[dhcp] Choose ProxyDHCP port based on presence of PXE options

If the ProxyDHCPOFFER already includes PXE options (i.e. option 60 is
set to "PXEClient" and option 43 is present) then assume that the
ProxyDHCPREQUEST can be sent to port 67, rather than port 4011.  This
is a reasonable assumption, since in that case the ProxyDHCP server
has already demonstrated by responding to the DHCPDISCOVER that it is
listening on port 67.  (If the ProxyDHCP server were not listening on
port 67, then the standard DHCP server would have been configured to
respond with option 60 set to "PXEClient" but no option 43 present.)

The PXE specification is ambiguous on this point; the specified
behaviour covers only the cases in which option 43 is *not* present in
the ProxyDHCPOFFER.  In these cases, we will continue to send the
ProxyDHCPREQUEST to port 4011.

This change is required in order to allow us to interoperate with
dnsmasq, which listens only on port 67.  (dnsmasq relies on
unspecified behaviour of the Intel PXE stack, which it seems will
retain the ProxyDHCPOFFER as an options source and never issue a
ProxyDHCPREQUEST, thereby enabling dnsmasq to omit listening on port
4011.)

src/net/udp/dhcp.c

 	int no_pxedhcp;
 	/** ProxyDHCP server */
 	struct in_addr proxy_server;
+	/** ProxyDHCP port */
+	uint16_t proxy_port;
 	/** ProxyDHCP server priority */
 	int proxy_priority;
 
 				  vci, sizeof ( vci ) );
 	has_pxeclient = ( ( vci_len >= ( int ) sizeof ( vci ) ) &&
 			  ( strncmp ( "PXEClient", vci, sizeof (vci) ) == 0 ));
+
+	/* Identify presence of vendor-specific options */
 	pxeopts_len = dhcppkt_fetch ( dhcppkt, DHCP_VENDOR_ENCAP, NULL, 0 );
 	has_pxeopts = ( pxeopts_len >= 0 );
 	if ( has_pxeclient )
 	}
 
 	/* Select as ProxyDHCP offer, if applicable */
-	if ( has_pxeclient && ( ! has_pxeopts ) && ( msgtype == DHCPOFFER ) &&
+	if ( has_pxeclient && ( msgtype == DHCPOFFER ) &&
 	     ( priority >= dhcp->proxy_priority ) ) {
+		/* If the offer already includes the PXE options, then
+		 * assume that we can send the ProxyDHCPREQUEST to
+		 * port 67 (since the DHCPDISCOVER that triggered this
+		 * ProxyDHCPOFFER was sent to port 67).  Otherwise,
+		 * send the ProxyDHCPREQUEST to port 4011.
+		 */
 		dhcp->proxy_server = server_id;
+		dhcp->proxy_port = ( has_pxeopts ? htons ( BOOTPS_PORT )
+				     : htons ( PXE_PORT ) );
 		dhcp->proxy_priority = priority;
 	}
 
 	}
 
 	/* Start ProxyDHCPREQUEST if applicable */
-	if ( dhcp->proxy_server.s_addr && ( ! dhcp->no_pxedhcp ) ) {
+	if ( dhcp->proxy_server.s_addr /* Have ProxyDHCP server */ &&
+	     ( ! dhcp->no_pxedhcp ) /* ProxyDHCP not disabled */ &&
+	     ( /* ProxyDHCP server is not just the DHCP server itself */
+	       ( dhcp->proxy_server.s_addr != dhcp->server.s_addr ) ||
+	       ( dhcp->proxy_port != htons ( BOOTPS_PORT ) ) ) ) {
 		dhcp_set_state ( dhcp, &dhcp_state_proxy );
 		return;
 	}
 			   struct sockaddr_in *peer ) {
 	int rc;
 
-	DBGC ( dhcp, "DHCP %p ProxyDHCP REQUEST to %s:%d\n",
-	       dhcp, inet_ntoa ( dhcp->proxy_server ), PXE_PORT );
+	DBGC ( dhcp, "DHCP %p ProxyDHCP REQUEST to %s:%d\n", dhcp,
+	       inet_ntoa ( dhcp->proxy_server ), ntohs ( dhcp->proxy_port ) );
 
 	/* Set server ID */
 	if ( ( rc = dhcppkt_store ( dhcppkt, DHCP_SERVER_IDENTIFIER,
 
 	/* Set server address */
 	peer->sin_addr = dhcp->proxy_server;
-	peer->sin_port = htons ( PXE_PORT );
+	peer->sin_port = dhcp->proxy_port;
 
 	return 0;
 }
 	DBGC ( dhcp, "\n" );
 
 	/* Filter out unacceptable responses */
-	if ( peer->sin_port != htons ( PXE_PORT ) )
+	if ( peer->sin_port != dhcp->proxy_port )
 		return;
 	if ( msgtype != DHCPACK )
 		return;