Function format_mac 

pub fn format_mac(bytes: &[u8]) -> String

Render a hardware address the way the bootp_request event reports it: lower-case hex with no separators, e.g. 001122334455.

The separator-free form is what tests/server/bootp/e2e_test.rs matches on, so it is the contract. send_bootp_reply’s client_mac parameter accepts either this or the colon-separated form when a handler supplies one explicitly.