How can I read consul SRV records in my go application? - dns

Question

I am trying to implement consul for service discovery, and I am having trouble with two things: connecting to a custom DNS server, and formatting my net.LookupSRV() request.
Here is what I'm trying to look up from within my go app:
$ dig #127.0.0.1 -p 8600 serviceb.service.consul SRV
; <<>> DiG 9.8.4-rpz2+rl005.12-P1 <<>> #127.0.0.1 -p 8600 serviceb.service.consul SRV
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 4511
;; flags: qr aa rd; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 2
;; WARNING: recursion requested but not available
;; QUESTION SECTION:
;serviceb.service.consul. IN SRV
;; ANSWER SECTION:
serviceb.service.consul. 0 IN SRV 1 1 80 az1-serviceb1.node.dc1.consul.
serviceb.service.consul. 0 IN SRV 1 1 80 az2-serviceb2.node.dc1.consul.
;; ADDITIONAL SECTION:
az1-serviceb1.node.dc1.consul. 0 IN A 10.6.41.22
az2-serviceb2.node.dc1.consul. 0 IN A 10.6.41.20
;; Query time: 6 msec
;; SERVER: 127.0.0.1#8600(127.0.0.1)
;; WHEN: Fri May 16 15:09:28 2014
;; MSG SIZE rcvd: 275
and here is the relevant code. (I know it's wrong, but just so you can see what I'm trying to do)
cname, addrs, err := net.LookupSRV("serviceb", "service", "consul")
log.Printf("%+v %+v %+v", cname, addrs, err)
and the output:
2014/05/16 15:24:31 [] lookup _serviceb._service.consul: no such host
Any help would be appreciated! thanks

Answer 1

Try to use a more sharp tool such as the github.com/miekg/dns package. Last time I looked at it, it allowed to control virtually every bit of the client-side setup to do DNS resolution.

Answer 2

Consul does support strict RFC 2782 and lookup can be done using the standard library only:
resolver := &net.Resolver{
Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
return (&net.Dialer{}).DialContext(ctx, network, "127.0.0.1:8600")
},
}
_, addrs, err := resolver.LookupSRV(
context.Background(), "svcname", "tcp", "consul",
)

Answer 3

While this doesn't answer your exact question, I find it's an easier way to access service data for greenfield apps.
It's quite easy to call the HTTP API with net/http:
package main
import (
"fmt"
"net/http"
"io/ioutil"
)
func main() {
resp, _ := http.Get("http://localhost:8500/v1/catalog/service/serviceb")
body, _ := ioutil.ReadAll(resp.Body)
fmt.Print(string(body))
}
The basics of the HTTP API are documented in the Services Guide.

Answer 4

The best way is to use PreparedQueries, as Posted here
import (
"fmt"
consulapi "github.com/hashicorp/consul/api"
)
func main() {
config := consulapi.DefaultConfig()
consul, err := consulapi.NewClient(config)
if err != nil {
fmt.Println(err)
}
preparedQuery := consul.PreparedQuery()
queryID, _, err := preparedQuery.Create(&consulapi.PreparedQueryDefinition{
Name: "DnsQuery",
Service: consulapi.ServiceQuery{
Service: "serviceb",
OnlyPassing: true,
},
}, &consulapi.WriteOptions{})
if err != nil {
fmt.Println(err)
}
res, _, _ := preparedQuery.Execute(queryID, &consulapi.QueryOptions{})
for _, node := range res.Nodes {
fmt.Println(node.Service.Address, node.Service.Port)
}