Reconsidering the Host File: The Last Line of Defense
Whether an architectural flaw or other hardware/software failure impedes access to the DNS, an up-to-date Host File can be the difference between continued operations and complete network shutdown. We show you how to always have a current Host File.
Introduction
The Domain Name System (DNS) serves as the internet’s comprehensive directory service, providing far more than simple name resolution. While the proven, decentralized, and redundant design makes DNS very reliable, failures can (and do) happen. Most people know DNS for translating human-readable hostnames into IP addresses (like converting www.example.com to 192.0.2.1), DNS handles numerous critical functions:
DNS Directory Services:
- Name Resolution (A/AAAA records): Hostname to IP address translation
- Mail Routing (MX records): Directing email to mail servers
- Service Discovery (SRV records): Locating services like LDAP, SIP, XMPP
- Domain Verification (TXT records): SPF, DKIM, DMARC for email authentication
- Certificate Authority Authorization (CAA records): Controlling which CAs can issue certificates
- Reverse DNS (PTR records): IP address to hostname lookup
- Aliases (CNAME records): Creating alternate names for services
- Name Server Delegation (NS records): Distributing DNS authority
DNS has been refined and battle-tested for decades, with the most widely-used DNS server software, BIND (Berkeley Internet Name Domain), having been in continuous development for over 40 years. BIND 9, the current major version released in 2000, was completely rewritten from scratch to improve security and auditability. In 2023, ISC (Internet Systems Consortium) commissioned X41 D-Sec, a specialized German security firm, to conduct a comprehensive 60-day security audit of BIND 9’s source code. This level of scrutiny and continuous development has made DNS infrastructure remarkably robust and trustworthy.
While DNS is remarkably robust, it remains a single point of failure in most networks. When DNS fails—whether due to software bugs, misconfigurations, or infrastructure issues—systems lose the ability to resolve hostnames, potentially bringing operations to a halt.
It turns out a backup (that pre-dates DNS) has been in place all along. The humble host file (/etc/hosts on Unix-like systems, C:\Windows\System32\drivers\etc\hosts on Windows) provides a simple yet powerful last line of defense for this vulnerability. By maintaining a properly configured host file with entries for critical infrastructure, organizations can ensure continued operations even when DNS services fail completely.
The 2025 AWS DNS Outage: A Case Study in Implementation Mistakes
On October 20, 2025, Amazon Web Services (AWS) experienced a massive 7-hour outage that affected 113 services and millions of users globally. The root cause was a race condition in AWS' DynamoDB DNS management system—specifically, a bug that occurred when two DNS Enactors simultaneously attempted to update DNS endpoints, triggering a cascade failure across Amazon’s network.
This incident highlighted a critical lesson about DNS architecture: the failure wasn’t a DNS limitation, but a failure to use DNS’s built-in redundancy.
DNS is Designed to Be Redundant: The DNS protocol was specifically designed to eliminate single points of failure through multiple nameservers, distributed architecture, and hierarchical delegation. Properly configured DNS environments run multiple DNS servers, and clients are configured with multiple DNS server addresses. If one DNS server fails, clients automatically query the next server in their list.
The AWS Implementation Failure: The AWS outage occurred because their DynamoDB DNS management software had a race condition that affected their entire internal DNS implementation, bypassing the built-in redundancy. This was not a failure of DNS as a protocol, but rather a failure to properly implement redundant DNS infrastructure. It’s analogous to having multiple backup generators that all share the same faulty control circuit—the redundancy exists on paper but fails in practice.
Organizations that maintained comprehensive host files on their critical systems were able to continue operations during the outage. Those relying solely on DNS found themselves unable to reach essential services, including:
- Management interfaces for cloud resources
- Database servers
- Authentication systems
- Monitoring and alerting platforms
- Backup systems
Key Lessons from the AWS Outage:
- Even correctly designed redundant systems can fail when implementations have flaws
- DNS redundancy requires truly independent DNS servers (different software, different infrastructure)
- Host files provide a final failsafe when even redundant DNS fails
- Critical infrastructure should never depend solely on any single service, even DNS
This real-world example demonstrates that DNS failures aren’t theoretical risks—they happen even to the largest and most sophisticated infrastructure providers, usually due to architectural or implementation failures rather than DNS protocol limitations.
