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consul/contributing/service-discovery/dns.md

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DNS Interface

The DNS interface allows users to find the IP address and port of services, using DNS queries. The DNS interface is in many ways similar to an HTTP API. The major difference is in the DNS protocol.

There are lots of guides to DNS, the following list is a short reference that should help you understand the parts that are relevant to the DNS interface in Consul. Full details about the DNS protocol can be found in the RFCs: RFC 1035, RFC 6891, RFC 2782, and others.

DNS Server

The DNS interface is implemented as a DNS server using miekg/dns and the handlers for requests are in agent/dns.go.

DNS Queries and Records

The DNS interface handles queries where OPCODE=0 (standard query). The following table describe the current DNS behaviour for different record types and domain names. The Domain names (along the top) are always in the form <prefix>.<name in table>.<domain>, where domain is generally documented as consul but can be set using the domain or alt_domain config fields. The prefix is an identifier (service name, node name, prepared query name, etc).

Type service connect ingress node query addr
SOA Supported Supported Supported Supported Supported Supported
NS Supported Supported Supported Supported Supported Supported
AXFR Not Implemented Not Implemented Not Implemented Not Implemented Not Implemented Not Implemented
A/AAAA Supported Supported Supported Supported Supported
ANY Supported (return A) Supported (return A) Supported (return A) Supported Supported (return A)
CNAME Supported (node cname) Supported (node cname) Supported (node cname) Supported (node cname) return empty with A as extra
OPT Supported (node OPT) Supported (node OPT) Supported (node OPT) Supported (node OPT) return empty with A as extra
PTR Supported (node PTR) Supported (node PTR) Supported (node PTR) Supported (node PTR) return empty with A as extra
SRV Supported (service SRV) Supported (service SRV) Supported (service SRV) No error but empty return empty with A as extra
TXT Answer A record (????) Answer A record (????) Answer A record (????) Supported return empty with A as extra