SSH - A Practical Introduction

If you are working through any wargame, CTF, or home lab, SSH is the first tool you need to be comfortable with. This post covers everything from basic connections to config aliases, the things you will actually use.

What SSH Is

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SSH (Secure Shell) is a protocol for connecting to remote machines over a network. It encrypts everything in transit, which means your commands and any data you send cannot be read by anyone intercepting the connection.

When you connect to a remote server, a VPS, a lab machine, a wargame server — SSH is almost always how you do it.

Basic Connection

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ssh username@hostname

If the server runs SSH on a non-standard port (default is 22), specify it with -p:

ssh username@hostname -p 2220

You will be prompted for a password. Type it and press Enter — the terminal will not show any characters while you type, that is normal.

The Known Hosts Warning

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The first time you connect to a server you will see something like:

The authenticity of host 'bandit.labs.overthewire.org' can't be established.
ED25519 key fingerprint is SHA256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Are you sure you want to continue connecting (yes/no/[fingerprint])?

This is SSH asking you to confirm you trust the server. Type yes and the server’s fingerprint gets saved to ~/.ssh/known_hosts. Next time you connect, SSH checks the fingerprint automatically, if it changed, SSH will warn you, which could indicate something is wrong.

Never blindly accept fingerprints on machines you don’t control in a production context. For lab and wargame servers it is fine.

Password vs Key Authentication

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SSH supports two ways to authenticate:

Password: you type a password when connecting. Simple but weaker. The server has to store password hashes and brute force is possible if the password is weak.

Key-based: you generate a key pair (public + private). The public key goes on the server, the private key stays on your machine. No password is ever sent over the network. This is the standard for anything serious.

Generate a key pair:

ssh-keygen -t ed25519 -C "your comment here"

This creates two files in ~/.ssh/:

• id_ed25519: your private key, never share this
• id_ed25519.pub: your public key, this goes on servers

Copy your public key to a server:

ssh-copy-id username@hostname

From that point on you connect without a password.

Useful Flags

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The SSH Config File

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Typing ssh [email protected] -p 2220 every time gets old fast. The config file at ~/.ssh/config lets you define aliases for any host.

Create or edit the file:

[greycipher@remnant ~]$ nano ~/.ssh/config

Add an entry:

Host bandit0
    HostName bandit.labs.overthewire.org
    User bandit0
    Port 2220

Now you connect with just:

[greycipher@remnant ~]$ ssh bandit0

You can add as many entries as you want. For the Bandit series you could add one per level, or use a wildcard:

Host bandit*
    HostName bandit.labs.overthewire.org
    Port 2220

Then ssh bandit0, ssh bandit1 and so on all work automatically — SSH fills in the hostname and port, you only specify the user via the host alias.

Copying Files Over SSH

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scp: copies files like cp but over SSH:

# local to remote
scp file.txt username@hostname:/remote/path/

# remote to local
scp username@hostname:/remote/file.txt ./local/path/

sftp: interactive file transfer session:

sftp username@hostname

Once connected you get an sftp> prompt where you can ls, cd, get and put files.

Troubleshooting

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Connection refused: the server is not running SSH, the port is wrong, or a firewall is blocking it. Double check the port with -p.

Permission denied: wrong username, wrong password, or your key is not on the server. Try -v to see exactly where authentication fails.

Host key verification failed: the server’s fingerprint changed since you last connected. If you trust the server, remove the old entry from ~/.ssh/known_hosts and reconnect:

ssh-keygen -R hostname

What’s Next

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If you are working through OverTheWire Bandit, you now have everything you need to get started. SSH is also the foundation for tunneling, proxying, and remote port forwarding, topics worth exploring once the basics are solid.