Tech Tutorial: 207.3 Securing a DNS Server #

Introduction #

DNS (Domain Name System) is a critical part of the internet infrastructure, providing a way to resolve hostnames into IP addresses and vice versa. However, due to its crucial role and widespread use, DNS is often a target for various types of attacks such as DNS spoofing, DDoS attacks, and DNS hijacking. In this tutorial, we will focus on securing a DNS server, specifically BIND (Berkeley Internet Name Domain), one of the most popular DNS server software used today.

Exam Objective: #

The objective of this tutorial is to cover the necessary steps and configurations to secure a BIND DNS server. We will address key knowledge areas including:

• DNSSEC (DNS Security Extensions)
• Chroot environment setup for BIND
• Configuration of logging and auditing
• Split DNS configurations

Utilities: #

• bind9
• dnssec-keygen
• dnssec-signzone
• named-checkconf
• named-checkzone
• rndc

Step-by-Step Guide #

Step 1: Installation of BIND #

Before securing the DNS server, you must first ensure that BIND is installed on your Linux system.

sudo apt update
sudo apt install bind9 bind9utils bind9-doc dnsutils

Step 2: Configuring BIND in a Chroot Environment #

Running BIND in a chroot environment can enhance security by limiting the potential damage in the event of a server compromise.

1. Set up the chroot environment:

sudo mkdir -p /chroot/bind/{etc, var/run, var/cache/bind, var/log}
sudo mv /etc/bind /chroot/bind/etc/
sudo ln -s /chroot/bind/etc/bind /etc/bind

2. Adjust permissions and ownership:

sudo chown -R bind:bind /chroot/bind/var/*
sudo chown -R bind:bind /chroot/bind/etc/bind

3. Modify systemd service file to run BIND in chroot:

Edit the BIND9 service file /lib/systemd/system/bind9.service to include the chroot path:

[Service]
ExecStart=/usr/sbin/named -f -t /chroot/bind

4. Reload systemd and restart BIND:

sudo systemctl daemon-reload
sudo systemctl restart bind9

Step 3: Implementing DNSSEC #

DNSSEC helps protect against certain types of attacks by digitally signing your DNS data.

1. Generate the Zone Signing Key (ZSK) and Key Signing Key (KSK):

dnssec-keygen -a RSASHA256 -b 2048 -n ZONE example.com
dnssec-keygen -a RSASHA256 -b 4096 -n ZONE -f KSK example.com

2. Sign the zone file with the generated keys:

dnssec-signzone -o example.com -k KSKexample.com.+007+12345.key example.com.zone ZSKexample.com.+007+67890.key

3. Update BIND configuration to use the signed zone file:

Edit /etc/bind/named.conf to reference the signed zone:

zone "example.com" {
    type master;
    file "/var/cache/bind/db.example.com.signed";
};

4. Reload BIND configuration:

sudo rndc reload

Step 4: Configuring Logging and Auditing #

Proper logging can help detect and respond to potential security incidents.

1. Configure logging in BIND:

Edit /etc/bind/named.conf to include a logging clause:

logging {
    channel default_log {
        file "/var/log/named/named.log" versions 3 size 5m;
        severity info;
        print-time yes;
    };
    category default {
        default_log;
    };
};

2. Restart BIND to apply changes:

sudo systemctl restart bind9

Step 5: Setting Up Split DNS #

Split DNS allows different DNS information to be served based on the source of the DNS request, enhancing security and privacy.

1. Configure internal and external views in BIND:

Edit /etc/bind/named.conf to define views:

view "internal" {
    match-clients { 192.168.0.0/24; };
    recursion yes;

    zone "example.com" {
        type master;
        file "/var/cache/bind/db.example.com.internal";
    };
};

view "external" {
    match-clients { any; };
    recursion no;

    zone "example.com" {
        type master;
        file "/var/cache/bind/db.example.com";
    };
};

2. Restart BIND for the changes to take effect:

sudo systemctl restart bind9

Conclusion #

Securing a DNS server is an ongoing process that involves implementing best practices and staying updated with the latest security advancements. By following the steps outlined in this tutorial, you should be able to enhance the security of your BIND DNS server, protecting it against common threats and ensuring it provides reliable service. Always remember to test your configurations in a staging environment before applying them to production.