Dns Configuration In Centos 7

DNS, or Domain Name System, is a critical component of the internet’s infrastructure, responsible for translating human-friendly domain names into IP addresses that computers use to identify each other on the network. In CentOS 7, configuring DNS correctly is essential for ensuring that your system can communicate with other servers and services on the internet or within a private network.

The Role of DNS in Network Communication

DNS acts as the phonebook of the internet, allowing users to access websites using domain names instead of numerical IP addresses. For instance, when you type www.example.com into your browser, DNS servers translate that domain into an IP address like 192.0.2.1, directing your request to the correct server.

DNS Configuration Files in CentOS 7

In CentOS 7, the primary configuration file for setting up DNS clients is /etc/resolv.conf. This file contains information about the nameservers that the system should query to resolve domain names. Additionally, the nsswitch.conf file dictates the order in which services are queried for name resolution.

Setting Up a DNS Client on CentOS 7

Configuring a CentOS 7 system as a DNS client involves specifying the DNS servers that it should use for name resolution. This process ensures that your system can find its way around the network by knowing where to send DNS queries.

Editing /etc/resolv.conf

To set up a DNS client, edit the /etc/resolv.conf file to include the IP addresses of your preferred DNS servers:

nameserver 8.8.8.8
nameserver 8.8.4.4

These entries point to Google’s public DNS servers, but you can replace them with any DNS servers of your choice.

Using Network Manager for DNS Configuration

CentOS 7 typically uses NetworkManager, which may overwrite manual changes to /etc/resolv.conf. To make persistent changes, use the NetworkManager command-line interface (nmcli) or graphical user interface (nmtui).

Installing and Configuring BIND as a DNS Server

BIND (Berkeley Internet Name Domain) is the most widely used DNS software on the internet. On CentOS 7, you can configure BIND to serve as your own DNS server, providing greater control over domain resolution.

Installing BIND Packages

First, install the necessary packages using yum:

yum install bind bind-utils

Configuring named.conf

The main configuration file for BIND is /etc/named.conf. Here, you define global options and specify zones that BIND will be authoritative for.

options {
    listen-on port 53 { 127.0.0.1; };
    directory   "/var/named";
    dump-file   "/var/named/data/cache_dump.db";
    ...
};

zone "example.com" IN {
    type master;
    file "named.example.com";
    allow-update { none; };
};

This example sets BIND to listen on localhost and defines a zone for example.com.

Creating Zone Files

For each zone declared in named.conf, create a corresponding zone file under /var/named. This file contains DNS records such as A, CNAME, and MX.

$TTL 86400
@   IN  SOA     ns1.example.com. admin.example.com. (
                2023010101 ; Serial
                3600       ; Refresh
                1800       ; Retry
                604800     ; Expire
                86400 )    ; Minimum

@   IN  NS      ns1.example.com.
@   IN  A       192.0.2.1
ns1 IN  A       192.0.2.2
www IN  CNAME   @
mail IN  A       192.0.2.3
@   IN  MX 10   mail.example.com.

This is a basic example of a zone file for example.com with various record types.

Securing Your DNS Server with Firewall and SELinux

Security is paramount when running a DNS server. You must configure firewalls and SELinux policies to protect against unauthorized access and attacks.

Configuring firewalld

Use firewalld to open the necessary ports for DNS traffic:

firewall-cmd --permanent --add-service=dns
firewall-cmd --reload

Adjusting SELinux Policies

Ensure that SELinux allows BIND to run properly by setting the correct boolean values:

setsebool -P named_write_master_zones 1

Troubleshooting Common DNS Issues

When DNS isn’t functioning as expected, there are several tools and logs you can check to diagnose and resolve issues.

Using dig and nslookup

The dig and nslookup commands are useful for testing DNS resolution and querying DNS servers directly.

dig @localhost example.com
nslookup example.com localhost

Checking Logs for Errors

BIND logs messages to /var/log/messages. Review this log for any errors or warnings that could indicate configuration issues.

Frequently Asked Questions

How do I restart the BIND service after making changes?
To apply changes, restart BIND using systemctl restart named.
Can I use multiple DNS servers for redundancy?
Yes, you can specify multiple nameservers in /etc/resolv.conf for failover purposes.
What is the difference between a caching and authoritative DNS server?
A caching DNS server stores recent lookups for faster response times, while an authoritative DNS server provides responses based on its own DNS zone files.
How can I ensure my DNS server only responds to queries from my local network?
In the named.conf options section, restrict the listen-on directive to your local network IP range.

References

CentOS Project. (n.d.). Documentation. Retrieved from https://www.centos.org/docs/
BIND 9 Documentation. (n.d.). Retrieved from https://bind9.readthedocs.io/en/latest/
Internet Systems Consortium. (n.d.). BIND. Retrieved from https://www.isc.org/bind/
Firewalld Project. (n.d.). Services: dns. Retrieved from https://firewalld.org/documentation/services/dns.html
Red Hat Enterprise Linux 7. (n.d.). SELinux Users and Administrators Guide. Retrieved from https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/selinux_users_and_administrators_guide/index