Add firewalld to doc/recipes.md

doc/recipes.md

@@ -45,16 +45,29 @@ but then reserved ports would be accessible by any user than can execute sōzu (
 could setup of TCP proxy for SSH, SMTP etc to their own software).
 The unit file is the recommended way.
 
-## iptables
+## Using unprivileged ports
 
-iptables can be used to route connections to reserved ports to other unprivileged
-ports. You can set it up as follows for 80 -> 8080 and 443 -> 8443 redirections:
+Different firewalls can be used to route connections from reserved ports to other unprivileged ports.
+Most common redirections follow 80 -> 8080 and 443 -> 8443.
+
+### iptables
+
+iptables can be utilized, using a simple nat.
 
 ```
 iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-ports 8080
 iptables -t nat -A PREROUTING -p tcp --dport 443 -j REDIRECT --to-ports 8443
 ```
 
+### firewalld
+
+firewalld's syntax is very similiar to iptables. It can be made permanent using `--permanent`.
+
+```
+firewall-cmd --direct --add-rule ipv4 nat PREROUTING 0 -p tcp --dport 80 -j REDIRECT --to-port 8080
+firewall-cmd --direct --add-rule ipv4 nat PREROUTING 0 -p tcp --dport 443 -j REDIRECT --to-port 8443
+```
+
 Note that any software running under the same uid as sōzu will be able to listen on
 the 8080 and 8443 ports, because those ports are unprivileged and sōzu sets up
 listen socket with the `SO_REUSEPORT` option.