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Secure Shell (ssh)

SSH provides a secure connection between the host machine and a remote server.

ssh -i <path-to-private-key> <host> where <path-to-private-key> can be ~/.ssh/id_rsa for instance.

Authentication key pairs

Algorithms and key size

Currently, the relevant algorithms are rsa and ed25519.

The algorithm is selected using the -t flag and key size using the -b option:

  • ssh-keygen -t rsa -b 4096
  • ssh-keygen -t ed25519 size is fixed for this algorithm, no need to specify it.

Copy pubkey to server

To use public key authentication, the public key must be copied to a server and installed in an authorized_keys file. This can be conveniently done using the ssh-copy-id tool.

ssh-copy-id -i ~/.ssh/id_rsa user@host

File tranfer

Copy a file from/to the remote host and vice-versa.

  • Local to host: scp file host:path
  • Host to local: scp host:path file

Configuration file

When connecting to multiple remote systems over SSH, it can be difficult to remember all of the remote IP addresses, different usernames and ports.

To better manage those connections, we can create a config file at /.ssh/config to shorten the connection command.

Example:

Host targaryen
    HostName 192.168.1.10
    User daenerys
    Port 7654
    IdentityFile ~/.ssh/targaryen.key

Host tyrell
    HostName 192.168.10.20

Host martell
    HostName 192.168.10.50

Host *ell
    user oberyn

Host * !martell
    LogLevel INFO

Host *
    User root
    Compression yes

Then we can simply do ssh targaryen to remotely connect to the 192.168.1.10 host.