Service Discovery

Giriş
Service Discovery şu temeller üzerine inşa edilir.
Service Registration : Yeni bir katılımcı kendisini bir şekilde anons eder.
Service Discovery: Bir hizmete ihtiyacı olan katılımcı hizmeti sağlayan katılımcıyı bulur.

Service Registry Yöntemi
Bu yöntemde yeni katılımcı merkezi bir kayıt alanına (service registry) kayıt olunur. Hizmete ihtiyacı olan katılımcı merkezi kayıt alanından sorgulama yapar.

Broadcast Yöntemi
Bu yöntemde yeni katılımcı varlığını ağa broadcast eder. Diğer katılımcılar tüm ağın bir listesini saklarlar. Hizmete ihtiyacı olan katılımcı katılımcıların listesi elinde olduğu için gerekli eşleşmeyi sağladığını düşündüğü katılımcıyla irtibata geçer.

How can I discover a local network appliance and connect to its built-in webserver?

Açıklaması şöyle

There are protocols / combinations of protocols designed for this very scenario. They broadly fall under the category of zero configuration networking aka zeroconf. Some popular implementations are Apple's Bonjour (formerly known as Rendezvous), mDNSResponder (an Open Source implementation by Apple), and Avahi, an Open Source cross-platform implementation compatible with Bonjour. Microsoft's protocol is called UPnP (Universal Plug and Play). There are also many implementations targeted at the IoT space for all kinds of footprints down to the smallest sensor platforms.

Apple Bonjour
Açıklaması şöyle. Eski ismi Rendezvous idi

Apple Bonjour protocol use Multicast DNS, which has been normalized in RFC6762 and RFC6763.

It uses multicast IPv4 address 224.0.0.251 or IPv6 address FF02::FB

An bonjour enabled device will announce itself by multicast; so what you have to do is to examine traffic sent to 224.0.0.251 (with TCDPUMP, wireshark or equivalent).

mDNS

Açıklaması şöyle. Yani DNS altyapısı gerektirmez.

mDNS (Multicast DNS) makes it possible to essentially just blindly send DNS Queries into the network as multicast UDP messages and allow anybody to answer them.

Açıklaması şöyle

In theory, mDNS can be used for zero-configuration networking:

- The appliance connects normally to the available network and receives an IP address via DHCP.

- The appliance listens to mDNS broadcasts on the local network.

- The user navigates to a page on the some-appliance.local domain. If configured properly, this should cause the user's operating system to try mDNS.

- The appliance receives the broadcast and returns a mDNS response with its IP address.

- The user's browser connects to port 80 on that IP address.

This is exactly your UDP multicast idea, except that the protocol already exists. Unfortunately, there are substantial differences in mDNS support between operating systems – test carefully. Domains in the .local TLD should use mDNS automatically, but most operating systems have mDNS disabled by default (as of 2021).

A further limitation of this approach is that you won't get certificates for .local domains, so you won't get HTTPS. This is a problem as browsers are moving to HTTPS by default and will show scary warning messages for HTTP sites.

DNS-SD (DNS Service Discovery)

Açıklaması şöyle. Yani DNS altyapısı gerektirir.

DNS-SD (DNS Service Discovery) uses special DNS entries to describe services. For example, to find a printer in the local network, you just resolve the SRV record _ipp._tcp.local.