ipconfig: send Client-identifier in DHCP requests

A dhcp server may provide parameters to a client from a pool of IP
addresses and using a shared rootfs, or provide a specific set of
parameters for a specific client, usually using the MAC address to
identify each client individually. The dhcp protocol also specifies
a client-id field which can be used to determine the correct
parameters to supply when no MAC address is available. There is
currently no way to tell the kernel to supply a specific client-id,
only the userspace dhcp clients support this feature, but this can
not be used when the network is needed before userspace is available
such as when the root filesystem is on NFS.

This patch is to be able to do something like "ip=dhcp,client_id_type,
client_id_value", as a kernel parameter to enable the kernel to
identify itself to the server.

Signed-off-by: Li RongQing <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/filesystems/nfs/nfsroot.txt

···
 		  both:        use both BOOTP and RARP but not DHCP
 		               (old option kept for backwards compatibility)
 
+		if dhcp is used, the client identifier can be used by following
+		format "ip=dhcp,client-id-type,client-id-value"
+
                 Default: any
 
   <dns0-ip>	IP address of first nameserver.

net/ipv4/ipconfig.c

···
 /* vendor class identifier */
 static char vendor_class_identifier[253] __initdata;
 
+#if defined(CONFIG_IP_PNP_DHCP)
+static char dhcp_client_identifier[253] __initdata;
+#endif
+
 /* Persistent data: */
 
 static int ic_proto_used;			/* Protocol used, if any */
···
 			*e++ = len;
 			memcpy(e, vendor_class_identifier, len);
 			e += len;
+		}
+		len = strlen(dhcp_client_identifier + 1);
+		/* the minimum length of identifier is 2, include 1 byte type,
+		 * and can not be larger than the length of options
+		 */
+		if (len >= 1 && len < 312 - (e - options) - 1) {
+			*e++ = 61;
+			*e++ = len + 1;
+			memcpy(e, dhcp_client_identifier, len + 1);
+			e += len + 1;
 		}
 	}
 
···
 		return 0;
 	}
 #ifdef CONFIG_IP_PNP_DHCP
-	else if (!strcmp(name, "dhcp")) {
+	else if (!strncmp(name, "dhcp", 4)) {
+		char *client_id;
+
 		ic_proto_enabled &= ~IC_RARP;
+		client_id = strstr(name, "dhcp,");
+		if (client_id) {
+			char *v;
+
+			client_id = client_id + 5;
+			v = strchr(client_id, ',');
+			if (!v)
+				return 1;
+			*v = 0;
+			if (kstrtou8(client_id, 0, dhcp_client_identifier))
+				DBG("DHCP: Invalid client identifier type\n");
+			strncpy(dhcp_client_identifier + 1, v + 1, 251);
+			*v = ',';
+		}
 		return 1;
 	}
 #endif