diff --git a/pkgs/build-support/fetchurl/default.nix b/pkgs/build-support/fetchurl/default.nix
index 385a0a4c650c..e4a4694404ea 100644
--- a/pkgs/build-support/fetchurl/default.nix
+++ b/pkgs/build-support/fetchurl/default.nix
@@ -23,4 +23,10 @@ stdenv.mkDerivation {
       if sha256 != "" then sha256 else if sha1 != "" then sha1 else md5;
   
   inherit url;
+
+  # We borrow these environment variables from the caller to allow
+  # easy proxy configuration.  This is impure, but a fixed-output
+  # derivation like fetchurl is allowed to do so since its result is
+  # by definition pure.
+  impureEnvVars = ["http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"];
 }