Discovery with SRV Records

About DNS SRV records

DNS SRV records are resources used to identify computers that host specific services. They are contained in the ANSWER section of DNS responses and have the following structure:

_service._proto.name. TTL class SRV priority weight port target

where:

Configure service discovery

1. Update your DNS nameserver to resolve a service name, such as myservice.example.local, to one or more hostnames and ports via SRV records. Those hostnames should resolve to IP addresses using A records.

   • Add DNS A records that resolve multiple hostnames, such as host1, host2 and host3, to different IP addresses.

   • Add the same number of SRV records that resolve a service name, such as _myservice._tcp.example.local, to the hostnames you defined and the port on the host where the service listens.

2. Query the nameserver directly with the dig tool to ensure that it returns the correct records, as follows:

   $ dig @192.168.50.30 -p 53 SRV _myservice._tcp.example.local
   
      ;; QUESTION SECTION:
      ;_myservice._tcp.example.local. IN
      ;; ANSWER SECTION:
      _myservice._tcp.example.local. 0 IN     SRV     0 0 8080 host1.
      _myservice._tcp.example.local. 0 IN     SRV     0 0 8081 host2.
      _myservice._tcp.example.local. 0 IN     SRV     0 0 8082 h
      ;; ADDITIONAL SECTION:
      host1.                  0       IN      A       192.168.50.31
      host2.                  0       IN      A       192.168.50.32
      host3.                  0       IN      A       192.168.50.33

3. Add a resolvers section in your HAProxy Enterprise configuration file to set the DNS nameservers to watch for changes.

   • Add one or more nameserver lines to specify the IP addresses and ports of your DNS nameservers.

   • Set the accepted_payload_size to 8192 to allow larger DNS payloads, which is required to receive more server IPs within a single DNS result.

   resolvers mydns
      nameserver dns1 192.168.50.30:53
      accepted_payload_size 8192

4. Use a server-template in a backend to set the template for the server lines when HAProxy Enterprise queries your DNS servers, as follows:

   backend webservers
      balance roundrobin
      server-template web 5 _myservice._tcp.example.local resolvers mydns check init-addr none

   In this example, the server-template directive:

   • Adds the specified number of servers (5) to the backend.

   • Appends "web" as a prefix to their names.

   • Queries the service name _myservice._tcp.example.local.

   • Have the SRV records fill in the ports.

   • Specifies the resolvers section mydns

   • The init-addr none argument means that HAProxy Enterprise can initialize without having to resolve the IP addresses at startup. It can resolve them during runtime.

   This is equivalent to adding a backend to HAProxy Enterprise that looks like this:

   backend webservers
       balance roundrobin
       server web1 192.168.50.31:8080 check
       server web2 192.168.50.32:8081 check
       server web3 192.168.50.33:8082 check
       server web4 192.168.50.34:80 check disabled
       server web5 192.168.50.35:80 check disabled

   When you add more records to your nameserver, they will automatically go into the backend to fill in the web4 and web5 slots.

Inspect servers with the Runtime API

Run the command show servers state [backend name] to see the servers loaded into memory:

$ echo "show servers state webservers" | sudo socat stdio unix-connect:/var/run/hapee-2.0/hapee-lb.sock

  # be_id  be_name  srv_id  srv_name  srv_addr       srv_fqdn  srv_port  srvrecord
  3        servers  1       web1      192.168.50.33  host3     8082      _myservice._tcp.example.local
  3        servers  2       web2      192.168.50.32  host2     8081      _myservice._tcp.example.local
  3        servers  3       web3      192.168.50.31  host1     8080      _myservice._tcp.example.local
  3        servers  4       web4      -              -         0         _myservice._tcp.example.local
  3        servers  5       web5      -              -         0         _myservice._tcp.example.local