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plugin/loop: Improve loop troubleshoot docs (#2363)

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* improve loop troubleshoot docs

* fix spelling
This commit is contained in:
Chris O'Haver
2018-12-04 06:58:20 -05:00
committed by Miek Gieben
parent e5f5da4297
commit b53cc51f53
1 changed files with 15 additions and 12 deletions
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plugin/loop/README.md
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@@ -67,19 +67,22 @@ to another DNS server that is forwarding requests back to CoreDNS. If `proxy` or
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.
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 proxy/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 `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.
* Add the following to `kubelet`: `--resolv-conf <path-to-your-real-resolv-conf-file>`. 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 `proxy . /etc/resolv.conf` with
the ip address of your upstream DNS, for example `proxy . 8.8.8.8`.
the ip address of your upstream DNS, for example `proxy . 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.