:source;
 	new_offer->offered_address=offer_packet->yiaddr;