[ipv6] Use given source address only if it is not the unspecified address

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

src/include/ipxe/in.h

 #define s6_addr32       in6_u.u6_addr32
 };
 
+#define IN6_IS_ADDR_UNSPECIFIED( addr )					\
+	( ( ( ( ( const uint32_t * ) (addr) )[0] ) |			\
+	    ( ( ( const uint32_t * ) (addr) )[1] ) |			\
+	    ( ( ( const uint32_t * ) (addr) )[2] ) |			\
+	    ( ( ( const uint32_t * ) (addr) )[3] ) ) == 0 )
+
 #define IN6_IS_ADDR_MULTICAST( addr )					\
 	( *( ( const uint8_t * ) (addr) ) == 0xff )
 

src/net/ipv6.c

 		rc = -ENETUNREACH;
 		goto err;
 	}
-	if ( sin6_src )
+	if ( sin6_src && ! IN6_IS_ADDR_UNSPECIFIED ( &sin6_src->sin6_addr ) )
 		src = &sin6_src->sin6_addr;
 	memcpy ( &iphdr->src, src, sizeof ( iphdr->src ) );