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

Source: munin-monitoring.org
There are a number of situations where you’d like to run munin-node on hosts not directly available to the Munin server. This article describes a few scenarios and different alternatives to set up monitoring. Monitoring hosts behind a non-routing server.

In this scenario, a *nix server sits between the Munin server and one or more Munin nodes. The server in-between reaches both the Munin server and the Munin node, but the Munin server does not reach the Munin node or vice versa.

To enable for Munin monitoring, there are several approaches, but mainly either using SSH tunneling or “bouncing” via the in-between server.

Bouncing

This workaround uses netcat and inetd/xinetd to forward the queries from the Munin server. All incoming connections to defined ports are automatically forwarded to the Munin node using netcat.

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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