[tcpip] Provide tcpip_netdev() to determine the transmitting network device

Provide the function tcpip_netdev() to allow external code to
determine the transmitting network device for a given socket address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tcpip.h

 		       struct sockaddr_tcpip *st_dest,
 		       struct net_device *netdev,
 		       uint16_t *trans_csum );
+	/**
+	 * Determine transmitting network device
+	 *
+	 * @v st_dest		Destination address
+	 * @ret netdev		Network device, or NULL
+	 */
+	struct net_device * ( * netdev ) ( struct sockaddr_tcpip *dest );
 };
 
 /** TCP/IP transport-layer protocol table */
 		      struct sockaddr_tcpip *st_dest,
 		      struct net_device *netdev,
 		      uint16_t *trans_csum );
+extern struct net_device * tcpip_netdev ( struct sockaddr_tcpip *st_dest );
 extern uint16_t generic_tcpip_continue_chksum ( uint16_t partial,
 						const void *data, size_t len );
 extern uint16_t tcpip_chksum ( const void *data, size_t len );

src/net/ipv4.c

 	return NULL;
 }
 
+/**
+ * Determine transmitting network device
+ *
+ * @v st_dest		Destination network-layer address
+ * @ret netdev		Transmitting network device, or NULL
+ */
+static struct net_device * ipv4_netdev ( struct sockaddr_tcpip *st_dest ) {
+	struct sockaddr_in *sin_dest = ( ( struct sockaddr_in * ) st_dest );
+	struct in_addr dest = sin_dest->sin_addr;
+	struct ipv4_miniroute *miniroute;
+
+	/* Find routing table entry */
+	miniroute = ipv4_route ( &dest );
+	if ( ! miniroute )
+		return NULL;
+
+	return miniroute->netdev;
+}
+
 /**
  * Check if IPv4 fragment matches fragment reassembly buffer
  *
 	.name = "IPv4",
 	.sa_family = AF_INET,
 	.tx = ipv4_tx,
+	.netdev = ipv4_netdev,
 };
 
 /** IPv4 ARP protocol */

src/net/ipv6.c

 	return NULL;
 }
 
+/**
+ * Determine transmitting network device
+ *
+ * @v st_dest		Destination network-layer address
+ * @ret netdev		Transmitting network device, or NULL
+ */
+static struct net_device * ipv6_netdev ( struct sockaddr_tcpip *st_dest ) {
+	struct sockaddr_in6 *sin6_dest = ( ( struct sockaddr_in6 * ) st_dest );
+	struct in6_addr *dest = &sin6_dest->sin6_addr;
+	struct ipv6_miniroute *miniroute;
+
+	/* Find routing table entry */
+	miniroute = ipv6_route ( sin6_dest->sin6_scope_id, &dest );
+	if ( ! miniroute )
+		return NULL;
+
+	return miniroute->netdev;
+}
+
 /**
  * Check that received options can be safely ignored
  *
 	.name = "IPv6",
 	.sa_family = AF_INET6,
 	.tx = ipv6_tx,
+	.netdev = ipv6_netdev,
 };
 
 /** IPv6 socket address converter */

src/net/tcpip.c

 
 FILE_LICENCE ( GPL2_OR_LATER );
 
-/** Process a received TCP/IP packet
+/**
+ * Process a received TCP/IP packet
  *
  * @v iobuf		I/O buffer
  * @v netdev		Network device
 	return -EPROTONOSUPPORT;
 }
 
-/** Transmit a TCP/IP packet
+/**
+ * Find TCP/IP network-layer protocol
+ *
+ * @v st_dest		Destination address
+ * @ret tcpip_net	TCP/IP network-layer protocol, or NULL if not found
+ */
+static struct tcpip_net_protocol *
+tcpip_net_protocol ( struct sockaddr_tcpip *st_dest ) {
+	struct tcpip_net_protocol *tcpip_net;
+
+	for_each_table_entry ( tcpip_net, TCPIP_NET_PROTOCOLS ) {
+		if ( tcpip_net->sa_family == st_dest->st_family )
+			return tcpip_net;
+	}
+
+	DBG ( "Unrecognised TCP/IP address family %d\n", st_dest->st_family );
+	return NULL;
+}
+
+/**
+ * Transmit a TCP/IP packet
  *
  * @v iobuf		I/O buffer
  * @v tcpip_protocol	Transport-layer protocol
 	struct tcpip_net_protocol *tcpip_net;
 
 	/* Hand off packet to the appropriate network-layer protocol */
-	for_each_table_entry ( tcpip_net, TCPIP_NET_PROTOCOLS ) {
-		if ( tcpip_net->sa_family == st_dest->st_family ) {
-			DBG ( "TCP/IP sending %s packet\n", tcpip_net->name );
-			return tcpip_net->tx ( iobuf, tcpip_protocol, st_src,
-					       st_dest, netdev, trans_csum );
-		}
+	tcpip_net = tcpip_net_protocol ( st_dest );
+	if ( tcpip_net ) {
+		DBG ( "TCP/IP sending %s packet\n", tcpip_net->name );
+		return tcpip_net->tx ( iobuf, tcpip_protocol, st_src, st_dest,
+				       netdev, trans_csum );
 	}
-	
-	DBG ( "Unrecognised TCP/IP address family %d\n", st_dest->st_family );
+
 	free_iob ( iobuf );
 	return -EAFNOSUPPORT;
 }
 
+/**
+ * Determine transmitting network device
+ *
+ * @v st_dest		Destination address
+ * @ret netdev		Network device, or NULL
+ */
+struct net_device * tcpip_netdev ( struct sockaddr_tcpip *st_dest ) {
+	struct tcpip_net_protocol *tcpip_net;
+
+	/* Hand off to the appropriate network-layer protocol */
+	tcpip_net = tcpip_net_protocol ( st_dest );
+	if ( tcpip_net )
+		return tcpip_net->netdev ( st_dest );
+
+	return NULL;
+}
+
 /**
  * Calculate continued TCP/IP checkum
  *