diff --git a/src/include/gpxe/tcpip.h b/src/include/gpxe/tcpip.h
index 9bc3cc4a..da89530e 100644
--- a/src/include/gpxe/tcpip.h
+++ b/src/include/gpxe/tcpip.h
@@ -82,6 +82,7 @@ struct tcpip_net_protocol {
 	 *
 	 * @v iobuf		I/O buffer
 	 * @v tcpip_protocol	Transport-layer protocol
+	 * @v st_src		Source address, or NULL to use default
 	 * @v st_dest		Destination address
 	 * @v netdev		Network device (or NULL to route automatically)
 	 * @v trans_csum	Transport-layer checksum to complete, or NULL
@@ -91,6 +92,7 @@ struct tcpip_net_protocol {
 	 */
 	int ( * tx ) ( struct io_buffer *iobuf,
 		       struct tcpip_protocol *tcpip_protocol,
+		       struct sockaddr_tcpip *st_src,
 		       struct sockaddr_tcpip *st_dest,
 		       struct net_device *netdev,
 		       uint16_t *trans_csum );
@@ -107,7 +109,8 @@ struct tcpip_net_protocol {
 extern int tcpip_rx ( struct io_buffer *iobuf, uint8_t tcpip_proto,
 		      struct sockaddr_tcpip *st_src,
 		      struct sockaddr_tcpip *st_dest, uint16_t pshdr_csum );
-extern int tcpip_tx ( struct io_buffer *iobuf, struct tcpip_protocol *tcpip, 
+extern int tcpip_tx ( struct io_buffer *iobuf, struct tcpip_protocol *tcpip,
+		      struct sockaddr_tcpip *st_src,
 		      struct sockaddr_tcpip *st_dest,
 		      struct net_device *netdev,
 		      uint16_t *trans_csum );
diff --git a/src/net/icmpv6.c b/src/net/icmpv6.c
index 7b7146c2..237fc4a6 100644
--- a/src/net/icmpv6.c
+++ b/src/net/icmpv6.c
@@ -60,7 +60,7 @@ int icmp6_send_solicit ( struct net_device *netdev, struct in6_addr *src __unuse
 	st_dest.sin6.sin6_addr.in6_u.u6_addr8[13] = 0xff;
 	
 	/* Send packet over IP6 */
-	return tcpip_tx ( iobuf, &icmp6_protocol, &st_dest.st,
+	return tcpip_tx ( iobuf, &icmp6_protocol, NULL, &st_dest.st,
 			  NULL, &nsolicit->csum );
 }
 
diff --git a/src/net/ipv4.c b/src/net/ipv4.c
index 63dcca28..335048c8 100644
--- a/src/net/ipv4.c
+++ b/src/net/ipv4.c
@@ -286,6 +286,7 @@ static int ipv4_ll_addr ( struct in_addr dest, struct in_addr src,
  *
  * @v iobuf		I/O buffer
  * @v tcpip		Transport-layer protocol
+ * @v st_src		Source network-layer address
  * @v st_dest		Destination network-layer address
  * @v netdev		Network device to use if no route found, or NULL
  * @v trans_csum	Transport-layer checksum to complete, or NULL
@@ -295,10 +296,12 @@ static int ipv4_ll_addr ( struct in_addr dest, struct in_addr src,
  */
 static int ipv4_tx ( struct io_buffer *iobuf,
 		     struct tcpip_protocol *tcpip_protocol,
+		     struct sockaddr_tcpip *st_src,
 		     struct sockaddr_tcpip *st_dest,
 		     struct net_device *netdev,
 		     uint16_t *trans_csum ) {
 	struct iphdr *iphdr = iob_push ( iobuf, sizeof ( *iphdr ) );
+	struct sockaddr_in *sin_src = ( ( struct sockaddr_in * ) st_src );
 	struct sockaddr_in *sin_dest = ( ( struct sockaddr_in * ) st_dest );
 	struct ipv4_miniroute *miniroute;
 	struct in_addr next_hop;
@@ -317,7 +320,11 @@ static int ipv4_tx ( struct io_buffer *iobuf,
 
 	/* Use routing table to identify next hop and transmitting netdev */
 	next_hop = iphdr->dest;
-	if ( ( miniroute = ipv4_route ( &next_hop ) ) ) {
+	if ( sin_src )
+		iphdr->src = sin_src->sin_addr;
+	if ( ( next_hop.s_addr != INADDR_BROADCAST ) &&
+	     ( ! IN_MULTICAST ( ntohl ( next_hop.s_addr ) ) ) &&
+	     ( ( miniroute = ipv4_route ( &next_hop ) ) != NULL ) ) {
 		iphdr->src = miniroute->address;
 		netdev = miniroute->netdev;
 	}
diff --git a/src/net/ipv6.c b/src/net/ipv6.c
index 3407d538..f7308bb4 100644
--- a/src/net/ipv6.c
+++ b/src/net/ipv6.c
@@ -176,6 +176,7 @@ void ipv6_dump ( struct ip6_header *ip6hdr ) {
  */
 static int ipv6_tx ( struct io_buffer *iobuf,
 		     struct tcpip_protocol *tcpip,
+		     struct sockaddr_tcpip *st_src __unused,
 		     struct sockaddr_tcpip *st_dest,
 		     struct net_device *netdev,
 		     uint16_t *trans_csum ) {
diff --git a/src/net/tcp.c b/src/net/tcp.c
index 4e0db464..0f91bcf9 100644
--- a/src/net/tcp.c
+++ b/src/net/tcp.c
@@ -491,7 +491,7 @@ static int tcp_xmit ( struct tcp_connection *tcp, int force_send ) {
 	DBGC ( tcp, "\n" );
 
