dns Archives - Networking HowTos

Get The Active DNS Servers On Linux

June 1, 2017 CentOS, Linux, Networking, Ubuntu

The Domain Name System (DNS for short) is one of the most important parts of the internet infrastructure. It converts the easy to remember domain names, into IP (and IPv6) addresses, that aren’t so friendly to the end user.
Regardless of the distribution of Linux that you are using (such as Ubuntu, CentOS, Debian, Arch, Redhat, etc), the process of determining what DNS servers are currently being used for domain name resolution is the same.
To determine what DNS servers are being used, you simply need to view the contents of the “/etc/resolv.conf” file. This can be done via a graphical editing tool such as gedit, or can easily be viewed from the command line with a simple “cat” of the file, to show the contents. The command below will outline how to determine the DNS servers in use.
Show the active DNS servers on Linux:

$ cat /etc/resolv.conf

You will end up with some output like this:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 8.8.8.8
nameserver 8.8.4.4

You may or may not have the comments at the top of the file, as that will depend on your distribution, but you should have at least one or more “nameserver” records. These will be the name server IP’s that your system will currently be using. In this example, 8.8.8.8 and 8.8.4.4.
Note that this must be an IP address, and cant be a domain name.

Prevent DNS Amplification Attacks with BIND

August 5, 2013 Linux

DNS amplification attacks are a common form of DDoS that makes used of misconfigured DNS servers on the internet. The attack involves sending a request to the misconfigured DNS server, with a spoofed source IP address, so the response goes back to a third party (the target/victim). The attacker will use the largest DNS query possible, so that more data is sent to the target IP address, flooding their connection. To help prevent these attacks, always make sure your DNS servers are set up to block recursive queries from the general public.
Restrict recursion to authorized networks:
Edit the bind configuration file (usually named.conf), and set up a new ACL. Specify the networks you wish to allow recursion.
Eg:

acl internalnetworks {
    127.0.0.1/32;
    192.168.0.0/24;
    10.0.0.0/8;
};

In this example, we are allowing 127.0.0.1, 192.168.0.0/24 and 10.0.0.0/8.
Once done, set up the options section to allow recursion only from the ACL set up above.

options {
    allow-query {
        any;
    };
    allow-recursion {
        internalnetworks;
    };
};

Save the file, and restart BIND to make these changes take affect.

Install the dig DNS lookup utility on CentOS

July 23, 2013 CentOS, Linux

The dig DNS lookup utility is a handy tool for performing DNS queries. The dig utility is part of the bind utilities package in CentOS. The following command can be used to install dig.
Install dig (the bind-utils package):

# yum install bind-utils

Note: Make sure you run the above command as a user with root privledges.
You should now be able to use dig to perform DNS lookups. See ‘man dig’ for more information.

Flush the DNS cache on Windows

February 7, 2013 Microsoft, Windows

Windows will cache DNS resolution results for a period of time. You may need to clear this cache if you need to make sure the DNS value is up to date. The command below can be used to flush the DNS cache on a Windows machine.
This can he useful if you are changing entries in the hosts file and you need to clear out any cached DNS resolution queries.
Clear the DNS cache on Windows:
Open the command prompt.
Run the following command:

ipconfig /flushdns

Check the reverse dns hostname for a IP on Linux

February 1, 2013 Linux

This guide will step you through how to take an IP address, and find out what its reverse dns host name is. This is useful for checking mail server DNS setup, etc.
The DNS system works two ways. It is used to convert host names to IP addresses, but it can also convert IP addresses to host names.
Two handy programs to perform DNS queries on Linux are ‘dig’ and ‘host’. If either of these aren’t installed on your system, they should be available in your distributions repositories.
Using ‘dig’:

$ dig -x 8.8.8.8 +short

Example:

$ dig -x 8.8.8.8 +short
google-public-dns-a.google.com.
$

Using ‘host’:

$ host 8.8.8.8

Example:

$ host 8.8.8.8
8.8.8.8.in-addr.arpa domain name pointer google-public-dns-a.google.com.
$