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Fallback DNS

Set up Fallback DNS, which acts as a fallback DNS to provide query services when the primary DNS fails or does not respond.

Use the -fallback parameter to designate a specific server as the fallback DNS.
```
server -fallback
```
Equivalent configuration method:

Use the -e -group fallback options together, which is equivalent to the above configuration.
```
server -e -group fallback
```