MSEARCH

MSEARCH is a request method used by the Simple Service Discovery Protocol (SSDP), which in turn is a key component of the Universal Plug and Play (UPnP) framework. It enables clients on a local IP network to discover devices and services without prior knowledge of their addresses.

In operation, a client constructs an M-SEARCH request and sends it over UDP to the SSDP multicast address 239.255.255.250 on port 1900. The request line is M-SEARCH * HTTP/1.1, followed by headers such as HOST, MAN: "ssdp:discover", MX, and ST. The MX header specifies the maximum wait time (in seconds) for responses, and the ST (search target) identifies the type of device or service the client is seeking (for example, ssdp:all to discover all devices, or urn:schemas-upnp-org:device:MediaServer:1 to find a specific device type).

Devices that match the search respond with a unicast UDP reply to the requester. The reply uses HTTP-like headers and includes details such as LOCATION (URL of device description), ST (same search target), and USN (unique service name). This mechanism allows clients to build a dynamic map of networked devices and their available services.

MSEARCH is widely used in UPnP and DLNA environments for initial discovery of media servers, printers, smart speakers, and other networked devices. It operates entirely within the local network and depends on UDP multicast delivery, which has implications for firewalls, NAT, and network segmentation. See also SSDP, UPnP, and DLNA.

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