Helpful Knowledge 🎓

A general repository for stuff I always keep forgetting how to do

Creating and Using SSH Keys for Passwordless Experience

Some well-written and thorough SSH information can be found at ssh.com

Concept

Using SSH Keys enables secure logins easily.

Generally concerned with two files:

id_rsa.pub (public and shared on remote system)
id_rsa(private on local system)

The files—usually referred to as keys—can be likened to a physical padlock 🔒 (.pub) and key 🔑

These files are usually kept inside a hidden directory in your home directory (i.e. ~/.ssh)

$ ls ~/.ssh/
id_rsa     id_rsa.pub

Like a 🔑, the id_rsa file is not to be public or shared. Conversely, the 🔒can be on multiple systems.

There's nothing particualarily special with the name id_rsa. You can rename when generating keys (e.g. id_rsa_home, my_coolsite.pub)

Generating Keys

To create a matching pair of keys 🔐(i.e. padlock & key), on Linux/MacOS use the ssh-keygen utility:

$ ssh-keygen -t rsa -C "note to help id the pub file entry"

Generating public/private rsa key pair.

Enter file in which to save the key (/Users/skube/.ssh/id_rsa): [enter some descriptive note] name_of_key_file

Enter passphrase (empty for no passphrase):
Enter same passphrase again:

Your identification has been saved in /Users/skube/.ssh/name_of_key_file
Your public key has been saved in     /Users/skube/.ssh/name_of_key_file.pub

Note: It is recommended to use a passphrase (e.g. orange you glad I didn't say banana) Note: Passphrase can be temporarily saved/bypassed using a utility (ssh-agent)

Info:

The contents of [] are optional, a note will automatically be appended to the *.pub file as user@machine

-t rsa is default tells ssh-keygen to create an RSA type encrpytion

-C overrides the default comment on the *.pub file (why?)

Authorized Keys

Now you have two files id_rsa.pub 🔒 and id_rsa 🔑. The 🔒 must be placed on the server and appended to the authorized_keys file.

Method 1

One way to easily copy the *.pub file to a remote server is to use the ssh-copy-id util:

# If util available, supply the _key_ if not using the default `id_rsa`

$ ssh-copy-id [-i name_of_key_file] user@remotehost

-i if you want to override the default identity file of id_rsa

Method 2

Another way is with a long, complicated but impressive looking shell command:

# Fancy unix one-liner

$ cat ~/.ssh/id_rsa.pub | ssh user@remoteserver 'mkdir -p ~/.ssh && cat >>  ~/.ssh/authorized_keys'

(Info: the -p flag on mkdir creates intermediate directories as required, not erroring if the directory already exists)

Method 3

Otherwise, you can simply manually copy the 🔒contents and paste through an online form

# Manually copy to clipboard 
$ pbcopy < ~/.ssh/id_rsa.pub

You can safely now delete the *.pub file as it has been appended to the authorize_keys file.

Using SSH

Now you can SSH into your remote server

# Method 1: 
ssh user@remoteserver -i ~/.ssh/id_rsa_<note>

Info: the -i is to specifcy a path to the proper key when not using the the default id_rsa

# Method 2: If not using multiplie keys (i.e. only one `id_rsa` key)
ssh user@remoteserver

Managing Multiple Keys

It's not uncommon to use multiple keys (not to mention it's easier to use a friendly-name). To facilitate this, you can use a ~/.ssh/config file.

# contents of ~/.ssh/config (replace <...>) 
Host <friendly-name>
    User <user>
    HostName <remoteserver.com>
    IdentityFile ~/.ssh/id_rsa_<note>

# macOS Sierra no longer persists keys
Host *
  AddKeysToAgent yes

That's it! You should* be able to SSH simply by into a remote server by: ssh friendly-name

Bypassing Passphrase with `ssh-agent`

If you supplied a passphrase, you may find it super annoying entering it each time.

The ssh-agent manages your SSH keys and remembers your passphrases.

To use ssh-agent and ssh-add, follow the steps below:

1. At the Unix prompt, enter:

eval `ssh-agent`

Note: make sure you use the backquote (`) rather than the single quote (').

2. Add the key to the agent:

ssh-add -K ~/.ssh/id_rsa_<custom>

Info: -K adds to keychain for persistance (tho may not work after macOS Sierra)

3. Kill the current agent:

ssh-agent -k

*Troubleshooting

Problem 1

Your local and remote systems may have different ssh versions. In that case, you need to set permissions on the .ssh directory and authorized_keys file of your remote system.

# method 1: one line
ssh user@remotehost 'chmod 700 ~/.ssh; chmod 640 ~/.ssh/authorized_keys'

# method 2: multi-line 
ssh user@remotehost
chmod 700 ~/.ssh
chmod 640 ~/.ssh/authorized_keys

(note: 421 corresponds to rwx, so chmod 640 means chmod u=rw,g=r,o-a)

Problem 2:

If you see a message "Agent admitted failure to sign using the key" then add your RSA or DSA identities to the authentication agent ssh-agent then execute the following command:

ssh-add

Using rsync with `ssh` identity file

Setting up the rsync command (trailing slashes seem to be important)

rsync --progress -avz -e "ssh -i ~/.ssh/id_rsa_<note>" /path/to/local/dir/ <user>@<hostname>:/path/to/remote/dir/

Create a shell script for the synchronization

vi ~/bin/some-super-cool-name.sh

Add to the shell script file's contents:

#!/bin/bash <copy-the above rsync line here>

Add proper permissions

chmod 700 ~/bin/some-super-cool-name.sh

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