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consulsrv-record

How consul constructs SRV record

Let say I registered a service in consul, so that I can query it by something like:

curl http://localhost:8500/v1/catalog/service/BookStore.US

and it returns

[
    {
        "ID": "xxxxx-xxx-...",
        "ServiceName": "BookStore.US",
        ...
    }
]

If I am using consul directly in my code, it is ok. But the problem is that when I want to use the SRV record directly, it does not work.

Normally, there is a service record created by consul with the name service_name.service.consul. In the above case, it is "BookStore.US.service.consul"

So that you can use "dig" command to get it.

dig @127.0.0.1 -p 8600 BookStore.US.service.consul SRV

But when I tried to "dig" it, it failed with 0 answer session.

My question:

How does consul construct the service/SRV name (pick up some fields in the registered consul record and concat them?)

Is there any way for me to search the SRV records with wildcards, so that at least I can search the SRV name by using the key word "BookStore"

Solution

  • The SRV lookup is not working because Consul is interpreting the . in the service name as domain separator in the hostname.

    Per https://www.consul.io/docs/discovery/dns#standard-lookup, service lookups in Consul can use the following format.

    [tag.]<service>.service[.datacenter].<domain>

    The tag and datacenter components are optional. The other components must be specified. Given the name BookStore.US.service.consul, Consul interprets the components to be:

    • Tag: BookStore
    • Service: US
    • Sub-domain: service
    • TLD: consul

    Since you do not have a service registered by the name US, the DNS server correctly responds with zero records.

    In order to resolve this, you can do one of two things.

    1. Register the service with a different name, such as bookstore-us.

      {
  "Name": "bookstore-us",
  "Port": 1234
}

    2. Specify the US location as a tag in the service registration.

      {
  "Name": "bookstore",
  "Tags": ["us"],
  "Port": 1234
}

    Note that in either case, the service name should be a valid DNS label. That is, it may contain only the ASCII letters a through z (in a case-insensitive manner), the digits 0 through 9, and the hyphen-minus character ('-').

    The SRV query should then successfully return a result for the service lookup.

    # Period in hostname changed to a hyphen
$ dig -t SRV bookstore-us.service.consul +short

# If `US` is a tag:

# Standard lookup
$ dig -t SRV us.bookstore.service.consul +short

# RFC 2782-style lookup
$ dig -t SRV _bookstore._us.service.consul +short