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SSH Command

SSH (Secure Shell) is a network communication protocol that runs on TCP. Its default port is 22.

Note: A client can establish as many connection as it wants and same for the Server, it can accept as many as it receives (as long as CPU doesnt giveup).

Jump into Terminal

# connect to this host using SSH with username being same as the username on your local machine (will ask for password)
ssh ssh.wani.com

# to login as a different user (2 ways)
ssh -l username hostname
ssh username@hostname

# use a private key to login
ssh -i keyFile username@hostname

# to use a non-default port (other than 22)
ssh -p 4422 -l username hostname

# facing issues logging in, use -v to enable debug messages (increase verbosity -vv -vvv)
ssh -v hostname

Using PK Auth

Private Key is stored on the client & Public key is to transferred to each server to which we want to connect.

# Generate a public-private keypair
ssh-keygen -t rsa -b 4096 -C "shoaibwani@gmail.com"

# send the public key to a remote server
ssh-copy-id -i ~/.ssh/previousKey username@server

# now use new key
ssh -i ~/.ssh/id_rsa username@server

Local Port Forwarding

# Forward a remote port to a local port
# remote_host:remote_port is resolved with respect to ssh server (hostname)
# -N means not to execute a remote command
# -f means run in background
ssh -L localp:remote_host:remote_port -N -f username@hostname

# like this example
ssh -L 8080:localhost:80 -N -f shoaib@ssh.wani.com        # forwards localhost:80 on ssh.wani.com to my machines 8080

Remote Port Forwarding

# This is done via the client, SERVER -> Client -> Dest
ssh -R remote_host:remote_port:dest_host:dest_port -N -f username@hostname

# example
# here ssh.shoaib.com is resolved from the client machine.
ssh -R localhost:80:ssh.shoaib.com:8080 -N -f shoaib@ssh.wani.com

Dynamic Port Forwarding

# It turns ssh client into a SOCKS proxy allowing us to tunnel all traffic through a remote server.
# APP(on local machine) -> localhost:port -> ssh.wani.com
ssh -D port username@ssh.wani.com

# It should be allowed by the server (make sure).

Learn SSH config File

This comes into picture when you are connecting to multiple hosts on a daily basis. Its hard to remember the hostname, username, port and etc.

We put all this info into the per-user Config file which is located in ~/.ssh/. Depending on the logged in user, the corresponding config is read.

We can create a system wide config for ssh in /etc/ssh/ssh_config.

Everything you will ever need to use in an ssh config file

# Global options
Host *
  # Use stronger ciphers for encryption
  Ciphers aes256-ctr,aes128-ctr
  # Enable compression for smaller data transfers
  Compression yes

# Host-specific configurations

# Server1 with password authentication
Host server1
  Hostname 192.168.1.10
  User shoaibwani
  Password itsmeyaar

# Server2 with public key authentication
Host server2
  Hostname example.com
  IdentityFile ~/.ssh/id_rsa
  Port 2222 # Non-standard port forwarding

# configure multiple hosts in single section
Host 192.168.* *.shoaib.com *.wani.in
    User shoaib
    Password letmein

# Group configuration for all servers in "dev" group
Match Group dev
  User devuser
  ForwardAgent yes

# Group definition for servers in "dev" group
Group dev
  server1
  server2

# Comments explaining specific options
# Port forwarding for web server on server2
Host webserver
  Hostname server2
  LocalForward 8080:localhost:80 # Expose local port 8080 to remote port 80