mirror of
https://github.com/coredns/coredns.git
synced 2025-10-30 17:53:21 -04:00
92 lines
4.2 KiB
Markdown
92 lines
4.2 KiB
Markdown
# loop
|
|
|
|
## 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 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
|
|
|
|
~~~ txt
|
|
loop
|
|
~~~
|
|
|
|
## Examples
|
|
|
|
Start a server on the default port and load the *loop* and *forward* plugins. The *forward* plugin
|
|
forwards to it self.
|
|
|
|
~~~ txt
|
|
. {
|
|
loop
|
|
forward . 127.0.0.1
|
|
}
|
|
~~~
|
|
|
|
After CoreDNS has started it stops the process while logging:
|
|
|
|
~~~ txt
|
|
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 `forward`s 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.
|