Tag: SSH Cheat Sheet

Add passphrase protection to ssh keyfile	ssh-keygen -p -f id_rsa
configure SSH to avoid trying all identity files	ssh -o IdentitiesOnly=yes -i id1.key [email protected]
Convert OpenSSL format to SSH-RSA format	ssh-keygen -f my_ssh.pub -i
Critical ssh files/folders	~/.ssh/authorized_keys, ~/.ssh/config, ~/.ssh/known_hosts
SSH config file	/etc/ssh/ssh_config, /etc/ssh/sshd_config
SSH key file permission	chmod 600 ~/.ssh/id_rsa
SSH folder permission	chmod 700 ~/.ssh, chown -R $USER:$USER ~/.ssh
Authorizedkeys file permission	chmod 644 ~/.ssh/authorized_keys
Mute Warning: Permanently added	ssh -o LogLevel=error

SSH port forward to a local port	ssh -N -i <ssh-keyfile> -f [email protected] -L *:18085:localhost:8085 -n /bin/bash
Reverse port forward to remote server	ssh -R *:40099:localhost:22 [email protected], ssh -p 40099 [email protected]
Setup ssh tunnel for your web browsing	sshuttle -r [email protected] 30.0.0.0/16 192.168.111.0/24 192.168.150.0/24 192.167.0.0/24

Disable ssh by password	sed -i 's/PasswordAuthentication yes/PasswordAuthentication no/g' /etc/ssh/sshd_config
Disable root login	sed -i 's/^PermitRootLogin yes/#PermitRootLogin yes/' /etc/ssh/sshd_config
Enable/Disable SSH Host Key Checking	StrictHostKeyChecking yes change ~/.ssh/config
Protect SSH server from brute force attacks	using fail2ban command-line tool

Download a remote folder	scp -r ec2-user@<ssh-host>:/home/letsencrypt-20180825 ./
Upload a file	scp -i <ssh-keyfile> /tmp/hosts ec2-user@<ssh-host>:/root/
Upload a folder	scp -r /tmp/abc/ ec2-user@<ssh-host>:/root/
Upload with timestamps/permissions kept	scp -rp /tmp/abc/ ec2-user@<ssh-host>:/root/
Mount remote directory as local folder	sshfs name@server:/path/remote_folder /path/local_folder

Events of ssh down	grep -R "ssh.*Received signal 15" /var/log/auth.log
Events of ssh up	grep -R "sshd.*Server listening" /var/log/auth.log
Events of ssh failed login	grep -R "sshd.*Failed password for invalid user" /var/log/auth.log
Events of ssh break-in attemp	grep -R "sshd.*POSSIBLE BREAK-IN ATTEMPT!" /var/log/auth.log
Events of ssh port scap	grep -R "sshd.*Bad protocol version identification" /var/log/auth.log
Events of ssh login by public key	grep -R "sshd.*Accepted publickey for" /var/log/auth.log
Events of ssh login by password	grep -R "sshd.*Accepted password for" /var/log/auth.log
Events of ssh logout event	grep -R "sshd.*pam_unix(sshd:session): session closed for" /var/log/auth.log

eval $(ssh-agent)       # Start agent on demand

ssh-add -l                      # List keys
ssh-add                         # Add default key
ssh-add ~/.ssh/id_rsa           # Add specific key
ssh-add -t 3600 ~/.ssh/id_rsa   # Add with timeout
ssh-add -D                      # Drop keys

ssh -A ...          # Enforce agent forwarding

Host unreachable_host
  ProxyCommand ssh -e none gateway_host exec nc %h %p

ssh -J Jumphost FooServer
ssh -J Jumphost vivek@FooServer
ssh -J hello.vpn.cyberciti.biz www.nixcraft.com
ssh -J hello.vpn.cyberciti.biz:22 [email protected]

Host <your jump host>
  ForwardAgent yes
  Hostname <your jump host>
  User <your user name on jump host>

# Note the server list can have wild cards, e.g. "webserver-* database*"
Host <server list>
  ForwardAgent yes
  User <your user name on all these hosts>
  ProxyCommand ssh -q <your jump host> nc -q0 %h 22

Host myserver
   User myuser
   Host myserver.com
   ProxyCommand bash -c '/usr/bin/knock %h 1000 2000 3000 4000; sleep 1; exec /bin/nc %h %p'

# Enforce a umask with SFTP

Subsystem sftp /usr/libexec/openssh/sftp-server -u 0002

# Parallel SSH on Debian

apt-get install pssh

=> using

pssh -h host_list.txt <command>
pssh -i -t 60 -h host_list.txt -- <command>   # 60s timeout, list output

# Clustered SSH on Debian

apt-get install clusterssh

=> using

cssh server1 server2

# Vim Remote File Editing:

vim scp:[email protected]//some/directory/file.txt

# Use GPG keys with SSH agent

monkeysphere subkey-to-ssh-agent -t 3600

# Knockd installation

apt update
apt install -y knockd iptables-persistent

# iptables rules

iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp --destination-port 22 -j DROP
iptables-save > /etc/iptables/rules.v4

These commands will do the following:

• Instruct iptables to keep existing connections alive.
• Instruct iptables to drop any connection to port tcp/22 (if your SSH daemon is listening on a port other than 22, you should modify the above command accordingly.)
• Save these two rules so they will persist after a reboot.

# Knockd configuration

Open the file /etc/knockd.conf

You will see:

[openSSH]
sequence    = 7000,8000,9000
seq_timeout = 5
command     = /sbin/iptables -A INPUT -s %IP% -p tcp --dport 22 -j ACCEPT
tcpflags    = syn

Change the opening sequence:

[openSSH]
sequence    = x,y,z

Change the command value :

command     = /sbin/iptables -I INPUT -s %IP% -p tcp --dport 22 -j ACCEPT

Change the closing sequence accordingly:

[closeSSH]
sequence    = z,y,x

Save your changes and exit, and open the file /etc/default/knockd

• Replace START_KNOCKD=0 with START_KNOCKD=1.
• Add the following line to the end of the file: KNOCKD_OPTS="-i ens3" (replace ens3 with the name of your public network interface if it differs.)
• Save and exit.

Start Knockd:

systemctl start knockd

~.	terminate the connection (and any multiplexed sessions)
~B	send a BREAK to the remote system
~C	open a command line
~R	Request rekey (SSH protocol 2 only)
~^Z	suspend ssh
~#	list forwarded connections
~&	background ssh (when waiting for connections to terminate)
~?	this message
~~	send the escape character by typing it twice