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.

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 a local DNS cache on the host node (e.g. systemd-resolved). For example, in certain configurations systemd-resolved will put the loopback address 127.0.0.53 as a nameserver into /etc/resolv.conf. Kubernetes (via kubelet) by default will pass this /etc/resolv.conf file to all Pods using the default dnsPolicy rendering them unable to make DNS lookups (this includes CoreDNS Pods). CoreDNS uses this /etc/resolv.conf as a list of upstreams to forward requests to. Since it contains a loopback 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 your kubelet config yaml: resolvConf: <path-to-your-real-resolv-conf-file> (or via command line flag --resolv-conf deprecated in 1.10). Your "real" resolv.conf is the one that contains the actual IPs of your upstream servers, and no local/loopback address. This flag tells kubelet to pass an alternate resolv.conf to Pods. For systems using systemd-resolved, /run/systemd/resolve/resolv.conf is typically the location of the "real" resolv.conf, although this can be different depending on your distribution.
Disable the local DNS cache on host nodes, and restore /etc/resolv.conf to the original.
A quick and dirty fix is to edit your Corefile, replacing forward . /etc/resolv.conf with the IP address of your upstream DNS, for example forward . 8.8.8.8. But this only fixes the issue for CoreDNS, kubelet will continue to forward the invalid resolv.conf to all default dnsPolicy Pods, leaving them unable to resolve DNS.