COREDNS-LOOP(7)

NAME
loop - detect simple forwarding loops and halt 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 is looping 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: Forwarding loop detected in "." zone. Exiting. See https://coredns.io/plugins/loop#troubleshooting. Probe query: "HINFO 5577006791947779410.8674665223082153551.".

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

•   the loop must be present at start up time.

•   the loop must occur for at least the HINFO query type.

TROUBLESHOOTING
When CoreDNS logs contain the message Forwarding 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. to 127.0.0.1 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 proxy or forward 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 proxy or forward are using a file (e.g. /etc/resolv.conf), make sure that file does not contain local addresses.

  TROUBLESHOOTING LOOPS IN KUBERNETES CLUSTERS
When a CoreDNS Pod deployed in Kubernetes detects a loop, the CoreDNS Pod will start to "CrashLoopBackOff". This is because Kubernetes will try to restart the Pod every time CoreDNS detects the loop and exits.

A common cause of forwarding loops in Kubernetes clusters is an interaction with systemd-resolved on the host node. systemd-resolved will, in certain configurations, put 127.0.0.53 as an upstream into /etc/resolv.conf. Kubernetes (kubelet) by default will pass this /etc/resolv/conf file to all Pods using the default dnsPolicy (this includes CoreDNS Pods). CoreDNS then uses this /etc/resolv.conf as a list of upstreams to proxy/forward requests to. Since it contains a local address, CoreDNS ends up forwarding requests to itself.

There are many ways to work around this issue, some are listed here: * Add the following to kubelet: --resolv-conf /run/systemd/resolve/resolv.conf. This flag tells kubelet to pass an alternate resolv.conf to Pods. For systemd-resolved, /run/systemd/resolve/resolv.conf is typically the location of the "original" /etc/resolv.conf. * Disable systemd-resolved on host nodes, and restore /etc/resolv.conf to the original. * A quick and dirty fix is to edit your Corefile, replacing proxy . /etc/resolv.conf with the ip address of your upstream DNS, for example proxy . 8.8.8.8.