# ssh tips

# default configuration per host
to not have to specify many options on the command-line, you can set the default user, port and more in the file ~/.ssh/config
~~~
Host testhost
Port 321
User testuser
IdentityFile ~/.ssh/private-key
~~~

# file transfer over ssh
mount a remote directory with sshfs

~~~
sshfs somehost:/ /mnt/somedirectory
~~~

or use "sftp" which can work interactively

~~~
sftp -P 22 someuser@somehost:/
~~~

the port option is an uppercase p unlike with the ssh command where it is lowercase
or use "scp"

# log in without having to enter a password
you need a private\public key pair. private keys in general can have an additional password themselves to protect the key.
use ssh-copy-id, or append the contents of the public key you want to use to the file ~/.ssh/authorised_keys on the server.
then configure ssh to use the key with IdentityFile option in .ssh/config or specify the path with a command-line option

## how to create a private\public key pair
~~~
ssh-keygen -t rsa
~~~
password can be empty, target is typically ~/.ssh/keyname
~~~
chmod 400 ~/.ssh/keyname*
~~~
ssh displays an error if the permissions are not limited enough

## how to copy the public key with ssh-copy-id
ssh-copy-id comes bundled with openssh

~~~
ssh-copy-id -i ~/.ssh/keyname someuser@somehost
~~~

## how to copy the public key manually to the server
append all contents of the file keyname.pub to the file .ssh/authorized_keys on the server. create the file or directory if it does not exist

## how to configure ssh to automatically use the key
in ~/.ssh/config
~~~
Host testserver
IdentityFile ~/.ssh/keyname
~~~

## how to select the key manually
~~~
ssh -i ~/.ssh/keyname testserver
~~~

# send shell commands to a remote server
~~~
echo "command-string" | ssh hostname
~~~

# sshd debugging
stop the sshd service. this normally does not close active connections
~~~
systemctl stop sshd
~~~
start sshd with debugging output
~~~
/usr/bin/sshd -d
~~~
the full path with /usr/bin is required or ssh will say so and exit
make connections and see the debug output displayed. to finish, exit with ctrl+c and do not forget to start the original sshd service again
~~~
systemctl start sshd
~~~