2021-06-21 17:00:17 +00:00
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# DNS Interface
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The DNS interface allows users to find the IP address and port of services, using DNS
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queries. The DNS interface is in many ways similar to an HTTP API. The major difference is
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in the DNS protocol.
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There are lots of guides to DNS, the following list is a short reference that should help you
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understand the parts that are relevant to the DNS interface in Consul. Full details about
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the DNS protocol can be found in the RFCs: [RFC 1035], [RFC 6891], [RFC 2782], and others.
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[RFC 1035]: https://tools.ietf.org/html/rfc1035
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[RFC 6891]: https://tools.ietf.org/html/rfc6891
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[RFC 2782]: https://tools.ietf.org/html/rfc2782
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* [wikipedia: DNS message format](https://en.wikipedia.org/wiki/Domain_Name_System#DNS_message_format)
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is a quick introduction to the format used for queries and replies
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* [RFC 1035 Section 4.1.1](https://datatracker.ietf.org/doc/html/rfc1035#section-4.1.1)
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is a good reference for when to use specific response codes and what the different header
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bits refer to.
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## DNS Server
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The DNS interface is implemented as a DNS server using [miekg/dns] and the handlers for
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requests are in `agent/dns.go`.
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[miekg/dns]: https://github.com/miekg/dns
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2021-06-23 16:29:02 +00:00
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## DNS Queries and Records
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The DNS interface handles queries where OPCODE=0 (standard query). The following table describe the current
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DNS behaviour for different record types and domain names. The Domain names (along the
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top) are always in the form `<prefix>.<name in table>.<domain>`, where `domain` is
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generally documented as `consul` but can be set using the `domain` or `alt_domain` config
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fields. The `prefix` is an identifier (service name, node name, prepared query name, etc).
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| Type | service | connect | ingress | node | query | addr |
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|---------|-------------------------|-------------------------|-------------------------|------------------------|-----------------|------------------------------|
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| SOA | Supported | Supported | Supported | Supported | Supported | Supported |
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| NS | Supported | Supported | Supported | Supported | Supported | Supported |
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| AXFR | Not Implemented | Not Implemented | Not Implemented | Not Implemented | Not Implemented | Not Implemented |
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| A/AAAA | Supported | Supported | Supported | Supported | | Supported |
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| ANY | Supported (return A) | Supported (return A) | Supported (return A) | Supported | | Supported (return A) |
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| CNAME | Supported (node cname) | Supported (node cname) | Supported (node cname) | Supported (node cname) | | return empty with A as extra |
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| OPT | Supported (node OPT) | Supported (node OPT) | Supported (node OPT) | Supported (node OPT) | | return empty with A as extra |
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| PTR | Supported (node PTR) | Supported (node PTR) | Supported (node PTR) | Supported (node PTR) | | return empty with A as extra |
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| SRV | Supported (service SRV) | Supported (service SRV) | Supported (service SRV) | No error but empty | | return empty with A as extra |
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| TXT | Answer A record (????) | Answer A record (????) | Answer A record (????) | Supported | | return empty with A as extra |
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