# HTTP/JSON Rather than connecting directly to the [control socket](api_control_sockets.html.md), there is a JSON over HTTP endpoint that proxies calls to the `smtpd` control socket. The JSON [request](api_reference.html.md) body (called *payload*) and [response](api_reference.html.md) are direct JSON representations of the respective Protobuf structures. The HTTP/JSON API takes *version*, *program*, *command* and *payload* as arguments, where *payload* contains the [request](api_reference.html.md) body. * [Installation](api_install.html.md) * [Distributions](api_install.html.md#distributions) * [Ubuntu](api_install.html.md#ubuntu) * [RHEL](api_install.html.md#rhel) * [Azure Linux](api_install.html.md#azure-linux) * [Startup](api_install.html.md#startup) * [Upgrade](api_upgrade.html.md) * [Configuration](api_config.html.md) * [Authentication directives](api_config.html.md#authentication-directives) * [Listener directives](api_config.html.md#listener-directives) * [Other directives](api_config.html.md#other-directives) * [Environment directives](api_config.html.md#environment-directives) * [Examples](api_examples.html.md)