##1. Create the anchor file:
sodu vim /etc/pf.anchors/com.liuxingruo
Inside the anchor file, enter:
rdr pass on lo0 inet proto tcp from any to self port 80 -> 127.0.0.1 port 9191
rdr pass on en0 inet proto tcp from any to any port 80 -> 127.0.0.1 port 9191
rdr pass on en1 inet proto tcp from any to any port 80 -> 127.0.0.1 port 9191