diff -up dhcp-4.0.0/client/dhclient.c.port dhcp-4.0.0/client/dhclient.c
--- dhcp-4.0.0/client/dhclient.c.port	2008-10-29 13:27:33.000000000 -1000
+++ dhcp-4.0.0/client/dhclient.c	2008-10-29 13:27:33.000000000 -1000
@@ -218,7 +218,7 @@ int main(int argc, char **argv, char **e
 		} else if (!strcmp (argv [i], "-p")) {
 			if (++i == argc)
 				usage ();
-			local_port = htons (atoi (argv [i]));
+			local_port = validate_port(argv[i]);
 			log_debug ("binding to user-specified port %d",
 			       ntohs (local_port));
 		} else if (!strcmp (argv [i], "-d")) {
diff -up dhcp-4.0.0/common/inet.c.port dhcp-4.0.0/common/inet.c
--- dhcp-4.0.0/common/inet.c.port	2007-07-12 20:43:41.000000000 -1000
+++ dhcp-4.0.0/common/inet.c	2008-10-29 13:27:33.000000000 -1000
@@ -604,3 +604,20 @@ piaddrcidr(const struct iaddr *addr, uns
 	return ret;
 }
 
+/* Check the port number specified */
+u_int16_t
+validate_port(char *port) {
+	u_int16_t local_port = 0;
+	int lower = 1;
+	int upper = 65535;
+
+	errno = 0;
+	local_port = strtol(port, NULL, 10);
+	if ((errno == ERANGE) || (errno == EINVAL))
+		log_fatal ("Invalid port number specification: %s", port);
+
+	if (local_port < lower || local_port > upper)
+		log_fatal("Port number specified is out of range (%d-%d).", lower, upper);
+
+	return htons(local_port);
+}
diff -up dhcp-4.0.0/includes/dhcpd.h.port dhcp-4.0.0/includes/dhcpd.h
--- dhcp-4.0.0/includes/dhcpd.h.port	2008-10-29 13:27:33.000000000 -1000
+++ dhcp-4.0.0/includes/dhcpd.h	2008-10-29 13:28:57.000000000 -1000
@@ -64,6 +64,7 @@
 #endif
 
 #include <setjmp.h>
+#include <errno.h>
 
 #include "cdefs.h"
 #include "osdep.h"
@@ -2432,6 +2433,7 @@ isc_result_t free_iaddrcidrnetlist(struc
 const char *piaddr PROTO ((struct iaddr));
 char *piaddrmask(struct iaddr *, struct iaddr *);
 char *piaddrcidr(const struct iaddr *, unsigned int);
+u_int16_t validate_port(char *port);
 
 /* dhclient.c */
 extern int nowait;
diff -up dhcp-4.0.0/relay/dhcrelay.c.port dhcp-4.0.0/relay/dhcrelay.c
--- dhcp-4.0.0/relay/dhcrelay.c.port	2008-10-29 13:27:33.000000000 -1000
+++ dhcp-4.0.0/relay/dhcrelay.c	2008-10-29 13:27:33.000000000 -1000
@@ -141,7 +141,7 @@ main(int argc, char **argv) {
 		if (!strcmp (argv [i], "-p")) {
 			if (++i == argc)
 				usage ();
-			local_port = htons (atoi (argv [i]));
+			local_port = validate_port(argv[i]);
 			log_debug ("binding to user-specified port %d",
 			       ntohs (local_port));
 		} else if (!strcmp (argv [i], "-d")) {
diff -up dhcp-4.0.0/server/dhcpd.c.port dhcp-4.0.0/server/dhcpd.c
--- dhcp-4.0.0/server/dhcpd.c.port	2008-10-29 13:27:33.000000000 -1000
+++ dhcp-4.0.0/server/dhcpd.c	2008-10-29 13:27:33.000000000 -1000
@@ -257,15 +257,7 @@ main(int argc, char **argv) {
 		if (!strcmp (argv [i], "-p")) {
 			if (++i == argc)
 				usage ();
-			for (s = argv [i]; *s; s++)
-				if (!isdigit ((unsigned char)*s))
-					log_fatal ("%s: not a valid UDP port",
-					       argv [i]);
-			status = atoi (argv [i]);
-			if (status < 1 || status > 65535)
-				log_fatal ("%s: not a valid UDP port",
-				       argv [i]);
-			local_port = htons (status);
+			local_port = validate_port(argv[i]);
 			log_debug ("binding to user-specified port %d",
 			       ntohs (local_port));
 		} else if (!strcmp (argv [i], "-f")) {
@@ -438,7 +430,7 @@ main(int argc, char **argv) {
 	if (!local_port)
 	{
 		if ((s = getenv ("DHCPD_PORT"))) {
-			local_port = htons (atoi (s));
+			local_port = validate_port(s);
 			log_debug ("binding to environment-specified port %d",
 				   ntohs (local_port));
 		} else {