 	/* Transmit packet */
-	return tcpip_tx ( iobuf, &tcp_protocol, &tcp->peer, NULL,
+	return tcpip_tx ( iobuf, &tcp_protocol, NULL, &tcp->peer, NULL,
 			  &tcphdr->csum );
 }
 
@@ -572,7 +572,7 @@ static int tcp_xmit_reset ( struct tcp_connection *tcp,
 	DBGC ( tcp, "\n" );
 
 	/* Transmit packet */
-	return tcpip_tx ( iobuf, &tcp_protocol, st_dest,
+	return tcpip_tx ( iobuf, &tcp_protocol, NULL, st_dest,
 			  NULL, &tcphdr->csum );
 }
 
diff --git a/src/net/tcpip.c b/src/net/tcpip.c
index 1bc8d1a3..d4542b05 100644
--- a/src/net/tcpip.c
+++ b/src/net/tcpip.c
@@ -64,14 +64,15 @@ int tcpip_rx ( struct io_buffer *iobuf, uint8_t tcpip_proto,
  *
  * @v iobuf		I/O buffer
  * @v tcpip_protocol	Transport-layer protocol
+ * @v st_src		Source address, or NULL to use route default
  * @v st_dest		Destination address
  * @v netdev		Network device to use if no route found, or NULL
  * @v trans_csum	Transport-layer checksum to complete, or NULL
  * @ret rc		Return status code
  */
 int tcpip_tx ( struct io_buffer *iobuf, struct tcpip_protocol *tcpip_protocol,
-	       struct sockaddr_tcpip *st_dest, struct net_device *netdev,
-	       uint16_t *trans_csum ) {
+	       struct sockaddr_tcpip *st_src, struct sockaddr_tcpip *st_dest,
+	       struct net_device *netdev, uint16_t *trans_csum ) {
 	struct tcpip_net_protocol *tcpip_net;
 
 	/* Hand off packet to the appropriate network-layer protocol */
@@ -79,8 +80,8 @@ int tcpip_tx ( struct io_buffer *iobuf, struct tcpip_protocol *tcpip_protocol,
 	      tcpip_net < tcpip_net_protocols_end ; tcpip_net++ ) {
 		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_dest,
-					       netdev, trans_csum );
+			return tcpip_net->tx ( iobuf, tcpip_protocol, st_src,
+					       st_dest, netdev, trans_csum );
 		}
 	}
 	
diff --git a/src/net/udp.c b/src/net/udp.c
index fddf81df..63c2d9ee 100644
--- a/src/net/udp.c
+++ b/src/net/udp.c
@@ -182,13 +182,13 @@ static void udp_close ( struct udp_connection *udp, int rc ) {
  *
  * @v udp		UDP connection
  * @v iobuf		I/O buffer
- * @v src_port		Source port, or 0 to use default
+ * @v src		Source address, or NULL to use default
  * @v dest		Destination address, or NULL to use default
  * @v netdev		Network device, or NULL to use default
  * @ret rc		Return status code
  */
 static int udp_tx ( struct udp_connection *udp, struct io_buffer *iobuf,
-		    unsigned int src_port, struct sockaddr_tcpip *dest,
+		    struct sockaddr_tcpip *src, struct sockaddr_tcpip *dest,
 		    struct net_device *netdev ) {
        	struct udp_header *udphdr;
 	size_t len;
@@ -201,8 +201,8 @@ static int udp_tx ( struct udp_connection *udp, struct io_buffer *iobuf,
 	}
 
 	/* Fill in default values if not explicitly provided */
-	if ( ! src_port )
-		src_port = udp->local.st_port;
+	if ( ! src )
+		src = &udp->local;
 	if ( ! dest )
 		dest = &udp->peer;
 
@@ -210,7 +210,7 @@ static int udp_tx ( struct udp_connection *udp, struct io_buffer *iobuf,
 	udphdr = iob_push ( iobuf, sizeof ( *udphdr ) );
 	len = iob_len ( iobuf );
 	udphdr->dest = dest->st_port;
-	udphdr->src = src_port;
+	udphdr->src = src->st_port;
 	udphdr->len = htons ( len );
 	udphdr->chksum = 0;
 	udphdr->chksum = tcpip_chksum ( udphdr, len );
@@ -221,7 +221,7 @@ static int udp_tx ( struct udp_connection *udp, struct io_buffer *iobuf,
 	       ntohs ( udphdr->len ) );
 
 	/* Send it to the next layer for processing */
-	if ( ( rc = tcpip_tx ( iobuf, &udp_protocol, dest, netdev,
+	if ( ( rc = tcpip_tx ( iobuf, &udp_protocol, src, dest, netdev,
 			       &udphdr->chksum ) ) != 0 ) {
 		DBGC ( udp, "UDP %p could not transmit packet: %s\n",
 		       udp, strerror ( rc ) );
@@ -399,22 +399,19 @@ static int udp_xfer_deliver_iob ( struct xfer_interface *xfer,
 				  struct xfer_metadata *meta ) {
 	struct udp_connection *udp =
 		container_of ( xfer, struct udp_connection, xfer );
-	struct sockaddr_tcpip *src;
+	struct sockaddr_tcpip *src = NULL;
 	struct sockaddr_tcpip *dest = NULL;
 	struct net_device *netdev = NULL;
-	unsigned int src_port = 0;
 
 	/* Apply xfer metadata */
 	if ( meta ) {
 		src = ( struct sockaddr_tcpip * ) meta->src;
-		if ( src )
-			src_port = src->st_port;
 		dest = ( struct sockaddr_tcpip * ) meta->dest;
 		netdev = meta->netdev;
 	}
 
 	/* Transmit data, if possible */
-	udp_tx ( udp, iobuf, src_port, dest, netdev );
+	udp_tx ( udp, iobuf, src, dest, netdev );
 
 	return 0;
 }