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92 lines
4.2 KiB
Markdown
92 lines
4.2 KiB
Markdown
# loop
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## Name
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*loop* - detects simple forwarding loops and halts the server.
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## Description
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The *loop* plugin will send a random probe query to ourselves and will then keep track of how many times
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we see it. If we see it more than twice, we assume CoreDNS has seen a forwarding loop and we halt the process.
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The plugin will try to send the query for up to 30 seconds. This is done to give CoreDNS enough time
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to start up. Once a query has been successfully sent, *loop* disables itself to prevent a query of
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death.
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The query sent is `<random number>.<random number>.zone` with type set to HINFO.
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## Syntax
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~~~ txt
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loop
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~~~
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## Examples
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Start a server on the default port and load the *loop* and *forward* plugins. The *forward* plugin
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forwards to it self.
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~~~ txt
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. {
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loop
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forward . 127.0.0.1
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}
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~~~
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After CoreDNS has started it stops the process while logging:
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~~~ txt
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plugin/loop: Loop (127.0.0.1:55953 -> :1053) detected for zone ".", see https://coredns.io/plugins/loop#troubleshooting. Query: "HINFO 4547991504243258144.3688648895315093531."
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~~~
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## Limitations
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This plugin only attempts to find simple static forwarding loops at start up time. To detect a loop,
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the following must be true:
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* the loop must be present at start up time.
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* the loop must occur for the `HINFO` query type.
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## Troubleshooting
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When CoreDNS logs contain the message `Loop ... detected ...`, this means that the `loop` detection
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plugin has detected an infinite forwarding loop in one of the upstream DNS servers. This is a fatal
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error because operating with an infinite loop will consume memory and CPU until eventual out of
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memory death by the host.
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A forwarding loop is usually caused by:
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* 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`
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* Less commonly, CoreDNS forwarding to an upstream server that in turn, forwards requests back to CoreDNS.
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To troubleshoot this problem, look in your Corefile for any `forward`s to the zone
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in which the loop was detected. Make sure that they are not forwarding to a local address or
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to another DNS server that is forwarding requests back to CoreDNS. If `forward` is
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using a file (e.g. `/etc/resolv.conf`), make sure that file does not contain local addresses.
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### Troubleshooting Loops In Kubernetes Clusters
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When a CoreDNS Pod deployed in Kubernetes detects a loop, the CoreDNS Pod will start to "CrashLoopBackOff".
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This is because Kubernetes will try to restart the Pod every time CoreDNS detects the loop and exits.
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A common cause of forwarding loops in Kubernetes clusters is an interaction with a local DNS cache
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on the host node (e.g. `systemd-resolved`). For example, in certain configurations `systemd-resolved` will
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put the loopback address `127.0.0.53` as a nameserver into `/etc/resolv.conf`. Kubernetes (via `kubelet`) by default
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will pass this `/etc/resolv.conf` file to all Pods using the `default` dnsPolicy rendering them
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unable to make DNS lookups (this includes CoreDNS Pods). CoreDNS uses this `/etc/resolv.conf`
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as a list of upstreams to forward requests to. Since it contains a loopback address, CoreDNS ends up forwarding
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requests to itself.
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There are many ways to work around this issue, some are listed here:
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* 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"
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`resolv.conf` is the one that contains the actual IPs of your upstream servers, and no local/loopback address.
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This flag tells `kubelet` to pass an alternate `resolv.conf` to Pods. For systems using `systemd-resolved`,
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`/run/systemd/resolve/resolv.conf` is typically the location of the "real" `resolv.conf`,
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although this can be different depending on your distribution.
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* Disable the local DNS cache on host nodes, and restore `/etc/resolv.conf` to the original.
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* A quick and dirty fix is to edit your Corefile, replacing `forward . /etc/resolv.conf` with
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the IP address of your upstream DNS, for example `forward . 8.8.8.8`. But this only fixes the issue for CoreDNS,
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kubelet will continue to forward the invalid `resolv.conf` to all `default` dnsPolicy Pods, leaving them unable to resolve DNS.
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