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Allow multipe DNS servers as comma-seperated argument for kubelet --dns
This PR explores how kubectls "--dns" could be extended to specify multiple DNS servers for in-cluster PODs. Testing on the local libvirt-coreos cluster shows that multiple DNS server are injected without issues.
Specifying multiple DNS servers increases resilience against
- Packet drops
- Single server failure
I am debugging services that do 50+ DNS requests for a single incoming interactive request, thus highly increase the chance of a slowdown (+5s) due to a single packet drop. Switching to two DNS servers will reduce the impact of the issues (roughly +1s on glibc, 0s on musl, error-rate goes down to error-rate^2).
Note that there is no need to change any runtime related code as far as I know. In the case of "default" dns the /etc/resolv.conf is parsed and multiple DNS server are send to the backend anyway. This only adds the same capability for the clusterFirst case.
I've heard from @thockin that multiple DNS entries are somehow considered. I've no idea what was considered, though. This is what I would like to see for our production use, though.
```release-note
NONE
```
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