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This article is a troubleshoot documentation about problems with SSH Server on the Ubuntu Server 18.04 operating system.
Problems:
When Ubuntu Server 18.04 has not been installed SSH Server then we install the new SSH Server package, but from the SSH Client it cannot connect and there is an error message as follows:

Unexpectedly closed network connection server
Network error: Software caused connection abort
Connection closed by 192.168.2.106 port 22
ssh: connect to host 192.168.20.106 port 22: Connection refused

Log output at: /var/log/auth.log
Nov 18 03:59:56 localhost sshd[10704]: error: key_load_private: invalid format
Nov 18 03:59:56 localhost sshd[10704]: error: key_load_public: invalid format
Nov 18 03:59:56 localhost sshd[10704]: error: Could not load host key: /etc/ssh/ssh_host_rsa_key
Cause:
Format host key rsa,ecdsa,ed25519 are invalid / wrong.
Solution:
Generate suitable new formats:
ssh-keygen -A
Restart SSH Server Services:
systemctl restart sshd

https://www.e-tinkers.com/2020/01/getting-started-with-stm32-and-things-you-need-to-be-aware-of/

Definisi Protokol menurut saya adalah aturan-aturan yang harus diikuti. Sehingga IOT Protocol adalah aturan-aturan yang harus dipatuhi agar suksesnya IOT. Adapun protokol-protokol yang mendukung IOT adalah
1. HTTP
2. CoAP
3. MQTT
4. WebSockets
HTTP merupakan kepanjangan dari Hypertext Transfer Protocol. Umumnya http ini dipakai untuk menjelajah informasi-informasi yang ada di internet dengan bantuan aplikasi browser, seperti Mozilla, IE, Opera dll. Berdasarkan pengalaman saya, script PHP juga harus dipasangkan juga, sehingga di server IOT tersebut tersedia webserver (misalnya Apache) untuk membuka jalur HTTP dan aplikasi PHP untuk menjalankan scriptnya. Script yang umum dipakai adalah get, post.
Constrained Application Protocol (CoAP) is a lightweight http protocol that reads and controls the sensors deployed for IoT. It has actions like get, post, put, delete, observe, discover. This coap can be accessed similar like http, For ex: to access a particular mote (Sensor) with IPv6 like this coap://[aaaa::212:7402:2:202] The above line will fetch the sensor boards peripherals and one can control from the browser itself. Firefox has a Cu plugin to enable CoAP within the browser. CoAP is already available for all the devices like IOS, Android, Windows, Linux, Mac, etc