Difference between iptables default policy to `DROP` and inserting a seperate policy in input chain to DROP all connections

I'm trying to DROP all incoming connections to my server, except from particular networks. At the same time I want to keep all outgoing connections from my server to external network (any network over any protocol) open. It is working by making below two changes in INPUT chain:

Note: the testing was done with 3 servers (VMs) with IPs on 192.168.0.0/24 network, and all rules defined on server3 (IP: 192.168.0.3)

a. iptables -P INPUT DROP
b. iptables -A INPUT -s 192.168.0.0/24 -j ACCEPT
[root@server3 ~]# iptables -nvL
Chain INPUT (policy DROP 124 packets, 22308 bytes) pkts bytes target prot opt in out source destination 265 34603 ACCEPT all -- * * 192.168.0.0/24 0.0.0.0/0
Chain FORWARD (policy DROP 0 packets, 0 bytes) pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 433 packets, 34693 bytes) pkts bytes target prot opt in out source destination 

However, my first question:

1. When I defined the first rule above (changing the default policy for INPUT chain), it stopped all outgoing connections(ssh , ping) from my server(IP: 192.168.0.3) as well. Why is this happening, if my default OUTPUT chain policy is still accept, and I don't have any rules defined under OUTPUT chain?

I wanted to achieve same thing by not changing the default policy for INPUT chain, like this:

c. iptables -I INPUT -j DROP
d. iptables -A INPUT -s 192.168.0.0/24 -j ACCEPT

But it still blocks all incoming/outgoing connections to/from my server3. This brings me to my second question:

2. How are the rules c. and a. working differently ? Kindly help in understanding, as I'm new to linux firewalls.

1 Answer

If you have a default DROP INPUT policy, even the response packets from your outgoint connections will get dropped.

To accept those, add this input rule:

iptables -I INPUT 1 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

As for your last question, in your c and d example (assuming empty rules before those commands) you are setting a first rule drop everything and a second rule that would accept traffic from a certain network. IPTABLES grabs a match as soon as it can, so the first rule always matches (no condition set) so every rule after that won't execute. Exceptions to a rule must be defined before the rule.

In the first example -P INPUT DROP, you are setting a last rule that will catch whatever was not matched before, so any exception added will be executed before that default rule (-P).

-I inserts into a certain position (for example, in my previous command, I am setting the ESTABLISHED,RELATED rule to be the first so it matches no matter what you set after that.

-A appends to the rule list, so if will be matched just before the default.

If you want to achieve the same as the first example with explicit rules (like c and d), you should exchange possitions of those.