SSH

Last modified on 01 Oct 2021.

How it works?

1. Local creates public_key (id_rsa.pub) & private_key (id_rsa).
2. Only private_key can understand public_key.
3. Remote sends messages encrypted based on public_key.
4. Local has to use private_key to understand (decrypt) remote’s messages.

Generate a public key

• Windows: Using below command, if it asks for a location, indicate C:\Users\dinha\.ssh\
• Linux: /home/thi/.ssh/

   ssh-keygen -t rsa -b 4096 -C "[email protected]"
  
   # without email
   ssh-keygen -t rsa -f ~/.ssh/id_rsa.home
  

Multiple ssh keys

1. Create key with different names, e.g. id_rsa.home, id_rsa.work.
2. Add to ~/.ssh/config,

   Host home
   Hostname home.example.com
   IdentityFile ~/.ssh/id_rsa.home
   User <your home acct>
   
   Host work
   Hostname work.example.com
   IdentityFile ~/.ssh/id_rsa.work
   User <your work acct>
   

3. Add to ssh-agent (don’t need to retype password again)

   eval "$(ssh-agent -s)"
   ssh-add ~/.ssh/id_rsa.home
   ssh-add ~/.ssh/id_rsa.work
   

4. Don’t forget to clone you repo with git instead of https.

Add public key to remote

Suppose that we wanna connect to a remote host [email protected] from a local machine.

1. On local machine, copy public key at C:/Users/dinha/.ssh (Windows) and ~/.ssh (Linux) (something like id_rsa.pub) (copy its content).
2. On remote server (Linux), go to ~/.ssh, open file authorized_keys by vim authorized_keys
   1. Be carefull, you can modify the current keys!
   2. Go to the end of this file (by W)
   3. Press I to enter to the editing mode, press Enter for a new line.
   4. Using mouse to copy/paste the key in the 1st step (on your local machine).
   5. Note that, each key stays in a separated line.
   6. ESC and then type :wq to quick and save.
   7. Try to connect again!

Connecting

ssh remote_username@remote_host
ssh remote_username@remote_host -p remote_port

# CHECK VERSION
ssh -V

# DISCONNECT
exit

# COPY FILE: LOCAL -> REMOTE
scp local_file user@remote-host:/var/tmp/

# multiple files, using wildcat "\*"

# REMOTE -> LOCAL
scp user@remote:/usr/local/bin/add.sh .

# multiple files, using wildcat "\*"

# pass inside the command
sudo apt-get install sshpass
sshpass -p your_password ssh user@hostname

# copy with sudo on remote
# 1. copy to a place you have permissions
scp * thi@remote:/home/thi/tmp/
# 2. move to the place you want
ssh thi@remote sudo mv /home/thi/tmp/\* /place/we/want

Command line parameters

# FOR EXAMPLE
ssh -C # use data compression

Usage: Access jupyter notebooks from remote server on local machine.

Below are some popular commands^[ref] :

# check the full list
man ssh

# exit background running
sudo apt install net-tools
netstat -lepunt

# kill a process, e.g. 29231/ssh
kill <pid> # eg. kill 29231

Errors

# REMOTE HOST IDENTIFICATION HAS CHANGED
# Offending ECDSA key in /home/thi/.ssh/known_hosts:21

# SOLUTION:
# Open /home/thi/.ssh/known_host and delete line 21

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