What protocol remains functional if a masquerade rule is improperly configured?

HTTP remains functional if a masquerade rule is improperly configured because it generally operates over TCP and primarily relies on a simple request-response model that can still be processed in many cases without the need for modifications in the NAT rules.

When masquerading is implemented, it is used to allow private IP addresses to be represented by a single public IP address for outbound connections. If the masquerade rule has a flaw or is misconfigured, it might still allow basic HTTP traffic to flow through because HTTP has inherent fallbacks in networking — many web applications can attempt to connect using alternative ports and mechanisms.

In contrast, other protocols such as FTP may require specific configuration to operate correctly behind a NAT due to the way they manage connections and data channels. DNS is integral to resolving hostnames to IP addresses but is usually not directly affected by NAT misconfigurations. SMTP, used for email transmission, can also face issues if not properly configured, particularly with regards to server communication and client/SMTP relay specifications.

Thus, among the given protocols, HTTP is likely to continue functioning even when masquerade settings are incorrect, making it the most dependable choice in this scenario.