Firewall – Check if IP address is allowed through firewall

Advantages of firewall:

1. You can filter outbound traffic.
2. Layer 7 firewalls (IPS) can protect against known application vulnerabilities.
3. You can block a certain IP address range and/or port centrally rather than trying to ensure that there is no service listening on that port on each individual machine or denying access using TCP Wrappers.
4. Firewalls can help if you have to deal with less security aware users/administrators as they would provide second line of defence. Without them one has to be absolutely sure that hosts are secure, which requires good security understanding from all administrators.
5. Firewall logs would provide central logs and help in detecting vertical scans. Firewall logs can help in determining whether some user/client is trying to connect to same port of all your servers periodically. To do this without a firewall one would have to combine logs from various servers/hosts to get a centralized view.
6. Firewalls also come with anti-spam / anti-virus modules which also add to protection.
7. OS independent security. Based on host OS, different techniques / methods are required to make the host secure. For example, TCP Wrappers may not be available on Windows machines.

Above all this if you do not have firewall and system is compromised then how would you detect it? Trying to run some command 'ps', 'netstat', etc. on local system can't be trusted as those binaries can be replaced. 'nmap' from a remote system is not guaranteed protection as an attacker can ensure that root-kit accepts connections only from selected source IP address(es) at selected times.

Hardware firewalls help in such scenarios as it is extremely difficult to change firewall OS/files as compared to host OS/files.

Disadvantages of firewall:

1. People feel that firewall will take care of security and do not update systems regularly and stop unwanted services.
2. They cost. Sometimes yearly license fee needs to be paid. Especially if the firewall has anti-virus and anti-spam modules.
3. Additional single point of failure. If all traffic passes through a firewall and the firewall fails then network would stop. We can have redundant firewalls, but then previous point on cost gets further amplified.
4. Stateful tracking provides no value on public-facing systems that accept all incoming connections.
5. Stateful firewalls are a massive bottleneck during a DDoS attack and are often the first thing to fail, because they attempt to hold state and inspect all incoming connections.
6. Firewalls cannot see inside encrypted traffic. Since all traffic should be encrypted end-to-end, most firewalls add little value in front of public servers. Some next-generation firewalls can be given private keys to terminate TLS and see inside the traffic, however this increases the firewall's susceptibility to DDoS even more, and breaks the end-to-end security model of TLS.
7. Operating systems and applications are patched against vulnerabilities much more quickly than firewalls. Firewall vendors often sit on known issues for years without patching, and patching a firewall cluster typically requires downtime for many services and outbound connections.
8. Firewalls are far from perfect, and many are notoriously buggy. Firewalls are just software running on some form of operating system, perhaps with an extra ASIC or FPGA in addition to a (usually slow) CPU. Firewalls have bugs, but they seem to provide few tools to address them. Therefore firewalls add complexity and an additional source of hard-to-diagnose errors to an application stack.

I note that you've done a great job tying down several different daemons, and from what you've said I think it unlikely that you'll expose yourself to trouble through those services you have already secured. This still leaves you in a "everything is permitted except that which I have forbidden" state, and you can't get out of that state by hunting down daemon after daemon and securing them one by one.

A firewall configured to DENY ANY ANY by default moves you to a "everything is forbidden except that which is permitted" mode of operation, and I have found over many years that they're better.

Right now, given a legitimate user with a legitimate shell on your system, she could decide to run some local unprivileged daemon for proxying web requests for the internet, or start file sharing on port 4662, or accidentally open up a listener by using -g with ssh port tunneling, not understanding what it does; or a sendmail install could leave you running an MUA on port 587 which was improperly configured despite all the work you'd done on securing the MTA sendail on port 25; or a hundred and one things could happen that bypass your careful and thoughtful security simply because they weren't around when you were thinking carefully about what to forbid.

Do you see my point? At the moment, you've put a lot of effort into securing all the things you know about, and it sounds like they won't bite you. What may bite you is the things you don't know about, or that aren't even there, right now.

A firewall which defaults to DENY ANY ANY is the sysadmin way of saying that if something new comes along and opens up a network listener on this server, noone will be able to talk to it until I have given explicit permission.