Defense in Depth: DNS Redundancy Plus Host Files
Proper DNS Architecture: DNS was designed with redundancy built-in, and organizations should leverage this:
- Multiple DNS Servers: Run at least 3 independent DNS servers
- Software Diversity Trade-offs: Two valid approaches exist:
- Approach 1 - Same Software (Recommended for most): Run BIND 9 on all servers. BIND has the best track record (40+ years, thoroughly audited), and staff only need to learn one system. Infrastructure diversity (different networks, AS, providers) is more important than software diversity.
- Approach 2 - Different Software (Maximum diversity): Use different implementations (BIND, PowerDNS, Unbound) to avoid software bugs affecting all servers. Requires staff training on multiple systems, but provides protection against software-specific vulnerabilities.
- Network Independence: DNS servers should be on different /24 networks to avoid single network failures
- Autonomous System Diversity: Place servers in different Autonomous Systems (AS) to avoid BGP routing issues
- Provider Diversity: Best practice is different infrastructure providers:
- Example: One server on AWS, one on colocation VM, one on GCP
- Avoids single-provider outages affecting all DNS
- Different data center locations and network paths
- Different power grids and infrastructure dependencies
- Geographic Distribution: Place DNS servers in different physical locations
- Independent Infrastructure: Truly separate servers, not just multiple instances on the same platform
Host Files as Final Failsafe: Even with properly redundant DNS, host files provide an essential last line of defense:
- Protocol-Independent: Work when DNS protocol issues occur
- Zero Dependencies: Require no network services to function
- Always Available: Local file access never depends on network state
- Fast Fallback: No timeout waiting for failed DNS queries
Layered Defense Strategy:
- Primary: Multiple independent DNS servers handle normal operations
- Secondary: DNS failover to alternate servers when primary fails
- Tertiary: Host files provide resolution when all DNS fails
- Recovery: Host files let you access DNS servers to fix DNS issues
Host File Capacity: Dispelling the Myths
A common misconception is that host files must be kept small due to performance constraints. In reality, operating systems have handled large host files well for decades—this isn’t a modern capability, but a long-standing characteristic of Unix-like systems:
Historical Performance: Unix systems have efficiently handled 10,000+ entry host files for over 20 years. Hardware that was current in 2005 had no trouble with large host files, and modern systems perform even better. This has never been a real limitation for infrastructure use cases.
Windows Systems:
- Can reliably handle up to 10,000 entries with minimal performance impact
- Very large files (100,000+ entries) may cause slowdowns but are rarely needed for infrastructure purposes
- Recommended maximum: 10,000 entries for general use
Linux, macOS, and Unix-like Systems:
- Superior performance characteristics compared to Windows
- Have handled 10,000 entries easily since the early 2000s on modest hardware
- Modern systems can handle 30,000+ entries without noticeable degradation
- Files over 50,000 entries still perform adequately on current hardware
- Better suited for large-scale host file deployments
Performance Considerations:
- Host files are read sequentially from top to bottom
- Place frequently accessed entries near the top for optimal performance
- Operating systems cache host file lookups, minimizing repeated file access
- Sequential scanning impacts are negligible for infrastructure-focused host files (typically < 500 entries)
DNS Servers: Low-Impact Infrastructure Services
An important consideration when planning DNS infrastructure is that DNS servers are remarkably lightweight services:
Resource Requirements:
- Minimal CPU Usage: DNS queries are simple lookups requiring minimal processing
- Low Memory Footprint: Even large DNS zones fit easily in RAM on modest hardware
- Minimal Network Bandwidth: DNS queries and responses are small (typically < 512 bytes)
- No Persistent Connections: Stateless protocol means no connection overhead
Practical Implications:
- DNS servers can run on very modest hardware (even Raspberry Pi devices)
- Multiple DNS servers add negligible infrastructure cost
- Virtual machines with minimal resource allocation work well
- DNS can coexist with other services without performance impact
Why This Matters for Redundancy:
- Running 3-5 redundant DNS servers is trivial from a resource perspective
- Cost and complexity arguments against DNS redundancy don’t hold up
- Organizations should run multiple independent DNS servers
Software Selection Considerations:
- BIND 9 (Recommended for most organizations): Best track record with 40+ years of development and thorough security audits. Staff only need to learn one system. When combined with infrastructure diversity (different networks, AS, providers), provides excellent resilience without operational complexity.
