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We enable special behavior under the `kv.sslip.io` subdomain: it can be
treated as a key-value store, the sub-subdomain being the key, and the
TXT record being the value.
For example, to write ("put") the value "12.0.1" to the key
"macos-version" on the `ns-gce.sslip.io.` nameserver, you'd use the
following `dig` command:
```shell
dig @ns-gce.sslip.io. txt put.12.0.1.macos-version.kv.sslip.io.
```
To read ("get") the value back, you'd write the following `dig` command:
```shell
dig @ns-gce.sslip.io. txt get.macos-version.kv.sslip.io.
```
Since "get" is the default behavior, you don't need to include it in the
domain name:
```shell
dig @ns-gce.sslip.io. txt macos-version.kv.sslip.io.
```
Finally, when you're done with the key-value, you can "delete" it:
```shell
dig @ns-gce.sslip.io. txt delete.macos-version.kv.sslip.io.
```
Notes:
- Keys are case-insensitive (to accommodate DNS convention). In other
words, `KEY.kv.sslip.io` and `key.kv.sslip.io` return the same TXT
record.
- Values are case-sensitive. `put.CamelCase.style.kv.sslip.io` sets the
TXT record to "CamelCase".
- `put` requests will return the TXT record being put; i.e.
`put.hello.world.kv.sslip.io` returns one TXT record of one string,
`hello`.
- `delete` requests will return the TXT record being deleted; i.e.
`delete.world.kv.sslip.io` returns one TXT record of one string,
`hello`. If the TXT record does not exist, no TXT records will be
returned.
- Values are limited to 63 bytes to mitigate using the sslip.io servers
in a [DNS amplification
attack](https://us-cert.cisa.gov/ncas/alerts/TA13-088A).
- Values are not persistent: if the server is restarted, all values
disappear. Poof.
- Values are not consistent. If a value is set in `ns-aws.sslip.io`, it
does not propagate to `ns-gce.sslip.io` nor `ns-azure.sslip.io`.