diff --git a/src/standard.c b/src/standard.c
index 0b16296..07aa5e4 100644
--- a/src/standard.c
+++ b/src/standard.c
@@ -10,6 +10,7 @@
  *
  */
 
+#include <ctype.h>
 #include <netdb.h>
 #include <stdlib.h>
 #include <string.h>
@@ -98,6 +99,24 @@
 	return 0;
 }
 
+/*
+ * Checks <name> for invalid characters. Valid chars are [A-Za-z0-9_:.-]. If an
+ * invalid character is found, a pointer to it is returned. If everything is
+ * fine, NULL is returned.
+ */
+const char *invalid_char(const char *name)
+{
+	if (!*name)
+		return name;
+
+	while (*name) {
+		if (!isalnum(*name) && *name != '.' && *name != ':' &&
+		    *name != '_' && *name != '-')
+			return name;
+		name++;
+	}
+	return NULL;
+}
 
 /*
  * converts <str> to a struct sockaddr_in* which is locally allocated.