- Alternative Software Options (for maximum diversity):
- PowerDNS: Modern alternative with excellent performance
- Unbound: Validating, recursive, caching DNS resolver
- Knot DNS: High-performance authoritative server
- NSD: Authoritative-only server focused on simplicity
- Trade-off: Software diversity adds operational complexity (multiple systems to learn, maintain, monitor) but protects against software-specific bugs. Most organizations find infrastructure diversity sufficient.
The Key Lesson: Infrastructure diversity (different networks, AS, providers) matters more than software diversity for most organizations. Running battle-tested BIND 9 on three servers across AWS, colocation, and GCP provides substantial resilience at minimal cost (< $30/month) and operational complexity.
Managed DNS Services: Convenience vs. Control
Many organizations use managed DNS services like GoDaddy, DNS Made Easy, Route53, Cloudflare, or similar providers instead of running their own DNS servers. These services offer convenience but come with important trade-offs.
Advantages of Managed DNS Services:
- No Infrastructure to Maintain: Provider handles servers, software updates, security patches
- Built-in Redundancy: Providers typically run geographically distributed infrastructure
- DDoS Protection: Large providers have substantial DDoS mitigation capabilities
- Web Interface: Easy-to-use control panels for DNS management
- API Access: Programmatic DNS updates for automation
Critical Limitations and Risks:
- You’re at the Mercy of the Provider: Complete dependence on their reliability, security, and policies
- Service Outages: When the provider goes down, ALL your DNS goes down
- GoDaddy has experienced multiple significant outages
- Route53 outages have impacted major websites
- No provider has perfect uptime, regardless of marketing claims
- Policy Changes: Providers can change terms, pricing, or features at will
- Account Lockout: Security incidents, billing issues, or mistakes can lock you out
- No Emergency Access: When their service fails, you have no way to fix DNS
- Vendor Lock-in: Migration to different provider can be complex and risky
- Data Sovereignty: Your DNS data resides on their infrastructure
Hybrid Approach Limitations: Most managed DNS providers (GoDaddy, Route53, DNS Made Easy, Cloudflare) expect to be the ONLY authoritative nameservers for your domain. They typically don’t allow mixing their NS records with your own self-hosted servers in a simple hybrid configuration.
Split Horizon as Alternative: You can use split horizon DNS to have separate nameservers for different audiences:
- External (public internet): Use managed service nameservers
- Internal (your network): Use self-hosted BIND 9 nameservers
- Benefit: Internal users can reach infrastructure even if managed service fails
- Limitation: External users still depend entirely on managed service
Critical Host File Consideration: When using managed DNS services, host files become even more critical because you have zero control when the provider fails:
- Cannot Fix Provider’s DNS: You can’t SSH to GoDaddy’s servers to fix their bugs
- Must Reach Your Infrastructure: Host files ensure access to your own systems during provider outages
- Only Failsafe: Since you can’t mix managed and self-hosted nameservers, host files are your only fallback for critical infrastructure access
ResorsIT Integration with Managed DNS Services:
The ResorsIT DNS/IP Management system provides seamless integration with major managed DNS providers, giving you the best of both worlds: convenience of managed services with control and visibility.
