Cvdupdate

➕ `cvd update` will now retry up to 3x if the downloaded content length is
less than the content-length in the response header. This is to resolve
issues with flakey connections.

➕ `cvd update` now has a `--debug-mode` (`-D`) option to print out the HTTP
headers to debug issues with the update process.

➕ The update process will now save the DNS TXT record containing version
metadata as `dns.txt` in the database directory so it may be served by the
private mirror.

Some common check scripts (on clients) use `dns.txt` to check if ClamAV is up
to date instead of using DNS or the HTTP CVD-header check.

🌌 CVDUpdate will now have a unique User-Agent: `CVDUPDATE/<version> (<UUID>)`

The UUID is randomly generated, and will help with anonymous usage metrics.

🐛 Fixed a couple issues with the `cvd update <specific database>` option.

Added

➕ Two ways to set a custom DNS nameserver.
DNS queries are required to check the latest available database versions.

1. Set the nameserver in the config. Eg:

bash
cvd config set --nameserver 208.67.222.222
cvd update


2. Set the environment variable `CVDUPDATE_NAMESERVER`. Eg:

bash
CVDUPDATE_NAMESERVER="208.67.222.222" cvd update


🐛 Error handling so `dns update` fails if a DNS query fails.

Acknowledgements

Special thanks to Michael Callahan for adding the custom nameserver feature.

First release!