Ubuntu/openssh

Most versions of linux come with sshd installed, however it is not configured as a server. The easiest way is to use apt-get to install openssh-server

sudo apt-get install openssh-server

backup the default sshd_config to your home directory

sudo cp /etc/ssh/sshd_config ~

edit the sshd_config to increase security

disable root login

PermitRootLogin no

only allow specific users

AllowUsers john

restart sshd

sudo restart ssh

check to see if the service is running and listening on port 22

sudo egrep -i sshd /var/log/*log|more

or

sudo netstat -lp

sftp

after sshd is installed: edit sshd_config:

Subsystem sftp /usr/lib/openssh/sftp-server 
#Subsystem sftp internal-sftp -f AUTH -1 VERBOSE

#Uncomment this line if already commented
UsePAM yes

AllowGroups john sftpusers
Match Group sftpusers
   ChrootDirectory %h
   AllowTCPForwarding no
   X11Forwarding no
   ForceCommand internal-sftp
Match

In the case above, I already had a group that the user was part of. Below you'll find instructions on what to do if that does not exist already

>sudo groupadd sftpusers
>sudo adduser sftpuser
>sudo usermod -a -G sftpusers sftpuser
>sudo passwd sftpuser 
>sudo chown root:sftpuser /home/sftpuser
>sudo chmod 750 /home/sftpuser
>sudo mkdir /home/sftpuser/public
>sudo chown sftpuser:sftpuser /home/sftpuser/public
>sudo chmod 777 /home/sftpuser/public

fixing quick time-out

~$ sudo vi /etc/sshd/sshd_config
uncomment or add:
ClientAliveInterval 30
ClientAliveCountMax 5

~$ sudo service sshd restart

ClientAliveInterval: number of seconds that the server will wait before sending a null packet to the client (to keep the connection alive).
ClientAliveCountMax: This is the limit of how long a client are allowed to stay unresponsive before being disconnected. The default value is 3

Create ed25519 ssh key

~$ ssh-keygen -t ed25519

Connecting through a ssh tunnel

This assumes the public and private keys are on the originating machine and the public key is added to the authorized key files on both the tunnel and destination servers.

~$ ssh -i ~/.ssh/id_ed25519 -J username@tunnel username@detination

scp through ssh proxy

~$ scp -i ~/.ssh/id_ed25519 -o ProxyJump=user@proxyserver ./sourcefile user@destinationserver:/destinationfile