secure shell (ssh)

ssh is a secure shell protocol that runs on port 22 and sometimes if in different environment its on 8022 like in tmux

Important commands you must know

Key generation

• So its the first thing to do after installing ssh.

• There are two type of key ed25519 and rsa but use ed25519 cause its secure compare to rsa.

  • How to generate the key

        ssh-keygen -t ed25519 -C "write anythingto identify"
    

    so here -t is for the key algorithm type.

    -C is for comment so it is used to indentify the public key cause most generated key are same format and to indentify public key we use the comment.

note: The ssh-keygen will generate two files which are public and private keys the pubic file will have .pub extension where as private doesn't have.

Difference between private and pubilc key

• Public key are like the lock pad

• To understand easily think it as a talcha(Lock pad ) so we add that public key to lock which in case is server so we just add the key in authorized_key file.

• Its just an asumption that we assume it as lockpad to understand easily.

• Private key are the actual key for the lockpad

• Like each lock have different key same goes here every public key have different privatekey.

How does it work

• To understand how ssh work we need to know about ssh daemon(sshd)

• Its a background process that listen for any ssh connection

• When we try to use ssh command to connect the sshd first check if the server have the public key for the provided ssh private key if its there then only authorized other wise cant access.

• once it is authorized it create a tty session which provide a remote terminal.

Note The public key should be in the authorized_key file not like usual file in ./ssh/edu.pub.

SSh connection

• To connect with ssh we need the server ipaddress,user, hostname, port if modified

ssh -i private_key_path -p <port> user@hostname

-i stands for identity-file -p stands for port user is the actual user you want to login hostname is the ipaddress

Config File

• There is Config file we can create inside the .ssh folder

• Its used to automate the long command we type for connecting on the server like on above bash command.

• The file is in this format

host <anything> // this is the name that u used to connect like ssh <anything> 
    hostname <ipaddress>
    user <user to login> 
    port <which port sshd is listening to>
    IdentityFile <path to private ssh key>
    ConnectTimeout <time>  // used to timeout automatically after given time so that it doesnt continuously try to connect

Copy file using scp

• We can also send file from our local to server using scp command.

• Command format is like:

scp -i <private_key_path> -P <port> "file_path_to_send" user@hostname:<server_path_to_save>

Note Here its -P not -p remember.

Copy the public key directly

• We can directly copy the ssh public key without any problem

• Only problem is PasswordAuthentication should be enabled first

ssh-copy-id -i <path_to_public_key> user@hostname

Note Require password login enabled

Key Notes

• ServerAliveInterval: used to send small packet on given time so that ssh connection doesnt dies.ServerAliveInterval 60 sends every 60 second

• ServerAliveCountMax: how many times it should send the alive packet ServerAliveCountMax 10