The purpose of the /etc/networks file

What is /etc/networks file? Also wondering what particular files do on Linux? One of those files we recently rediscovered during auditing is the /etc/networks file. For some reason it was always there, yet we never change it. When looking at the man page of networks(5) we learn its purpose (almost instantly): It translates between IP ranges and network names It is used for tools like netstat and route It only works on class A, B, or C networks It does […]

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Is your /etc/hosts file healthy?

Audit your /etc/hosts file The /etc/hosts file is one of the few files you will always find on a Linux system. It stores the ‘hosts’ database, and can be used to resolve between IP addresses and hostnames. Although the file is very simple structured, it is still common to see minor issues with name resolving on systems.¬†Guess what, your /etc/hosts file might be causing more trouble than you think. A regular check up won’t hurt. Order matters in name resolving […]

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Linux DNS Tuning for Performance and Resilience

DNS Configuration on Linux We often don’t realize the importance of DNS, or name resolving in our infrastructure. The impact when things go (slightly) wrong is huge. Time to have a good look at improving our DNS configuration. How DNS resolving works When your Linux system needs to know the IP address of a particular host, it will use gethostbyname(3) function. This will use the nsswitch configuration stored in /etc/nsswitch.conf. For the related hosts line, it will determine how to […]

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Linux Security Guide for Hardening IPv6

Linux Security Guide for Hardening IPv6 Version 6 of Internet Protocol is now 20+ years available. You would think it is widely available now, right? Not exactly. Still many internet providers don’t have it deployed for their customers. Hosting companies are not always eager to deploy it either. Mostly because of lacking knowledge. To get at east more knowledge shared on the security side of IPv6, we have crafted this guide. Hopefully it will be a practical guide for your […]

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List network interfaces on Linux

The network configuration is a common place to start during system configuration, security¬†audits, and troubleshooting. It can reveal useful information like MAC and IP addresses. This guide helps you to gather this information on Linux, including listing all available network interfaces and its details. Show network interfaces Linux Every Linux distribution is using its own way of configuring the network configuration details. Therefore, it is good to know which tools can be used to query these details in a generic […]

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Which Linux process is using a particular network port?

Most network related services have to open up a network socket, so they can start listening for incoming network requests. It is common to find the TCP or UDP being used as the main communication protocol. In this article, we will check what ports are used by which Linux process. Auditing processes and network services Find out what process is listening to a port Only one process can actively listen to a TCP or UDP port. We usually only discover […]

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Alternative for netstat: ss tool

Alternative for netstat System administrators and security professionals searching for listening ports on a server, are definitely familiar with the netstat command. However, newer distributions do not have the tool default installed anymore. Time to start using ss besides our beloved netstat command. ss Socket statistics, or ss for short, is an easy replacement command for netstat. One way to use it, is with parameters ss -aut -a: show listening and non-listening sockets -u: show UDP -t: show TCP [root@archlinux […]

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Filtering ARP traffic with Linux arptables

Filtering ARP traffic with Linux arptables Most Linux system administrators will be familiar with iptables on Linux. Less known is the arptables utility, which controls filtering arp packets. Installation The arptables utility is easy to set-up, as the main functionality is already implemented in the Linux kernel. Just install the arptables package on your favorite Linux distribution. Red Hat / CentOS / Fedora yum install arptables Debian / Ubuntu apt-get install arptables Configuration example To show the effect of filtering […]

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How to clear the ARP cache on Linux?

How to clear the ARP cache on Linux? In some cases, you might need to clear your ARP cache. There are two common ways on Linux, using the arp or ip utility. Depending on your Linux distribution, it might be preferred to use the ip utility. Clearing cache with arp The arp utility does not accept an option to clear the full cache. Instead, it allows to flush out entries found with the -d option. root@ubuntu:~# arp -d 192.168.1.1 After […]

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