template

template allows for dynamic responses based on the incoming query.

Description

The template plugin allows you to dynamically respond to queries by just writing a (Go) template.

Syntax

template CLASS TYPE [ZONE...] {
    [match REGEX...]
    [answer RR]
    [additional RR]
    [authority RR]
    [...]
    [rcode CODE]
    [upstream [ADDRESS...]]
    [fallthrough [ZONE...]]
}
  • CLASS the query class (usually IN or ANY).
  • TYPE the query type (A, PTR, … can be ANY to match all types).
  • ZONE the zone scope(s) for this template. Defaults to the server zones.
  • REGEX Go regexp that are matched against the incoming question name. Specifying no regex matches everything (default: .*). First matching regex wins.
  • answer|additional|authority RR A RFC 1035 style resource record fragment built by a Go template that contains the reply.
  • rcode CODE A response code (NXDOMAIN, SERVFAIL, ...). The default is SUCCESS.
  • upstream [ADDRESS…] defines the upstream resolvers used for resolving CNAME. If no ADDRESS is given, CoreDNS will resolve CNAMEs against itself. ADDRESS can be an IP, an IP:port, or a path to a file structured like resolv.conf.
  • fallthrough Continue with the next plugin if the zone matched but no regex matched. If specific zones are listed (for example in-addr.arpa and ip6.arpa), then only queries for those zones will be subject to fallthrough.

At least one answer or rcode directive is needed (e.g. rcode NXDOMAIN).

Also see contains an additional reading list.

Templates

Each resource record is a full-featured Go template with the following predefined data

  • .Zone the matched zone string (e.g. example.).
  • .Name the query name, as a string (lowercased).
  • .Class the query class (usually IN).
  • .Type the RR type requested (e.g. PTR).
  • .Match an array of all matches. index .Match 0 refers to the whole match.
  • .Group a map of the named capture groups.
  • .Message the complete incoming DNS message.
  • .Question the matched question section.

The output of the template must be a RFC 1035 style resource record (commonly referred to as a “zone file”).

WARNING there is a syntactical problem with Go templates and CoreDNS config files. Expressions like {{$var}} will be interpreted as a reference to an environment variable by CoreDNS (and Caddy) while {{ $var }} will work. See Bugs and corefile(5).

Metrics

If monitoring is enabled (via the prometheus directive) then the following metrics are exported:

  • coredns_template_matches_total{server, regex} the total number of matched requests by regex.
  • coredns_template_template_failures_total{server, regex,section,template} the number of times the Go templating failed. Regex, section and template label values can be used to map the error back to the config file.
  • coredns_template_rr_failures_total{server, regex,section,template} the number of times the templated resource record was invalid and could not be parsed. Regex, section and template label values can be used to map the error back to the config file.

Both failure cases indicate a problem with the template configuration. The server label indicates the server incrementing the metric, see the metrics plugin for details.

Examples

Resolve everything to NXDOMAIN

The most simplistic template is

. {
    template ANY ANY {
      rcode NXDOMAIN
    }
}
  1. This template uses the default zone (. or all queries)
  2. All queries will be answered (no fallthrough)
  3. The answer is always NXDOMAIN

Resolve .invalid as NXDOMAIN

The .invalid domain is a reserved TLD (see RFC 2606 Reserved Top Level DNS Names) to indicate invalid domains.

. {
    proxy . 8.8.8.8

    template ANY ANY invalid {
      rcode NXDOMAIN
      authority "invalid. 60 {{ .Class }} SOA ns.invalid. hostmaster.invalid. (1 60 60 60 60)"
    }
}
  1. A query to .invalid will result in NXDOMAIN (rcode)
  2. A dummy SOA record is sent to hand out a TTL of 60s for caching purposes
  3. Querying .invalid in the CH class will also cause a NXDOMAIN/SOA response
  4. The default regex is .*

Block invalid search domain completions

Imagine you run example.com with a datacenter dc1.example.com. The datacenter domain is part of the DNS search domain. However something.example.com.dc1.example.com would indicate a fully qualified domain name (something.example.com) that inadvertently has the default domain or search path (dc1.example.com) added.

. {
    proxy . 8.8.8.8

    template IN ANY example.com.dc1.example.com {
      rcode NXDOMAIN
      authority "{{ .Zone }} 60 IN SOA ns.example.com hostmaster.example.com (1 60 60 60 60)"
    }
}

A more verbose regex based equivalent would be

. {
    proxy . 8.8.8.8

    template IN ANY example.com {
      match "example\.com\.(dc1\.example\.com\.)$"
      rcode NXDOMAIN
      authority "{{ index .Match 1 }} 60 IN SOA ns.{{ index .Match 1 }} hostmaster.{{ index .Match 1 }} (1 60 60 60 60)"
      fallthrough
    }
}

The regex-based version can do more complex matching/templating while zone-based templating is easier to read and use.

