diff -up vsftpd-2.0.5/sysutil.c.underscore_uname vsftpd-2.0.5/sysutil.c
--- vsftpd-2.0.5/sysutil.c.underscore_uname	2007-11-30 13:14:50.000000000 +0100
+++ vsftpd-2.0.5/sysutil.c	2007-11-30 13:12:25.000000000 +0100
@@ -905,7 +905,7 @@ vsf_sysutil_isprint(int the_char)
 int
 vsf_sysutil_isalnum(int the_char)
 {
-  return isalnum(the_char);
+  return isalnum(the_char) || the_char == '_' || the_char == '.';
 }
 
 int