Supported Managed DNS Providers:
- GoDaddy: Full API integration for DNS record management
- Cloudflare: Complete DNS sync and push operations
- DigitalOcean: DNS management integration
- Route53: AWS DNS service integration
- DNS Made Easy: Provider support (planned)
- Additional providers added based on customer demand
How ResorsIT Works with Managed DNS:
- Single Source of Truth: Maintain all DNS records in ResorsIT
- Push to Provider: Sync DNS records to GoDaddy/Cloudflare/DigitalOcean/ Route53 via API
- Pull from Provider: Import existing DNS records from provider
- Compare/Diff: See differences between local and provider records before pushing
- Backup Before Changes: Automatic backup of provider state before updates
- Change Tracking: SaaS has audit logs; Platform has Git-based revision control for all changes
Benefits of ResorsIT with Managed DNS:
- Don’t Manage DNS in Provider’s Web Interface: Work in ResorsIT, push to provider
- Change Auditing: SaaS provides audit logs; Platform provides complete Git history for version control
- Multi-Provider Management: Manage GoDaddy, Cloudflare, DigitalOcean, and Route53 from one interface
- Host File Generation: Generate host files from same data that goes to managed DNS
- Easy Migration: Move DNS between providers without recreating records
- Validation Before Push: Catch errors before they reach production
- Hybrid Configuration: Manage both self-hosted and managed DNS from one tool
Example Workflow:
- Update DNS records in ResorsIT interface
- Review changes in audit log (SaaS) or Git diff (Platform)
- Push to GoDaddy for domains using their nameservers
- Push to Cloudflare for domains using their nameservers
- Push to DigitalOcean for domains using their nameservers
- Push to Route53 for domains using AWS nameservers
- Update self-hosted BIND 9 servers for domains using your nameservers
- Generate host files for critical infrastructure from same data source
This approach gives you the convenience of managed DNS providers while maintaining control, visibility, and the ability to switch providers without losing your DNS data or change history.
Host File Details
For details on what content to include in your Host File, click here.
How ResorsIT DNS/IP Management Simplifies Host File Maintenance
The ResorsIT DNS/IP Management system serves as a single source of truth for all DNS and IP address data in your infrastructure. This centralized approach dramatically simplifies host file generation and maintenance:
Single Source of Truth:
- All DNS records and IP subnet allocations maintained in one place
- Eliminates inconsistencies between DNS, IPAM, and host files
- Changes propagate from a single authoritative source
- No more hunting through spreadsheets, multiple tools, or outdated documentation
Automated Host File Generation:
- Export critical infrastructure entries in standard /etc/hosts format
- User selects which hosts to include based on criticality assessment
- Automatic formatting with proper IPv4/IPv6 syntax and aliases
- Role-specific templates: Different host file sets for servers, workstations, network equipment
- Subnet-aware generation: Automatically include all hosts in specific critical subnets
- One-click updates: Regenerate host files when infrastructure changes
Real-World Recovery Scenario
Scenario: DNS server software bug causes complete DNS failure
Without Host Files:
- DNS queries begin failing across infrastructure
- Services lose ability to resolve hostnames
- Administrators cannot connect to DNS servers by name
- Manual lookup of IP addresses in documentation required
- Extended downtime while issues are diagnosed and resolved
- Potential data loss if services cannot reach dependencies
With Proper Host Files:
- DNS queries fail, but host file entries take precedence
- Critical services continue using host file resolution
- Administrators immediately connect to DNS servers using host file entries
- DNS issue diagnosed and resolved quickly
- Minimal service disruption
- All critical dependencies remain accessible
This scenario isn’t hypothetical—it describes exactly what happened during the 2025 AWS DNS outage. Organizations with comprehensive host files maintained operations while others experienced extended downtime.
