Add DNS_CHECK directive

[pandax381]

Keepalived supports DNS health checker since 1.3.0.

keepalived.conf(5)

...
# DNS healthchecker
DNS_CHECK
{  
    # ======== generic connection options
    # Optional IP address to connect to.
    # The default is the realserver IP
    connect_ip <IP ADDRESS>
    # Optional port to connect to
    # The default is the realserver port
    connect_port <PORT>
    # Optional interface to use to
    # originate the connection
    bindto <IP ADDRESS>
    # Optional source port to
    # originate the connection from
    bind_port <PORT>
    # Optional connection timeout in seconds.
    # The default is 5 seconds
    connect_timeout <INTEGER>
    # Optional fwmark to mark all outgoing
    # checker packets with
    fwmark <INTEGER>

    # Number of times to retry a failed check
    # The default is 3 times.
    retry <INTEGER>
    # DNS query type
    #   A|NS|CNAME|SOA|MX|TXT|AAAA
    # The default is SOA
    type <STRING>
    # Domain name to use for the DNS query
    # The default is . (dot)
    name <STRING>
}
...

[yuuki]

LGTM! Thanks!

[yuuki]

I have released v0.3.5 including this PR. https://github.com/yuuki/gokc/releases/tag/v0.3.5