--- a/sys/libkern/inet_ntop.c
+++ b/sys/libkern/inet_ntop.c
@@ -21,6 +21,7 @@
 #include <sys/param.h>
 #include <sys/socket.h>
 #include <sys/systm.h>
+#include <sys/errno.h>
 
 #include <netinet/in.h>
 
@@ -49,6 +50,7 @@
 	case AF_INET6:
 		return (inet_ntop6(src, dst, size));
 	default:
+		errno = EAFNOSUPPORT;
 		return (NULL);
 	}
 	/* NOTREACHED */
@@ -74,6 +76,7 @@
 
 	l = ksnprintf(tmp, sizeof(tmp), fmt, src[0], src[1], src[2], src[3]);
 	if (l <= 0 || (socklen_t) l >= size) {
+		errno = ENOSPC;
 		return (NULL);
 	}
 	strlcpy(dst, tmp, size);
@@ -155,8 +158,10 @@
 		if (i == 6 && best.base == 0 && (best.len == 6 ||
 		    (best.len == 7 && words[7] != 0x0001) ||
 		    (best.len == 5 && words[5] == 0xffff))) {
-			if (!inet_ntop4(src+12, tp, sizeof tmp - (tp - tmp)))
+			if (!inet_ntop4(src+12, tp, sizeof tmp - (tp - tmp))) {
+				errno = ENOSPC;
 				return (NULL);
+			}
 			tp += strlen(tp);
 			break;
 		}
@@ -172,6 +177,7 @@
 	 * Check for overflow, copy, and we're done.
 	 */
 	if ((socklen_t)(tp - tmp) > size) {
+		errno = ENOSPC;
 		return (NULL);
 	}
 	strcpy(dst, tmp);