DNS Security Extensions (DNSSEC) protect against cache poisoning and spoofing by cryptographically signing DNS records. On RHEL 8, you can enforce DNSSEC validation using either BIND 9 or Unbound, both available in the default repositories. This tutorial walks through configuring a validating resolver, testing zone signatures, and detecting failures. Securing your resolver is a foundational step for any hardened RHEL 8 environment.

Prerequisites

  • RHEL 8 server with a static IP address
  • Root or sudo access
  • bind or unbound package available via dnf
  • Outbound UDP/TCP port 53 permitted by firewalld
  • Basic familiarity with DNS resolution concepts

Step 1 — Install BIND 9 and Enable DNSSEC Validation

Install BIND and its utilities, then open the main configuration file to enable DNSSEC globally.

dnf install -y bind bind-utils
systemctl enable --now named

Edit /etc/named.conf and add the following lines inside the options { } block:

options {
    listen-on port 53 { 127.0.0.1; };
    recursion yes;
    dnssec-enable yes;
    dnssec-validation yes;
    dnssec-lookaside auto;
    managed-keys-directory "/var/named/dynamic";
};

Restart BIND to apply the changes:

named-checkconf /etc/named.conf
systemctl restart named

Step 2 — Install Unbound as an Alternative Validating Resolver

Unbound is a lightweight, security-focused resolver that ships in the RHEL 8 repositories and is well-suited for workstations and edge nodes.

dnf install -y unbound
systemctl enable --now unbound

Edit /etc/unbound/unbound.conf to enable DNSSEC hardening:

server:
    verbosity: 1
    interface: 127.0.0.1
    access-control: 127.0.0.0/8 allow
    trust-anchor-file: "/var/lib/unbound/root.key"
    auto-trust-anchor-file: "/var/lib/unbound/root.key"
    harden-dnssec-stripped: yes
    harden-glue: yes
    harden-large-queries: yes
    val-clean-additional: yes

Fetch the root trust anchor and restart Unbound:

unbound-anchor -a /var/lib/unbound/root.key
systemctl restart unbound
unbound-checkconf /etc/unbound/unbound.conf

Step 3 — Point the System Resolver at the Local Validator

Update /etc/resolv.conf (or NetworkManager) so all local queries pass through the validating resolver.

# Using nmcli to set DNS permanently
nmcli connection modify "System eth0" ipv4.dns "127.0.0.1"
nmcli connection modify "System eth0" ipv4.ignore-auto-dns yes
nmcli connection up "System eth0"

# Verify
cat /etc/resolv.conf

Step 4 — Test DNSSEC Validation with dig

The +dnssec flag requests RRSIG records. A valid response includes the ad (Authenticated Data) flag in the header when your resolver trusts the chain of trust.

# Request DNSSEC records for a signed zone
dig +dnssec example.com @127.0.0.1

# Verify the AD flag is present
dig +dnssec +short SOA dnssec-failed.org @127.0.0.1

# Trace the full DNSSEC chain
dig +sigchase +trusted-key=/var/lib/unbound/root.key dnssec.works

# Check DS records
dig +short DS google.com @127.0.0.1

A response containing flags: qr rd ra ad confirms the resolver is validating signatures successfully.

Step 5 — Detect DNSSEC Failures

Intentionally query a domain with a broken DNSSEC chain to confirm your resolver rejects invalid signatures. The test domain dnssec-failed.org is maintained specifically for this purpose.

# This should return SERVFAIL if validation is working
dig dnssec-failed.org @127.0.0.1
# Expected: status: SERVFAIL

# Compare against a non-validating resolver
dig dnssec-failed.org @8.8.8.8
# Expected: status: NOERROR (Google does not validate by default)

# Check Unbound logs for validation events
journalctl -u unbound -n 50 --no-pager | grep -i "dnssec|bogus|fail"

Step 6 — Allow DNS Through firewalld

If this server also acts as a resolver for the local network, open port 53 through firewalld.

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

Conclusion

You now have a DNSSEC-validating resolver running on RHEL 8 using either BIND 9 or Unbound. Both configurations enforce cryptographic validation of DNS responses, reject forged records, and log DNSSEC failures. Regular monitoring of resolver logs ensures you catch validation breakdowns caused by expired signatures or misconfigured zones before they impact users.

Next steps: How to Configure a Private DNS Server with BIND on RHEL 8, How to Harden Web Servers with Security Headers on RHEL 8, and How to Set Up Tripwire for File Integrity Monitoring on RHEL 8.