coredns/plugin/secondary/README.md

# secondary

## Name

*secondary* - enables serving a zone retrieved from a primary server.

## Description

With *secondary* you can transfer (via AXFR) a zone from another server. The retrieved zone is
*not committed* to disk (a violation of the RFC). This means restarting CoreDNS will cause it to
 retrieve all secondary zones.

~~~
secondary [ZONES...]
~~~

* **ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
    are used. Note that without a remote address to *get* the zone from, the above is not that useful.

A working syntax would be:

~~~
secondary [zones...] {
    transfer from ADDRESS
    transfer to ADDRESS
    upstream ADDRESS...
}
~~~

* `transfer from` specifies from which address to fetch the zone. It can be specified multiple times;
    if one does not work, another will be tried.
* `transfer to` can be enabled to allow this secondary zone to be transferred again.
* `upstream` defines upstream resolvers to be used resolve external names found (think CNAMEs)
  pointing to external names. This is only really useful when CoreDNS is configured as a proxy, for
  normal authoritative serving you don't need *or* want to use this. **ADDRESS** can be an IP
  address, and IP:port or a string pointing to a file that is structured as /etc/resolv.conf.

When a zone is due to be refreshed (Refresh timer fires) a random jitter of 5 seconds is
applied, before fetching. In the case of retry this will be 2 seconds. If there are any errors
during the transfer the transfer fails; this will be logged.

## Examples

Transfer `example.org` from 10.0.1.1, and if that fails try 10.1.2.1.

~~~ corefile
example.org {
    secondary {
        transfer from 10.0.1.1
        transfer from 10.1.2.1
    }
}
~~~

Or re-export the retrieved zone to other secondaries.

~~~ corefile
. {
    secondary example.net {
        transfer from 10.1.2.1
        transfer to *
    }
}
~~~

## Bugs

Only AXFR is supported and the retrieved zone is not committed to disk.