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SSH Cheat Sheet

Step 1 - Setup public SSH keys

On our origin server, we will generate public SSH keys with no password:

# ECDSA...
ssh-keygen -t ecdsa -b 521 -C "mail@example.com"

# OR RSA...
ssh-keygen -f ~/.ssh/id_rsa -q -P ""
# or if you'd like a windows (putty) compatible one:
ssh-keygen -m PEM -t rsa -b 4096 -P ""
# to generate a pem (private file), import it into puttygen saving it as pem afterwards.
cat ~/.ssh/id_rsa.pub

This is our public SSH key that can be placed on other hosts to give us access:

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDLVDBIpdpfePg/a6h8au1HTKPPrg8wuTrjdh0QFVPpTI4KHctf6/FGg1NOgM++hrDlbrDVStKn/b3Mu65//tuvY5SG9sR4vrINCSQF++a+YRTGU6Sn4ltKpyj3usHERvBndtFXoDxsYKRCtPfgm1BGTBpoSl2A7lrwnmVSg+u11FOa1xSZ393aaBFDSeX8GlJf1SojWYIAbE25Xe3z5L232vZ5acC2PJkvKctzvUttJCP91gbNe5FSwDolE44diYbNYqEtvq2Jt8x45YzgFSVKf6ffnPwnUDwhtvc2f317TKx9l2Eq4aWqXTOMiPFA5ZRM/CF0IJCqeXG6s+qVfRjB root@cloudads

Copy this key to your clipboard and login to your destination server. Place this SSH key into your ~/.ssh/authorized_keys file: If your SSH folder does not exist, create it manually:

mkdir ~/.ssh
chmod 0700 ~/.ssh
touch ~/.ssh/authorized_keys
chmod 0644 ~/.ssh/authorized_keys

# a id_rsa file should have the following permissions:
chmod 0600 ~/.ssh/id_rsa
# and the public key:
chmod 0644 ~/.ssh/id_rsa.pub

Alternatively you can use this command to copy the local key to the remote server:

ssh-copy-id -i ~/.ssh/mykey user@host
# Example for a key called servers_rsa.pub and radagast:
# The 'comment' in the public key is arbitrary. Feel free to change that with your favorite text editor before copying it to the server:
ssh-copy-id -i ~/code/ssh/servers_rsa.pub zebra@radagast.zebra-servers
# Then you have to authenticate yourself in order for the copy operation to succeed.

Config File

You can have a config file in your .ssh/ directory to ease some burdens of typing. It contains your host-names and the connection credentials (paths to the key-files). The benefit is that you'll have host-name completion by pressing TAB when using the ssh-command and you'll never have to enter credentials manually (not that I insinuate that you've done that up till now). You find the complete documentation of that file here.

Here's how to create it:

sudo nano ~/.ssh/config
sudo chmod 0600 ~/.ssh/config

# now you may fill that file with your configuration.

Example:

Host logger-proto3
  User cms
  HostName 192.166.15.121
  IdentityFile ~/.ssh/logger_proto1_ecdsa

Host hetzner-*
  User root
  IdentityFile ~/.ssh/hetzner_rsa

Host hetzner-main
  HostName 91.117.166.175

Host hetzner-proxy
  HostName 86.9.1.15

Host vela
  HostName 120.81.13.14
  User psilo
  IdentityFile ~/.ssh/kube_ecdsa

Host saturn
  HostName 10.11.12.13
  User psilo
  ProxyJump vela
  IdentityFile ~/.ssh/kube_ecdsa

Multiple Local RSA Keys

From .ssh/config:

Host myshortname realname.example.com
    HostName realname.example.com
    IdentityFile ~/.ssh/realname_rsa # private key for realname
    User remoteusername

Host myother realname2.example.org
    HostName realname2.example.org
    IdentityFile ~/.ssh/realname2_rsa
    User remoteusername

PUTTYgen

In order to use the public key in PUTTY and to connect to a linux host in that way, you have to convert it from OpenSSH format (.pem) to putty-format (.ppk).

In order to do that, start PUTTYgen, load your OpenSSH key and choose 'save public key as...' and save it as .ppk file.

Import that in putty and add it under 'auth' in the connection preferences. When logging in, you just have to specify the right username and you're connected.

rsync

sudo rsync -avrPe "ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -i $HOME/.ssh/epsilon3_rsa -p 65493" --rsync-path="sudo rsync" -avRHP --delete /mnt/Backup1/ pi@www.unterrainer.info:/mnt/Backup1B1 >> /var/log/rsyncToUnterrainerInformatik.log 2>&1

Security Issues

For rsync to be able to copy all data in that partition that could come from rsnapshot for instance, you'll have do sudo the rsync call. Otherwise you won't have the permission to read the files on your backup volume since the data there comes from many different users and the user-privileges

To be able to do that in a script (otherwise it would prompt you for a password, which would be moot since it's a script), you have to change the /etc/sudoers list so that the sudo call to rsync on your copy-source location (where you start the script) doesn't need a password-entry.

# Let's say your user running the script will be 'zebra'...
sudo nano /etc/sudoers
# Add the following line:
zebra ALL=NOPASSWD:/usr/bin/rsync
# Then you're able to change your script to:
sudo rsync -avrPe "ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -i $HOME/.ssh/epsilon3_rsa -p 65493" --rsync-path="sudo rsync" --progress /mnt/Backup1/* pi@www.unterrainer.info:/mnt/Backup1B1

Stacking Executions As cron job

When executing as cron job the script could be started a second time, although the last instance is already running (because it didn't finish by now). To circumvent that, you can use a lock-file.

Write a wrapper-script that calls your original script like so:

flock -n /tmp/rsync-to-unterrainer.lockfile ~/rsyncToUnterrainerInformatik.sh
# The -n option tells it to just abort the new instance if no lock could be obtained.

Now let's build a cron job for that wrapper.

crontab -e
# Edits the cron-file for the current user
crontab -u <user_name> -e
# Edits the cron-file for the given user

# Add to log to dev/null (error&stdout): > /dev/null 2>&1
# Add log to syslog:  | /usr/bin/logger -t <command_display_name>
#    you then can follow it with tail -f /var/log/syslog
00 23 * * TUE /home/zebra/rsyncToUnterrainerInformatikLocked.sh | /usr/bin/logger -t copy_backup_to_unterrainer
# (starts every 23:00 on every tuesday)
*/05 * * * * /home/zebra/rsyncToUnterrainerInformatikLocked.sh | /usr/bin/logger -t copy_backup_to_unterrainer
# (starts every 5 minutes)

Now you can watch the script at work, along with the error output, by tailing the syslog. And you can see if the script is currently running with:

ll /tmp
# As long as the file rsync-to-unterrainer.lockfile is present, no other task will start