diff --git a/README.md b/README.md
index c7d14db..51baf2a 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,13 @@ HOWTO use.
 Run the desired DROP script for the networks you would like to DROP or
 run `DROP-all` to DROP all the networks.
 
+The facebook script uses `whois` to get their networks, so that needs
+to be installed.
+
+```
+sudo apt install whois
+```
+
 The output of the scripts get written to /tmp.
 Copy them from there into your main firewall rules...