diff --git a/pkgs/applications/networking/browsers/chromium/source/default.nix b/pkgs/applications/networking/browsers/chromium/source/default.nix
index 12a691db633..74cc692919e 100644
--- a/pkgs/applications/networking/browsers/chromium/source/default.nix
+++ b/pkgs/applications/networking/browsers/chromium/source/default.nix
@@ -4,7 +4,10 @@
 }:
 
 with stdenv.lib;
-with getAttr channel (import ./sources.nix);
+
+with (import ./update.nix {
+  inherit (stdenv) system;
+}).getChannel channel;
 
 stdenv.mkDerivation {
   name = "chromium-source-${version}";
diff --git a/pkgs/applications/networking/browsers/chromium/source/sources.nix b/pkgs/applications/networking/browsers/chromium/source/sources.nix
index 49b5e2b4221..dceab16d38c 100644
--- a/pkgs/applications/networking/browsers/chromium/source/sources.nix
+++ b/pkgs/applications/networking/browsers/chromium/source/sources.nix
@@ -2,17 +2,14 @@
 {
   dev = {
     version = "35.0.1883.0";
-    url = "http://commondatastorage.googleapis.com/chromium-browser-official/chromium-35.0.1883.0.tar.xz";
     sha256 = "0qbv6prxl18y5824pfd13ng9798g561gzb6nypwp502hqr45jvb6";
   };
   beta = {
     version = "34.0.1847.60";
-    url = "http://commondatastorage.googleapis.com/chromium-browser-official/chromium-34.0.1847.60.tar.xz";
     sha256 = "1na5d6z4a0wkabn7cj62vyiv3mmvcb6qdvrkyy6fj79h7gk2hb7k";
   };
   stable = {
     version = "34.0.1847.116";
-    url = "http://commondatastorage.googleapis.com/chromium-browser-official/chromium-34.0.1847.116.tar.xz";
     sha256 = "04cpfav5rqa117igvzmrw0045r2ljxg5fqb46qgqvkgff30pjrfx";
   };
 }
diff --git a/pkgs/applications/networking/browsers/chromium/source/update.nix b/pkgs/applications/networking/browsers/chromium/source/update.nix
new file mode 100644
index 00000000000..841dfcccb54
--- /dev/null
+++ b/pkgs/applications/networking/browsers/chromium/source/update.nix
@@ -0,0 +1,58 @@
+{ system ? builtins.currentSystem }:
+
+let
+  inherit (import <nixpkgs> {}) lib writeText;
+
+  sources = if builtins.pathExists ./sources.nix
+            then import ./sources.nix
+            else null;
+
+  bucketURL = "http://commondatastorage.googleapis.com/"
+            + "chromium-browser-official";
+
+  tryChannel = channel: let
+    chanAttrs = builtins.getAttr channel sources;
+  in if sources != null then ''
+    oldver="${chanAttrs.version}";
+    echo -n "Checking if $oldver ($channel) is up to date..." >&2;
+    if [ "x$(get_newest_ver "$version" "$oldver")" != "x$oldver" ];
+    then
+      echo " no, getting sha256 for new version $version:" >&2;
+      sha256="$(nix-prefetch-url "$url")" || return 1;
+    else
+      echo " yes, keeping old sha256." >&2;
+      sha256="${chanAttrs.sha256}";
+    fi;
+  '' else ''
+    sha256="$(nix-prefetch-url "$url")" || return 1;
+  '';
+
+  caseChannel = channel: ''
+    ${channel}) ${tryChannel channel};;
+  '';
+
+in rec {
+  getChannel = channel: let
+    chanAttrs = builtins.getAttr channel sources;
+  in {
+    url = "${bucketURL}/chromium-${chanAttrs.version}.tar.xz";
+    inherit (chanAttrs) version sha256;
+  };
+
+  updateHelpers = writeText "update-helpers.sh" ''
+    get_sha256()
+    {
+      channel="$1";
+      version="$2";
+      url="${bucketURL}/chromium-$version.tar.xz";
+
+      case "$channel" in
+        ${lib.concatMapStrings caseChannel [ "stable" "dev" "beta" ]}
+      esac;
+
+      sha_insert "$version" "$sha256";
+      echo "$sha256";
+      return 0;
+    }
+  '';
+}
diff --git a/pkgs/applications/networking/browsers/chromium/source/update.sh b/pkgs/applications/networking/browsers/chromium/source/update.sh
index 0c4881bb396..a2e1e268f4f 100755
--- a/pkgs/applications/networking/browsers/chromium/source/update.sh
+++ b/pkgs/applications/networking/browsers/chromium/source/update.sh
@@ -3,16 +3,9 @@
 channels_url="http://omahaproxy.appspot.com/all?csv=1";
 history_url="http://omahaproxy.appspot.com/history";
 bucket_url="http://commondatastorage.googleapis.com/chromium-browser-official/";
-output_file="$(cd "$(dirname "$0")" && pwd)/sources.nix";
+base_path="$(cd "$(dirname "$0")" && pwd)";
 
