Databases, Systems & Networks

You would like to block outgoing access to particular remote host/ip or port for all or selected service/port. In this quick tutorial I will explain how to use iptables to block outgoing access.

Block Access To Outgoing IP Address

The following rule will block ip address 202.54.1.22 from making any outgoing connection:

iptables -A OUTPUT -d 202.54.1.22 -j DROP

The above will block chat server ip address or site having dangerous contains such as viruses or malware.

Block Access To Outgoing IP TCP / UDP Port Number

To block specific port number such tcp port # 5050, enter:
iptables -A OUTPUT -p tcp --dport 5050 -j DROP

To block tcp port # 5050 for an IP address 192.168.1.2 only, enter:
iptables -A OUTPUT -p tcp -d 192.168.1.2 --dport 5050 -j DROP

Finally, you need to save your firewall rules. Under CentOS / RHEL / Fedora Linux, enter:
# /sbin/service iptables save
OR
# /etc/init.d/iptables save

Source: Mellowhost

Firewalls

Iptables Examples For New SysAdmins

Linux comes with a host based firewall called Netfilter. According to the official project site:

netfilter is a set of hooks inside the Linux kernel that allows kernel modules to register callback functions with the network stack. A registered callback function is then called back for every packet that traverses the respective hook within the network stack.

This Linux based firewall is controlled by the program called iptables to handles filtering for IPv4, and ip6tables handles filtering for IPv6. I strongly recommend that you first read our quick tutorial that explains how to configure a host-based firewall called Netfilter (iptables) under CentOS / RHEL / Fedora / Redhat Enterprise Linux. This post list most common iptables solutions required by a new Linux user to secure his or her Linux operating system from intruders.

Linux

IPTABLES Rules Example

• Most of the actions listed in this post are written with the assumption that they will be executed by the root user running the bash or any other modern shell. Do not type commands on remote system as it will disconnect your access.
• For demonstration purpose I’ve used RHEL 6.x, but the following command should work with any modern Linux distro.
• This is NOT a tutorial on how to set iptables. See tutorial here. It is a quick cheat sheet to common iptables commands.

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1. Testing ICMP:

In this example hping3 will behave like a normal ping utility, sending ICMP-echo und receiving ICMP-reply

hping3 -1 0daysecurity.com

2. Traceroute using ICMP:

This example is similar to famous utilities like tracert (Windows) or traceroute (Linux) who uses ICMP packets increasing every time in 1 its TTL value.

hping3 --traceroute -V -1 0daysecurity.com

3. Checking port:

Here hping3 will send a SYN packet to a specified port (80 in our example). We can control also from which local port will start the scan (5050).

hping3 -V -S -p 80 -s 5050 0daysecurity.com

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Hping

Wikipedia defines hping as :

On ubuntu hping can be installed from synaptic manager.

$ sudo apt-get install hping3

Syn flood

To send syn packets use the following command at terminal

$ sudo hping3 -i u1 -S -p 80 192.168.1.1

The above command would send TCP SYN packets to 192.168.1.1
sudo is necessary since the hping3 create raw packets for the task , for raw sockets/packets root privilege is necessary on Linux.

S – indicates SYN flag
p 80 – Target port 80
i u1 – Wait for 1 micro second between each packet

More options

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