diff --git a/ndhc/ndhc.c b/ndhc/ndhc.c
index 9ef4c3f..88fa1b2 100644
--- a/ndhc/ndhc.c
+++ b/ndhc/ndhc.c
@@ -296,10 +296,8 @@ int main(int argc, char **argv)
                 len = strlen(optarg) > 64 ? 64 : strlen(optarg);
                 if (client_config.clientid)
                     free(client_config.clientid);
-                client_config.clientid = xmalloc(len + 3);
-                client_config.clientid[OPT_CODE] = DHCP_CLIENT_ID;
-                client_config.clientid[OPT_LEN] = len + 1;
-                memcpy(client_config.clientid + 3, optarg, len);
+                client_config.clientid =
+                    alloc_dhcp_client_id_option(0, (unsigned char *)optarg, len);
                 break;
             case 'f':
                 client_config.foreground = 1;
@@ -317,10 +315,8 @@ int main(int argc, char **argv)
                 len = strlen(optarg) > 64 ? 64 : strlen(optarg);
                 if (client_config.hostname)
                     free(client_config.hostname);
-                client_config.hostname = xmalloc(len + 3);
-                client_config.hostname[OPT_CODE] = DHCP_HOST_NAME;
-                client_config.hostname[OPT_LEN] = len + 1;
-                memcpy(client_config.hostname + 3, optarg, len);
+                client_config.hostname =
+                    alloc_option(DHCP_HOST_NAME, (unsigned char *)optarg, len);
                 break;
             case 'i':
                 client_config.interface = optarg;
@@ -376,11 +372,8 @@ int main(int argc, char **argv)
     }
 
     if (!client_config.clientid) {
-        client_config.clientid = xmalloc(6 + 3);
-        client_config.clientid[OPT_CODE] = DHCP_CLIENT_ID;
-        client_config.clientid[OPT_LEN] = 7;
-        client_config.clientid[OPT_DATA] = 1;
-        memcpy(client_config.clientid + 3, client_config.arp, 6);
+        client_config.clientid =
+            alloc_dhcp_client_id_option(1, client_config.arp, 6);
     }
 
     if (chdir(chroot_dir)) {
diff --git a/ndhc/options.c b/ndhc/options.c
index ea538ed..e6d83e8 100644
--- a/ndhc/options.c
+++ b/ndhc/options.c
@@ -8,9 +8,11 @@
 #include <stdlib.h>
 #include <string.h>
 
-#include "log.h"
 #include "options.h"
 
+#include "log.h"
+#include "malloc.h"
+
 /* supported options are easily added here */
 struct dhcp_option options[] = {
     /* name[10]     flags                                   code */
@@ -56,6 +58,55 @@ int option_lengths[] = {
 	[OPTION_S32] =      4
 };
 
+size_t sizeof_option(unsigned char code, size_t datalen)
+{
+	if (code == DHCP_PADDING || code == DHCP_END)
+		return 1;
+	return 2 + datalen;
+}
+
+// optdata can be NULL
+size_t set_option(unsigned char *buf, size_t buflen, unsigned char code,
+				  unsigned char *optdata, size_t datalen)
+{
+	if (!optdata)
+		datalen = 0;
+	if (code == DHCP_PADDING || code == DHCP_END) {
+		if (buflen < 1)
+			return 0;
+		buf[0] = code;
+		return 1;
+	}
+
+	if (datalen > 255 || buflen < 2 + datalen)
+		return 0;
+	buf[0] = code;
+	buf[1] = datalen;
+	memcpy(buf + 2, optdata, datalen);
+	return 2 + datalen;
+}
+
+unsigned char *alloc_option(unsigned char code, unsigned char *optdata,
+							size_t datalen)
+{
+	unsigned char *ret;
+	size_t len = sizeof_option(code, datalen);
+	ret = xmalloc(len);
+	set_option(ret, len, code, optdata, datalen);
+	return ret;
+}
+
+// This is tricky -- the data must be prefixed by one byte indicating the
+// type of ARP MAC address (1 for ethernet) or 0 for a purely symbolic
+// identifier.
+unsigned char *alloc_dhcp_client_id_option(unsigned char type,
+										   unsigned char *idstr, size_t idstrlen)
+{
+	unsigned char data[idstrlen + 1];
+	data[0] = type;
+	memcpy(data + 1, idstr, idstrlen);
+	return alloc_option(DHCP_CLIENT_ID, data, sizeof data);
+}
 
 /* Get an option with bounds checking (warning, result is not aligned) */
 uint8_t* get_option(struct dhcpMessage *packet, int code)
@@ -161,6 +212,7 @@ int add_simple_option(unsigned char *optionptr, unsigned char code,
 			length = option_lengths[options[i].flags & TYPE_MASK];
 		}
 
+	log_line("aso(): code=0x%02x length=0x%02x", code, length);
 	option[OPT_CODE] = code;
 	option[OPT_LEN] = (unsigned char)length;
 
diff --git a/ndhc/options.h b/ndhc/options.h
index 92da936..6af6dda 100644
--- a/ndhc/options.h
+++ b/ndhc/options.h
@@ -88,6 +88,15 @@ struct option_set {
 extern struct dhcp_option options[];
 extern int option_lengths[];
 
+size_t sizeof_option(unsigned char code, size_t datalen);
+size_t set_option(unsigned char *buf, size_t buflen, unsigned char code,
+				  unsigned char *optdata, size_t datalen);
+unsigned char *alloc_option(unsigned char code, unsigned char *optdata,
+							size_t datalen);
+
+unsigned char *alloc_dhcp_client_id_option(unsigned char type,
+										   unsigned char *idstr, size_t idstrlen);
+
 uint8_t *get_option(struct dhcpMessage *packet, int code);
 int end_option(uint8_t *optionptr);
 int add_option_string(unsigned char *optionptr, unsigned char *string);