[ipv4] Extend routing mechanism to handle non-default routes

Extend the definition of an IPv4 routing table entry to allow for the
expression of non-default gateways for specified off-link subnets, and
of on-link secondary subnets (where we can send directly to the
destination address even though our source address is not within the
subnet).

This more precise definition also allows us to correctly handle
routing in the (uncommon for iPXE) case when multiple network
interfaces are open concurrently and more than one interface has a
default gateway.

The common case of a single IPv4 address/netmask and a default gateway
now results in two routing table entries.  To retain backwards
compatibility with existing documentation (and to avoid on-screen
clutter), the "route" command prints default gateways on the same line
as the locally assigned address.  There is therefore no change in
output from the "route" command unless explicit additional (off-link
or on-link) routes are present.

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

src/include/ipxe/ip.h

@@ -54,38 +54,83 @@ struct ipv4_pseudo_header {
 	uint16_t len;
 };
 
-/** An IPv4 address/routing table entry */
+/** An IPv4 address/routing table entry
+ *
+ * Routing table entries are maintained in order of specificity.  For
+ * a given destination address, the first matching table entry will be
+ * used as the egress route.
+ */
 struct ipv4_miniroute {
 	/** List of miniroutes */
 	struct list_head list;
 
-	/** Network device */
+	/** Network device
+	 *
+	 * When this routing table entry is matched, this is the
+	 * egress network device to be used.
+	 */
 	struct net_device *netdev;
 
-	/** IPv4 address */
+	/** IPv4 address
+	 *
+	 * When this routing table entry is matched, this is the
+	 * source address to be used.
+	 *
+	 * The presence of this routing table entry also indicates
+	 * that this address is a valid local destination address for
+	 * the matching network device.
+	 */
 	struct in_addr address;
+	/** Subnet network address
+	 *
+	 * A subnet is a range of addresses defined by a network
+	 * address and subnet mask.  A destination address with all of
+	 * the subnet mask bits in common with the network address is
+	 * within the subnet and therefore matches this routing table
+	 * entry.
+	 */
+	struct in_addr network;
 	/** Subnet mask
 	 *
-	 * An address with all of these bits in common with our IPv4
-	 * address is in the local subnet.
+	 * An address with all of these bits in common with the
+	 * network address matches this routing table entry.
 	 */
 	struct in_addr netmask;
+	/** Gateway address, or zero
+	 *
+	 * When this routing table entry is matched and this address
+	 * is non-zero, it will be used as the next-hop address.
+	 *
+	 * When this routing table entry is matched and this address
+	 * is zero, the subnet is local (on-link) and the next-hop
+	 * address will be the original destination address.
+	 */
+	struct in_addr gateway;
 	/** Host mask
 	 *
-	 * An address in the local subnet with all of these bits set
-	 * to zero represents the network address, and an address in
-	 * the local subnet with all of these bits set to one
-	 * represents the directed broadcast address.  All other
-	 * addresses in the local subnet are valid host addresses.
+	 * An address in a local subnet with all of these bits set to
+	 * zero represents the network address, and an address in a
+	 * local subnet with all of these bits set to one represents
+	 * the local directed broadcast address.  All other addresses
+	 * in a local subnet are valid host addresses.
 	 *
-	 * For most subnets, this is the inverse of the subnet mask.
-	 * In a small subnet (/31 or /32) there is no network address
-	 * or directed broadcast address, and all addresses in the
-	 * subnet are valid host addresses.
+	 * For most local subnets, this is the inverse of the subnet
+	 * mask.  In a small subnet (/31 or /32) there is no network
+	 * address or directed broadcast address, and all addresses in
+	 * the subnet are valid host addresses.
+	 *
+	 * When this routing table entry is matched and the subnet is
+	 * local, a next-hop address with all of these bits set to one
+	 * will be treated as a local broadcast address.  All other
+	 * next-hop addresses will be treated as unicast addresses.
+	 *
+	 * When this routing table entry is matched and the subnet is
+	 * non-local, the next-hop address is always a unicast
+	 * address.  The host mask for non-local subnets is therefore
+	 * set to @c INADDR_NONE to allow the same logic to be used as
+	 * for local subnets.
 	 */
 	struct in_addr hostmask;
-	/** Gateway address, or zero for no gateway */
-	struct in_addr gateway;
 };
 
 extern struct list_head ipv4_miniroutes;