Compliance and Audit Considerations
Many compliance frameworks require documentation of single points of failure and remediation strategies. Properly maintained host files demonstrate:
Risk Management:
- Identification of critical dependencies
- Documented failover mechanisms
- Reduced impact of DNS failures
Business Continuity:
- Demonstrated resilience planning
- Reduced recovery time objectives (RTO)
- Alternative resolution mechanisms
Documentation Requirements:
- Track what infrastructure is deemed critical
- Justify host file entries with business impact analysis
- Regular testing and validation of failover capabilities
Implementation Roadmap
For organizations looking to implement comprehensive host file management:
Phase 1: Assessment (Week 1):
- Identify critical infrastructure components
- Document current DNS dependencies
- Determine priority for host file inclusion
- Establish baseline host file templates
Phase 2: Development (Weeks 2-3):
- Create role-based host file templates (servers, workstations, network gear)
- Develop deployment automation (Ansible, Puppet, etc.)
- Create validation and testing procedures
- Document maintenance processes
Phase 3: Deployment (Week 4):
- Deploy to test systems first
- Validate functionality and performance
- Roll out to production in phases
- Monitor and adjust as needed
Phase 4: Maintenance (Ongoing):
- Regular quarterly reviews of entries
- Updates when infrastructure changes
- Testing of emergency scenarios
- Audit trail maintenance
Conclusion
The host file is far more than a legacy feature from early networking days—it’s a critical component of modern infrastructure resilience. We have seen that sophisticated systems fail when DNS architecture lacks true redundancy.
Defense in Depth is Essential: Proper infrastructure resilience requires multiple layers:
- Primary Defense: Run multiple independent DNS servers (self-hosted BIND 9 across diverse infrastructure, optionally supplemented with managed services)
- Final Failsafe: Maintain comprehensive host files for critical infrastructure
- Recovery Capability: Include DNS server IPs in host files so you can fix DNS when it fails
- Managed Service Caution: If using GoDaddy, DNS Made Easy, Route53, etc., combine with self-hosted DNS for true independence
Key Takeaways:
- DNS Redundancy Through Infrastructure Diversity: BIND 9 on different /24 networks, Autonomous Systems, and providers (AWS, colocation, GCP) provides robust resilience at minimal cost (< $30/month)
- Host Files Are Critical Failsafe: Include DNS server IPs so you can reach them when DNS fails—the 2025 AWS outage proved this prevents extended downtime
- ResorsIT Simplifies Everything: Single source of truth for DNS/IP data, generates host files, manages both self-hosted and managed DNS (GoDaddy, Cloudflare, Route53)
- Host Files Scale: 10,000 entries (Windows), 30,000+ (Linux/Unix)—this has worked well for 20+ years
- Managed DNS Trade-offs: Convenient but at provider’s mercy—use hybrid approach with self-hosted DNS for true independence
Don’t wait for a DNS failure to discover the value of properly maintained host files. Implement comprehensive host file management today, run multiple independent DNS servers across diverse infrastructure, and ensure your critical infrastructure remains accessible when DNS inevitably fails.
Additional Resources
Related Documentation:
- RFC 952: DOD Internet Host Table Specification
- DNS Split Horizon article (complementary DNS architecture)
Tools and Automation:
- Ansible hosts module: automate host file management
- Puppet host resource type: centralized host file configuration
- SwitchHosts: cross-platform GUI editor for development use
ResorsIT DNS/IP Management System:
- Single Source of Truth: Centralized DNS and subnet management eliminates data inconsistencies
- Automated Host File Generation: Export infrastructure data in /etc/hosts format with one click
- Role-Based Templates: Different host file sets for servers, workstations, network equipment
- Tag-Based Selection: User controls which hosts are included based on criticality
- API Integration: REST API enables automated regeneration via Ansible/Puppet/scripts
- Change Tracking: SaaS provides audit logs; Platform provides Git-backed revision control
- Visual Subnet Management: Tree and table views help identify critical infrastructure
- IPv4/IPv6 Support: Automatic generation of both address families
- Validation and Conflict Detection: Prevents errors before host file deployment
- Learn More: Contact ResorsIT for demos and implementation guidance