NAME¶

loop - detects simple forwarding loops and halts the server.

DESCRIPTION¶

The loop plugin will send a random probe query to ourselves and will then keep track of how many times we see it. If we see it more than twice, we assume CoreDNS has seen a forwarding loop and we halt the process.

The plugin will try to send the query for up to 30 seconds. This is done to give CoreDNS enough time to start up. Once a query has been successfully sent, loop disables itself to prevent a query of death.

The query sent is <random number>.<random number>.zone with type set to HINFO.

SYNTAX¶

loop

EXAMPLES¶

Start a server on the default port and load the loop and forward plugins. The forward plugin forwards to it self.

. {


    loop


    forward . 127.0.0.1
}

After CoreDNS has started it stops the process while logging:

plugin/loop: Loop (127.0.0.1:55953 -> :1053) detected for zone ".", see https://coredns.io/plugins/loop#troubleshooting. Query: "HINFO 4547991504243258144.3688648895315093531."

LIMITATIONS¶

This plugin only attempts to find simple static forwarding loops at start up time. To detect a loop, the following must be true:

• the loop must be present at start up time.
• the loop must occur for the HINFO query type.

TROUBLESHOOTING¶

When CoreDNS logs contain the message Loop ... detected ..., this means that the loop detection plugin has detected an infinite forwarding loop in one of the upstream DNS servers. This is a fatal error because operating with an infinite loop will consume memory and CPU until eventual out of memory death by the host.

A forwarding loop is usually caused by:

• Most commonly, CoreDNS forwarding requests directly to itself. e.g. via a loopback address such as 127.0.0.1, ::1 or 127.0.0.53
• Less commonly, CoreDNS forwarding to an upstream server that in turn, forwards requests back to CoreDNS.

To troubleshoot this problem, look in your Corefile for any forwards to the zone in which the loop was detected. Make sure that they are not forwarding to a local address or to another DNS server that is forwarding requests back to CoreDNS. If forward is using a file (e.g. /etc/resolv.conf), make sure that file does not contain local addresses.