DNS based service discovery for Nomad (Using CoreDNS). Workaround for https://github.com/hashicorp/nomad/issues/12588

Corefile

.: {
	log
	loadbalance

	# auto: automatically pick up file changes
	auto service.nomad {
		directory local/zones
		reload 1s
	}
}

db.nomad.service

$TTL 0
$ORIGIN service.nomad.

@                                             IN    SOA    ns1.service.nomad.   hostmaster.service.nomad.(
             {{ timestamp "unix" }}   ; Serial number
             28800        ; Refresh period (8 hours)
             7200         ; Retry interval (2 hour)
             864000       ; Expiration time (10 days)
             3600         ; Minimum TTL (1 hour)
             )

                                              IN    NS ns1.service.nomad.

{{- $rr_a := sprig_list -}}
{{- $rr_srv := sprig_list -}}

{{- /* Iterate over all of the registered Nomad services */ -}}
{{- range nomadServices -}}
    {{ $service := . }}

    {{- /* Iterate over all of the instances of a services */ -}}
    {{- range nomadService $service.Name -}}
        {{ $svc := . }}

        {{- /* Generate a uniq label for IP */ -}}
        {{- $node := $svc.Address | md5sum | sprig_trunc 8 }}

        {{- /* Record A & SRV RRs */ -}}
        {{- $rr_a = sprig_append $rr_a (sprig_list $svc.Name $svc.Address) -}}
        {{- $rr_a = sprig_append $rr_a (sprig_list $node $svc.Address) -}}
        {{- $rr_srv = sprig_append $rr_srv (sprig_list $svc.Name $svc.Port $node) -}}

        {{- /* Iterate over tags */ -}}
        {{- range $svc.Tags -}}
            {{- $tag := . -}}

            {{- /* Record A & SRV RRs */ -}}
            {{- $rr_a = sprig_append $rr_a (sprig_list (printf "%s.%v" $tag $svc.Name) $svc.Address) -}}
            {{- $rr_srv = sprig_append $rr_srv (sprig_list (printf "%s.%v" $tag $svc.Name) $svc.Port $node) -}}
        {{- end -}}
    {{- end -}}
{{- end -}}

{{- /* Iterate over lists and print everything */ -}}
{{- range $rr_a | sprig_uniq -}}
{{ printf "%-45s %s %4s %s" (index . 0) "IN" "A" (sprig_last . ) }}
{{ end }}

{{ range $rr_srv -}}
{{ printf "%-45s %s %4s %d %d %6d %s" (index . 0) "IN" "SRV" 0 0 (index . 1) (index . 2) }}
{{ end -}}

example.output

$TTL 0
$ORIGIN service.nomad.

@                                             IN    SOA    ns1.service.nomad.   hostmaster.service.nomad.(
             1681488575   ; Serial number
             28800        ; Refresh period (8 hours)
             7200         ; Retry interval (2 hour)
             864000       ; Expiration time (10 days)
             3600         ; Minimum TTL (1 hour)
             )

                                              IN    NS ns1.service.nomad.

dc865212                                      IN    A 172.30.28.31
6c7eb8e0                                      IN    A 172.30.59.68
b8d6f155                                      IN    A 172.30.46.148
whoami                                        IN    A 172.30.46.148
bar.whoami                                    IN    A 172.30.46.148
foo.whoami                                    IN    A 172.30.46.148
baz.whoami                                    IN    A 172.30.46.148
whoami                                        IN    A 172.30.59.68
bar.whoami                                    IN    A 172.30.59.68
foo.whoami                                    IN    A 172.30.59.68
baz.whoami                                    IN    A 172.30.59.68


whoami                                        IN  SRV 0 0  23396 b8d6f155
bar.whoami                                    IN  SRV 0 0  23396 b8d6f155
foo.whoami                                    IN  SRV 0 0  23396 b8d6f155
whoami                                        IN  SRV 0 0  31486 b8d6f155
baz.whoami                                    IN  SRV 0 0  31486 b8d6f155
foo.whoami                                    IN  SRV 0 0  31486 b8d6f155
whoami                                        IN  SRV 0 0  23242 6c7eb8e0
bar.whoami                                    IN  SRV 0 0  23242 6c7eb8e0
foo.whoami                                    IN  SRV 0 0  23242 6c7eb8e0
whoami                                        IN  SRV 0 0  28488 6c7eb8e0
baz.whoami                                    IN  SRV 0 0  28488 6c7eb8e0
foo.whoami                                    IN  SRV 0 0  28488 6c7eb8e0

nomad service info

$ nomad service info whoami
Job ID  Address              Tags       Node ID   Alloc ID
whoami  172.30.46.148:23396  [foo,bar]  3b23e0a5  45d301b5
whoami  172.30.46.148:31486  [foo,baz]  3b23e0a5  45d301b5
whoami  172.30.59.68:23242   [foo,bar]  aa074fe2  506a2bf0
whoami  172.30.59.68:28488   [foo,baz]  aa074fe2  506a2bf0

I've had a hard time with some empty service names breaking the DNS file. If anyone else comes accross this, simple add an if statement around the record block – or do whatever you want to skip the service with an empty name 😁

        {{- if $svc.Name -}}
            {{- /* Record A & SRV RRs */ -}}
            {{- $rr_a = sprig_append $rr_a (sprig_list $svc.Name $svc.Address) -}}
            {{- $rr_a = sprig_append $rr_a (sprig_list $node $svc.Address) -}}
            {{- $rr_srv = sprig_append $rr_srv (sprig_list $svc.Name $svc.Port $node) -}}
        {{- end -}}