Resolve A/PTR for .example

. {
    proxy . 8.8.8.8

    # ip-a-b-c-d.example.com A a.b.c.d

    template IN A example {
      match (^|[.])ip-10-(?P<b>[0-9]*)-(?P<c>[0-9]*)-(?P<d>[0-9]*)[.]example[.]$
      answer "{{ .Name }} 60 IN A 10.{{ .Group.b }}.{{ .Group.c }}.{{ .Group.d }}"
      fallthrough
    }

    # d.c.b.a.in-addr.arpa PTR ip-a-b-c-d.example

    template IN PTR 10.in-addr.arpa. {
      match ^(?P<d>[0-9]*)[.](?P<c>[0-9]*)[.](?P<b>[0-9]*)[.]10[.]in-addr[.]arpa[.]$
      answer "{{ .Name }} 60 IN PTR ip-10-{{ .Group.b }}-{{ .Group.c }}-{{ .Group.d }}.example.com."
    }
}

An IPv4 address consists of 4 bytes, a.b.c.d. Named groups make it less error-prone to reverse the IP address in the PTR case. Try to use named groups to explain what your regex and template are doing.

Note that the A record is actually a wildcard: any subdomain of the IP address will resolve to the IP address.

Having templates to map certain PTR/A pairs is a common pattern.

Fallthrough is needed for mixed domains where only some responses are templated.

Resolve multiple ip patterns

. {
    proxy . 8.8.8.8

    template IN A example {
      match "^ip-(?P<a>10)-(?P<b>[0-9]*)-(?P<c>[0-9]*)-(?P<d>[0-9]*)[.]dc[.]example[.]$"
      match "^(?P<a>[0-9]*)[.](?P<b>[0-9]*)[.](?P<c>[0-9]*)[.](?P<d>[0-9]*)[.]ext[.]example[.]$"
      answer "{{ .Name }} 60 IN A {{ .Group.a}}.{{ .Group.b }}.{{ .Group.c }}.{{ .Group.d }}"
      fallthrough
    }
}

Named capture groups can be used to template one response for multiple patterns.

Resolve A and MX records for IP templates in .example

. {
    proxy . 8.8.8.8

    template IN A example {
      match ^ip-10-(?P<b>[0-9]*)-(?P<c>[0-9]*)-(?P<d>[0-9]*)[.]example[.]$
      answer "{{ .Name }} 60 IN A 10.{{ .Group.b }}.{{ .Group.c }}.{{ .Group.d }}"
      fallthrough
    }
    template IN MX example {
      match ^ip-10-(?P<b>[0-9]*)-(?P<c>[0-9]*)-(?P<d>[0-9]*)[.]example[.]$
      answer "{{ .Name }} 60 IN MX 10 {{ .Name }}"
      additional "{{ .Name }} 60 IN A 10.{{ .Group.b }}.{{ .Group.c }}.{{ .Group.d }}"
      fallthrough
    }
}

Adding authoritative nameservers to the response

. {
    proxy . 8.8.8.8

    template IN A example {
      match ^ip-10-(?P<b>[0-9]*)-(?P<c>[0-9]*)-(?P<d>[0-9]*)[.]example[.]$
      answer "{{ .Name }} 60 IN A 10.{{ .Group.b }}.{{ .Group.c }}.{{ .Group.d }}"
      authority  "example. 60 IN NS ns0.example."
      authority  "example. 60 IN NS ns1.example."
      additional "ns0.example. 60 IN A 203.0.113.8"
      additional "ns1.example. 60 IN A 198.51.100.8"
      fallthrough
    }
    template IN MX example {
      match ^ip-10-(?P<b>[0-9]*)-(?P<c>[0-9]*)-(?P<d>[0-9]*)[.]example[.]$
      answer "{{ .Name }} 60 IN MX 10 {{ .Name }}"
      additional "{{ .Name }} 60 IN A 10.{{ .Group.b }}.{{ .Group.c }}.{{ .Group.d }}"
      authority  "example. 60 IN NS ns0.example."
      authority  "example. 60 IN NS ns1.example."
      additional "ns0.example. 60 IN A 203.0.113.8"
      additional "ns1.example. 60 IN A 198.51.100.8"
      fallthrough
    }
}

Also see

Bugs

CoreDNS supports caddyfile environment variables with notion of {$ENV_VAR}. This parser feature will break Go template variables notations like{{$variable}}. The equivalent notation {{ $variable }} will work. Try to avoid Go template variables in the context of this plugin.