-nix_getattr()
-{
-    input_file="$1";
-    attr="$2";
-
-    var="$(nix-instantiate --eval-only -A "$attr" "$output_file")";
-    echo "$var" | tr -d '\\"';
-}
+source "$(nix-build --no-out-link "$base_path/update.nix" -A updateHelpers)";
 
 ### poor mans key/value-store :-) ###
 
@@ -53,39 +46,6 @@ get_newest_ver()
     fi;
 }
 
-if [ -e "$output_file" ];
-then
-    get_sha256()
-    {
-        channel="$1";
-        version="$2";
-        url="$3";
-
-        oldver="$(nix_getattr "$output_file" "$channel.version")";
-
-        echo -n "Checking if $oldver ($channel) is up to date..." >&2;
-
-        if [ "x$(get_newest_ver "$version" "$oldver")" != "x$oldver" ];
-        then
-            echo " no, getting sha256 for new version $version:" >&2;
-            sha256="$(nix-prefetch-url "$url")" || return 1;
-        else
-            echo " yes, keeping old sha256." >&2;
-            sha256="$(nix_getattr "$output_file" "$channel.sha256")" \
-                || return 1;
-        fi;
-
-        sha_insert "$version" "$sha256";
-        echo "$sha256";
-        return 0;
-    }
-else
-    get_sha256()
-    {
-        nix-prefetch-url "$3";
-    }
-fi;
-
 fetch_filtered_history()
 {
     curl -s "$history_url" | sed -nr 's/^'"linux,$1"',([^,]+).*$/\1/p';
@@ -99,9 +59,8 @@ get_prev_sha256()
     for version in $(fetch_filtered_history "$channel");
     do
         [ "x$version" = "x$current_version" ] && continue;
-        url="${bucket_url%/}/chromium-$version.tar.xz";
-        sha256="$(get_sha256 "$channel" "$version" "$url")" || continue;
-        echo "$sha256:$version:$url";
+        sha256="$(get_sha256 "$channel" "$version")" || continue;
+        echo "$sha256:$version";
         return 0;
     done;
 }
@@ -113,25 +72,21 @@ get_channel_exprs()
         channel="${chline%%,*}";
         version="${chline##*,}";
 
-        url="${bucket_url%/}/chromium-$version.tar.xz";
-
         echo -n "Checking if sha256 of version $version is cached..." >&2;
         if sha256="$(sha_lookup "$version")";
         then
             echo " yes: $sha256" >&2;
         else
             echo " no." >&2;
-            sha256="$(get_sha256 "$channel" "$version" "$url")";
+            sha256="$(get_sha256 "$channel" "$version")";
             if [ $? -ne 0 ];
             then
                 echo "Whoops, failed to fetch $version, trying previous" \
                      "versions:" >&2;
 
-                sha_ver_url="$(get_prev_sha256 "$channel" "$version")";
-                sha256="${sha_ver_url%%:*}";
-                ver_url="${sha_ver_url#*:}";
-                version="${ver_url%%:*}";
-                url="${ver_url#*:}";
+                sha_ver="$(get_prev_sha256 "$channel" "$version")";
+                sha256="${sha_ver%:*}";
+                version="${sha_ver#*:}";
             fi;
         fi;
 
@@ -139,7 +94,6 @@ get_channel_exprs()
 
         echo "  $channel = {";
         echo "    version = \"$version\";";
-        echo "    url = \"$url\";";
         echo "    sha256 = \"$sha256\";";
         echo "  };";
     done;
@@ -151,7 +105,7 @@ omaha="$(curl -s "$channels_url")";
 versions="$(echo "$omaha" | sed -nr -e 's/^linux,([^,]+,[^,]+).*$/\1/p')";
 channel_exprs="$(get_channel_exprs "$versions")";
 
-cat > "$output_file" <<-EOF
+cat > "$base_path/sources.nix" <<-EOF
 # This file is autogenerated from update.sh in the same directory.
 {
 $channel_exprs