Configuring Radware DefensePro for DNS Flood Mitigation
DNS is a small, essential service with a large operational impact. A flood of queries can exhaust authoritative name servers, saturate links, consume CPU, or overload recursive resolvers long before application traffic appears abnormal. Radware DefensePro can help reduce this exposure by identifying abnormal DNS behavior and enforcing rate, protocol, and traffic policies close to the network edge.
Effective protection depends on preparation as much as appliance configuration. Administrators need to understand which DNS services are public, how normal query volume changes throughout the day, and where DefensePro sits in the traffic path. A policy built from that information is easier to tune and less likely to block legitimate clients.
For related guidance on network security, automation, and infrastructure operations, the practical infrastructure guides on IT Diversified provide useful supporting context.
Define The Protected DNS Service
Begin by documenting the authoritative and recursive DNS endpoints that require protection. Record their public IP addresses, VLANs, interfaces, transport protocols, and expected traffic flows. Most DNS requests use UDP port 53, but TCP port 53 is also required for zone transfers, truncated responses, and some larger DNS responses.
Separate authoritative servers from recursive resolvers when possible. Their traffic patterns differ significantly: authoritative systems receive queries from broad Internet populations, while recursive servers typically serve a known user, branch, data center, or application population. A single threshold for both roles can create unnecessary alerts or inadequate protection.
Confirm whether DefensePro is deployed inline or receives redirected traffic through routing or another traffic-management method. The appliance must see enough context to identify the protected destination and enforce the intended policy. Preserve client source addresses where possible, since source-based analysis becomes less useful when all requests appear to originate from an intermediate device.
Build A DNS Protection Policy
Create a dedicated virtual server, network class, or protected-object definition for the DNS addresses. Apply a DNS-aware security profile rather than relying solely on a generic UDP flood rule. Protocol validation can help identify malformed packets, invalid flags, unusual opcodes, and traffic that does not match expected DNS behavior.
A useful policy commonly combines several controls. Volumetric thresholds limit the total request rate, while source-based thresholds identify clients generating excessive queries. Signature or anomaly protections can detect malformed DNS requests, repeated patterns, and suspicious query behavior. If DefensePro supports separate handling for UDP and TCP DNS traffic in the installed release, configure them independently.
Take care with legitimate exceptions. Monitoring systems, resolvers, content delivery platforms, and large enterprise NAT gateways may generate substantial query bursts from a small number of addresses. Trusted source definitions can be appropriate, but they should be narrow, documented, and monitored rather than used as a general bypass.
Tune Thresholds From Real Traffic
Thresholds should reflect observed traffic, not a generic number copied from another environment. Collect several days of DNS metrics that include business hours, overnight activity, maintenance windows, and known traffic events. Establish both a normal average and a realistic high-water mark before enabling aggressive mitigation.
| Control Area | What To Measure | Practical Starting Point | Tuning Signal |
|---|---|---|---|
| Total DNS rate | Queries per second to each service | Set above the normal peak with a safety margin | Sustained elevation or link saturation |
| Per-source rate | Requests from one address or NAT group | Account for resolver and enterprise NAT behavior | Repeated high-rate sources |
| UDP and TCP ratio | Transport distribution | Preserve expected TCP usage | Sudden protocol imbalance |
| Query validity | Flags, opcodes, lengths, and format | Block clearly malformed traffic | Increased anomaly events |
| Response behavior | Errors, drops, and latency | Compare with resolver and server logs | Rising SERVFAIL or timeout rates |
Use alert or report mode first when the platform and operational design allow it. Review event logs, dropped-packet counters, top talkers, query patterns, and protected-server performance. This phase reveals whether a threshold is detecting an attack, a resolver misconfiguration, a software rollout, or a predictable traffic spike.
When mitigation is enabled, prefer graduated actions. Rate limiting can absorb a moderate surge while preserving service, whereas dropping all traffic from a broad source category may cause an outage. Apply stricter treatment to malformed packets and clearly abusive sources, and retain enough logging to explain each enforcement decision.
Verify Deployment And Detection
Testing should confirm both traffic direction and policy behavior. Generate controlled DNS requests from approved test hosts, verify that normal UDP and TCP queries reach the server, and confirm that DefensePro records the expected protected object. Do not use an uncontrolled public flood tool against production infrastructure.
Check the appliance dashboard and logs for policy matches, dropped packets, source statistics, and mitigation state. Compare those results with DNS server logs and interface counters. A mismatch can indicate asymmetric routing, an incorrect virtual server definition, NAT obscuring the client address, or traffic bypassing the appliance entirely.
Validate failure behavior as well. Confirm that a saturated or unavailable DefensePro path has an approved operational response, whether that means fail-open, fail-closed, traffic diversion, or escalation to an upstream provider. The correct choice depends on the role of the DNS service and the organization’s availability requirements.
Operational Recommendations For Stable Protection
DNS flood defense works best as part of a layered design. DefensePro can filter malicious traffic near the edge, but upstream bandwidth protection, authoritative DNS redundancy, resolver hardening, and provider-level DDoS services may still be necessary for large attacks. Keep zone data available on geographically separate servers and avoid making every DNS dependency rely on one network path.
Use the following practices when moving from testing to production:
- Save the working policy and export configuration before major changes.
- Monitor DNS latency, SERVFAIL responses, packet drops, and CPU utilization together.
- Review thresholds after resolver, application, or zone changes.
- Keep trusted-source exceptions time-limited and supported by ownership records.
- Synchronize DefensePro time and forward security events to centralized logging.
Automate recurring checks where practical. PowerShell, monitoring APIs, or scheduled reports can compare current query rates with historical baselines and identify threshold drift. Configuration backups should include policy names, protected addresses, interface assignments, and the software version used to create them.
Troubleshoot False Positives And Gaps
A sudden increase in blocked traffic does not automatically prove that the policy is working correctly. Inspect representative events and determine whether the blocked packets are malformed, excessive, or simply from a legitimate resolver. If valid queries are being dropped, adjust the relevant rate or anomaly control rather than disabling the entire DNS profile.
Common gaps include protecting only UDP while overlooking TCP, defining the wrong public address, or applying a policy to a management interface instead of the service path. Another frequent issue is asymmetric routing: requests pass through DefensePro, but responses take another route, making stateful inspection and traffic accounting unreliable.
When events appear normal but the DNS server remains overloaded, compare DefensePro counters with upstream interface statistics. The attack may be bypassing the appliance, hidden behind an aggregation device, or arriving as a protocol pattern not covered by the selected profile. Review routing, NAT, policy precedence, and any upstream scrubbing rules before changing thresholds.
Apply the policy in a controlled maintenance window, observe real DNS behavior, and document each threshold and exception. Continuous measurement turns DefensePro from a reactive appliance into a repeatable part of DNS availability engineering.