LDNS

Stands for: local DNS resolver

The DNS server that queries a GSLB system on a client's behalf, and whose address the GSLB takes as the client's location.

A load-balancing term rather than a DNS standard one: RFC 9499 has no entry for it, and the role is usually played by a recursive resolver. A GSLB system such as BIG-IP DNS sees only the LDNS address, so a client behind a distant resolver can get the answer meant for the resolver's location.

F5's BIG-IP DNS manuals use the word throughout. In Topology load balancing, the request source statement of each topology record names the origin LDNS of a request, as an IP subnet, a region, an ISP, a state, a country or a continent, and the system places the LDNS address with an IP geolocation database. Longest Match sorting, the default, puts the most specific LDNS subnet first.

The proxy holds only while resolver and client are close. RFC 7871 (May 2016, Informational) observes that most queries reach authoritative servers from intermediate recursive resolvers, carrying the resolver's address, and that centralised resolvers, often far from the clients they serve, draw less than desirable answers from topology-sensitive servers. Its remedy is the EDNS Client Subnet option, code 8: the resolver forwards a truncated client prefix, and the reply's scope prefix length says how much of that prefix the answer covers.

BIG-IP DNS uses it when topology-prefer-edns0-client-subnet is enabled, globally or per wide IP: topology then matches on the ECS address and falls back to the source address when a query carries none. The price is privacy and cache pressure. RFC 7871 recommends the feature be off by default in nameserver software, asks resolvers to truncate IPv4 addresses to 24 bits and IPv6 to 56, and admits that, designed from scratch, it would probably look different.

Also known as: ldns, local dns, local dns server, local dns resolver

Sources

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