[master] Ignore empty host name option when parsing v4 packets

Merges in rt43786

RELNOTES

@@ -1168,6 +1168,12 @@ dhcp-users@lists.isc.org.
   with implicit function declarations errors for POST() and INSIST().
   [ISC-bugs #46332]
 
+- Added to code ignore empty IPv4 host name option (code 12). While RFC 2132
+  states the option cannot be empty, some clients are apparently capable of
+  sending it. Prior to this the server was attempting to use it and store it
+  in the lease file causing issues with DDNS and so forth.
+  [ISC-bugs #43786]
+
 			Changes since 4.2.0 (new features)
 
 - If a client renews before 'dhcp-cache-threshold' percent of its lease

common/options.c

@@ -202,6 +202,16 @@ int parse_option_buffer (options, buffer, length, universe)
 							     universe, NULL);
 		}
 
+		if (universe == &dhcp_universe && code == DHO_HOST_NAME &&
+		    len == 0) {
+			/* non-compliant clients can send it
+			 * we'll just drop it and go on */
+			log_debug ("Ignoring empty DHO_HOST_NAME option");
+			option_dereference(&option, MDL);
+			offset += len;
+			continue;
+		}
+
 		op = lookup_option(universe, options, code);
 		if (op == NULL) {
 			/* If we don't have an option create one */