CatchallDomains

CatchallDomains is a term used to describe configuring one or more domains to capture traffic that would otherwise go unaddressed, routing it to a defined destination rather than rejecting it. The concept appears in both email hosting and DNS/web hosting contexts and is used to ensure unhandled queries are not lost and to provide a predictable handling path for misaddressed or unknown requests.

In email hosting, a catch-all domain (or catch-all mailbox) accepts messages sent to any address at the domain, delivering them to a single mailbox or distribution list. This can prevent important messages from bouncing due to non-existent addresses and can aid monitoring or branding efforts. However, it also increases the potential for spam, requiring robust filtering and resource planning. Implementation typically involves a catch-all alias or a wildcard alias in the mail system, such as configurations available in Postfix, Exim, or other MTAs.

In DNS and web hosting, a catch-all is often implemented as a wildcard DNS record (*.domain) or a wildcard virtual host, directing requests for unknown subdomains to a specific server or application. This supports handling new subdomains, testing environments, or brand-consistent landing pages, and may be used in conjunction with wildcard TLS certificates. Caveats include security implications, certificate management challenges, and potential logging complications, along with unintended traffic to the catch-all destination.

Administrators should weigh convenience against risk, and document the intended handling for catch-all traffic. See also wildcard DNS, wildcard SSL, catch-all mailbox, and email aliasing.