iptable rules to allow outgoing DNS lookups, outgoing icmp (ping) requests, outgoing connections to configured package servers, outgoing connections to all ips on port 22, all incoming connections to port 22, 80 and 443 and everything on localhost

iptables.sh

#!/bin/bash
IPT="/sbin/iptables"

# Server IP
SERVER_IP="$(ip addr show eth0 | grep 'inet ' | cut -f2 | awk '{ print $2}')"

# Your DNS servers you use: cat /etc/resolv.conf
DNS_SERVER="8.8.4.4 8.8.8.8"

# Allow connections to this package servers
PACKAGE_SERVER="ftp.us.debian.org security.debian.org"

echo "flush iptable rules"
$IPT -F
$IPT -X
$IPT -t nat -F
$IPT -t nat -X
$IPT -t mangle -F
$IPT -t mangle -X

echo "Set default policy to 'DROP'"
$IPT -P INPUT   DROP
$IPT -P FORWARD DROP
$IPT -P OUTPUT  DROP

## This should be one of the first rules.
## so dns lookups are already allowed for your other rules
for ip in $DNS_SERVER
do
	echo "Allowing DNS lookups (tcp, udp port 53) to server '$ip'"
	$IPT -A OUTPUT -p udp -d $ip --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
	$IPT -A INPUT  -p udp -s $ip --sport 53 -m state --state ESTABLISHED     -j ACCEPT
	$IPT -A OUTPUT -p tcp -d $ip --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
	$IPT -A INPUT  -p tcp -s $ip --sport 53 -m state --state ESTABLISHED     -j ACCEPT
done

echo "allow all and everything on localhost"
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT

for ip in $PACKAGE_SERVER
do
	echo "Allow connection to '$ip' on port 21"
	$IPT -A OUTPUT -p tcp -d "$ip" --dport 21  -m state --state NEW,ESTABLISHED -j ACCEPT
	$IPT -A INPUT  -p tcp -s "$ip" --sport 21  -m state --state ESTABLISHED     -j ACCEPT

	echo "Allow connection to '$ip' on port 80"
	$IPT -A OUTPUT -p tcp -d "$ip" --dport 80  -m state --state NEW,ESTABLISHED -j ACCEPT
	$IPT -A INPUT  -p tcp -s "$ip" --sport 80  -m state --state ESTABLISHED     -j ACCEPT

	echo "Allow connection to '$ip' on port 443"
	$IPT -A OUTPUT -p tcp -d "$ip" --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT
	$IPT -A INPUT  -p tcp -s "$ip" --sport 443 -m state --state ESTABLISHED     -j ACCEPT
done


#######################################################################################################
## Global iptable rules. Not IP specific

echo "Allowing new and established incoming connections to port 21, 80, 443"
$IPT -A INPUT  -p tcp -m multiport --dports 21,80,443 -m state --state NEW,ESTABLISHED -j ACCEPT
$IPT -A OUTPUT -p tcp -m multiport --sports 21,80,443 -m state --state ESTABLISHED     -j ACCEPT

echo "Allow all outgoing connections to port 22"
$IPT -A OUTPUT -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
$IPT -A INPUT  -p tcp --sport 22 -m state --state ESTABLISHED     -j ACCEPT

echo "Allow outgoing icmp connections (pings,...)"
$IPT -A OUTPUT -p icmp -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
$IPT -A INPUT  -p icmp -m state --state ESTABLISHED,RELATED     -j ACCEPT

echo "Allow outgoing connections to port 123 (ntp syncs)"
$IPT -A OUTPUT -p udp --dport 123 -m state --state NEW,ESTABLISHED -j ACCEPT
$IPT -A INPUT  -p udp --sport 123 -m state --state ESTABLISHED     -j ACCEPT

# Log before dropping
$IPT -A INPUT  -j LOG  -m limit --limit 12/min --log-level 4 --log-prefix 'IP INPUT drop: '
$IPT -A INPUT  -j DROP

$IPT -A OUTPUT -j LOG  -m limit --limit 12/min --log-level 4 --log-prefix 'IP OUTPUT drop: '
$IPT -A OUTPUT -j DROP

exit 0

When I enter these rules:
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT
the iptables -L -n shows that the following line for both INPUT and OUTPUT chain
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
This would actually mean that it accept all input and output. Therefore, following rules would not be ignored. If I removed those two commands, I can't perform DNS resolution and hence, no internet. Can you help to clarify this?

Greetings!
Good script, Thank You!
Some remarks about "SERVER_IP" variable.
1). This is unused var.
2). The eth0 name is only one of possible names. It may be for example enp0s31f6.

Line 69 should be
$IPT -A OUTPUT -p icmp -m state --state NEW -j ACCEPT

Why is forward policy drop?

Why is forward policy drop?

Assuming you're not a router. Besides, drop all and allow what you need.

as state in the documentation:

Please note that specifying any name to be resolved with a remote query such as DNS is a really bad idea

why we are looping ips instead we can provide ips in comma-separated right? It reduces multiple command execution.

can anyone suggest to me, how to allow only apt-get/apt command in my cloud server for outbound traffic?