Add IP address range blocker script

pre-receive-hooks/block_ip_range.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+
+#
+# Pre-receive hook that will reject all pushes received from IP addresses
+# between `IP_LOW` and `IP_HIGH`
+#
+# More details on pre-receive hooks and how to apply them can be found on
+# https://help.github.com/enterprise/admin/guides/developer-workflow/managing-pre-receive-hooks-on-the-github-enterprise-appliance/
+#
+# NOTE: Use at your own risk!
+
+function ip2dec {
+  # see http://stackoverflow.com/a/10768196/1525223
+  local a b c d ip=$@
+  IFS=. read -r a b c d <<< "$ip"
+  echo "$((a * 256 ** 3 + b * 256 ** 2 + c * 256 + d))"
+}
+
+# define lower IP limit
+IP_LOW="0.0.0.0"
+IP_LOW_INT=$(ip2dec "${IP_LOW}")
+
+# define upper IP limit
+IP_HIGH="255.255.255.255"
+IP_HIGH_INT=$(ip2dec "${IP_HIGH}")
+
+# get IP from pre-receive hook variable
+IP_IN="${GITHUB_USER_IP}"
+IP_INT=$(ip2dec "${IP_IN}")
+
+# reject push if `IP_IN` is between `IP_LOW` and IP_HIGH
+if [ "${IP_INT}" -ge "${IP_LOW_INT}" ] && [ "${IP_INT}" -le "${IP_HIGH_INT}" ]; then
+  echo "Hello there! We have restricted pushes from your IP (${IP_IN}) address. Please see Dave in IT to discuss alternatives."
+  exit 1
+fi
+
+exit 0