I was accessing a server running Ubuntu 12.04 Server using SSH and managed to lock myself out of it. I'm still wondering how that happened:

1. The firewall was enabled by default;
2. sudo ufw status did not show any rules (but I could SSH to the server normally);

3. I tried explicitly allowing ports 80 and 443 using the commands:

   sudo ufw allow 80
   sudo ufw allow 443
   

4. sudo ufw status now showed something like:

   Status: active
   
   To                         Action      From
   --                         ------      ----
   80                         ALLOW       Anywhere
   80                         ALLOW       Anywhere (v6)
   443                        ALLOW       Anywhere
   443                        ALLOW       Anywhere (v6)
   

   (Recalling from memory and seeing some examples; I can't access the server to see the exact output, so I might be mistaken)

5. After logging out of SSH, now I can't log in anymore (connection timeout).

What just happened? There were no DENY rules previously (AFAIK), neither I introduced any. How could SSH be previously available and now it's not? Does ufw (or more precisely iptables) allow everything by default, unless you explicitly allow something, then it denies everything by default? Or did I do something wrong, that broke the existing rules somehow?