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DNS Flow Basic

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The diagram illustrates a high-availability setup for coreDNS using ECMP (Equal-Cost Multi-Path) routing and Anycast VIPs (Virtual IP addresses).

Here's a breakdown:

Clients (represented by the snowflake icons): These are the systems making DNS queries.​

  1. ECMP: This signifies that the client's requests can be routed to any of the available coreDNS servers via multiple equal-cost paths. The routers responsible for forwarding traffic implement ECMP, distributing the load across the different paths.

  2. coreDNS (the blue shaded area): This represents the group of coreDNS servers providing the DNS service.

  3. VIP1 (on each coreDNS server): This is an Anycast IP address. The same IP address (VIP1) is configured on all coreDNS servers. Due to the nature of Anycast, traffic directed to this VIP will be routed to the "closest" (in terms of network topology) coreDNS server.

  4. bird (on each coreDNS server): This likely refers to the BIRD Internet Routing Daemon, a common dynamic routing software. BIRD is used on each coreDNS server to announce the VIP1 via BGP (Border Gateway Protocol) to the upstream routers. This allows the routers to learn the multiple paths to VIP1 and implement ECMP.

coreDNS srv 1, srv 2, ... srv n: These are the individual coreDNS server instances. Each server runs its own instance of coreDNS and shares the same VIP1.

How it works:​

  1. A client sends a DNS query to VIP1.

  2. ECMP on the upstream routers distributes the traffic across the available paths to the different coreDNS servers. Each server advertises the same VIP1, but the routers choose the path based on factors like network proximity and link health.

  3. The "closest" coreDNS server receives the request and responds.

  4. If one coreDNS server fails, the routers automatically detect the failure (because BGP updates will show that the VIP is no longer reachable via that path) and stop sending traffic to that server. Traffic is seamlessly redirected to the remaining healthy servers, ensuring high availability.

Benefits:​

  1. High Availability: If one server fails, others continue to operate without interruption.
  2. Load Balancing: ECMP distributes the load across multiple servers, preventing overload on a single server.
  3. Scalability: Adding more coreDNS servers is relatively easy. Just configure them with the same VIP1 and announce it via BGP.
  4. Simplicity: Clients only need to know a single VIP for DNS resolution.

This architecture is commonly used for critical infrastructure services like DNS to ensure high availability and resilience.