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Using vTM 18.1 Service Discovery DNS plugin with internally-hosted domains
Pulse Virtual Traffic Manager v18.1 has introduced plugin-based Service Discovery that shipped bundled with two plugins: for Google Cloud, and for DNS.
The included DNS plugin was designed to help with a specific use case, where an authoritative DNS server returns a *subset* of the records every time it is queried, instead of the full set of records.
An example of this is AWS Route53 serving A-records for a large Elastic Load Balancer (ELB). Route53 will return up to 8 healthy records. This means that for ELBs with more than 8 nodes DNS query will only ever return 8, which a non-authoritative DNS server will cache and return for all subsequent queries.
If a regular DNS resolver is used to populate vTM pool nodes, this Route53 behaviour may lead to excessive vTM pool node churn. Additionally, traffic will only be ever sent to a maximum of 8 nodes.
To work around this issue, the bundled DNS plugin implements the following behaviour:
- For the hostname specified, find the authoritative DNS server(s) - Send a query for hostname's A-records directly to the discovered authoritative DNS servers - Cache the received results, along with each record's TTL - Check the cache for any existing records with TTL that hasn't expired - Combine the new records with the cached records, and return that superset as the result for vTM to use.
This behaviour has a side effect for some publicly registered domains with name servers that can't resolve the records within the domain, such as internally used domains. In this case, DNS resolver plugin will fail to work because in the process of discovering authoritative DNS servers upper level domain servers will respond with nameservers that can't resolve the names in question.
To work around this situation, DNS plugin can be run with an option "--nameservers" with IP address(es) of the internal authoritative DNS servers for the internal domain. This will bypass the logic for authoritative nameserver discovery.