From RFC 1034: "The strategy is to cycle around all of the addresses for all of the servers with a timeout between each transmission." Note that it does not s

From RFC 1034: "The strategy is to cycle around all of the addresses for all of the servers with a timeout between each transmission." Note that it does not specify an order -- the resolver determines that, and RFC 1035 even recommends keeping a weighted list of nameservers prioritized by response time: "To complete initialization of SLIST, the resolver attaches whatever history information it has to the each address in SLIST. This will usually consist of some sort of weighted averages for the response time of the address, and the batting average of the address (i.e., how often the address responded at all to the request)." ...which is exactly what systemd is doing. The glibc behavior is a naive simplest-implementation approach, and if your infrastructure depends on an implementation detail of a specific libc resolver, you should install proper configurable resolvers and use those instead. The fact that all nameservers are expected to return identical results is inherent in the design of DNS. It's one namespace, and if you need to split it, the client resolver is the wrong place to do that too.