Split-horizon DNS
Sometimes the same name should resolve differently depending on who is asking — a private address inside your network, a public one from outside. That is split-horizon DNS.
Why serve two answers
A common case: internal clients should reach a service over the private network at its internal address, while the outside world gets the public one. It keeps internal traffic off the public path and lets you use one name everywhere instead of hard-coding addresses in applications.
Run it cleanly
The usual shape is a split authoritative setup: an internal view for queries from your own ranges, an external view for everyone else. Keep the two in step — the surprises come from a record updated in one view and forgotten in the other. Use your own resolver for internal clients so they consistently get the internal view.
The pitfalls
- A record that exists in one view and not the other, so something resolves inside but not out.
- Caching that mixes the two — mind your TTLs.
- Clients on the wrong resolver getting the wrong view.
When a name "works for me but not for them", split-horizon is the first thing to check.
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