Adding an extra url for nc6. The previous was failing.

pkgs/tools/networking/nc6/default.nix

···
   name = "nc6-1.0";
 
   src = fetchurl {
-    url = ftp://ftp.deepspace6.net/pub/ds6/sources/nc6/nc6-1.0.tar.bz2;
+    urls = [
+      ftp://ftp.deepspace6.net/pub/ds6/sources/nc6/nc6-1.0.tar.bz2
+      http://fossies.org/linux/privat/nc6-1.0.tar.bz2
+    ];
     sha256 = "01l28zv1yal58ilfnz6albdzqqxzsx3a58vmc14r9gv0bahffdgb";
   };