Since the documentation doesn't clearly state how the redirect URL should look like, I'm assuming it will follow RFC3986 rules.

1. Trying to redirect to a IPv6 address doesn't work:

From RFC3689:

"A host identified by an IPv6 literal address is represented inside the square brackets without a preceding version flag"

set global redirect_url="mariadb://[::1]:3306";

ERROR 1231 (42000): Variable 'redirect_url' can't be set to the value of 'mariadb://[::1]:3306'

2. Reserved characters

Any reserved characters for URLs (for example, /, :, @, (, ), [, ], &, #, =, ?, and space) that appear in any part of the connection URL must be percent encoded.

set global redirect_url="mariadb://@host@:3306";

Query OK, 0 rows affected (0.001 sec)

3. Disallow to redirect to the same host/port

Don't allow to set redirect_url to the same host/port the server is running on - this might end up in endless connection loops.