What considerations would I need when configuring iptables?

Synopsis

I am looking at learning more about Server Administration. I have been running my own web server for a few years although many things were set-up by my friend and now I wish to learn more about these tasks.

I'm mainly curious about Security, Network security and firewalls and would love to find a good resource/book to learn more into this.

Nevertheless, my intentions are:

• To block all incoming connections; except specific ports


• To understand any security related tasks I need to research

I have a short list of ports to allow incoming connections for:

$ iptables -A INPUT -p icmp -j ACCEPT            # allows anyone to ping?
$ iptables -A INPUT -p tcp --dport 22 -j ACCEPT  # allow public ssh
$ iptables -A INPUT -p tcp --dport 80 -j ACCEPT  # allow public apache
$ iptables -A INPUT -p tcp --dport 443 -j ACCEPT  # allow public apache (ssl)
$ iptables -A INPUT -p tcp --dport 9001 -j ACCEPT  # allow a public service
$ iptables -A INPUT -j DROP                      # default drop

My list of rules above theoretically seem secure enough to block everything except 22, 80, 443 and 9001 - but when these rules are active I cannot ssh into my server. Then I flush the rules (iptables -F) and I can ssh again.