4.10.2. How Does It Work?
SSH uses a variety of ciphers to encrypt your data as it travels across the network. The exact ciphers used can be configured in /etc/ssh/ssh_config (for the client) and /etc/ssh/sshd_config (for the server). Configuring a stronger cipher will provide better protection, but will use more CPU power and possibly reduce communication speed; the default settings are a good compromise between security and performance.
Public-key authentication relies upon the fact that two extremely large numbersthe public key and private key , which are derived mathematically from a single large random number, can be used with cryptographic formulas to encrypt and decrypt data. Anything encrypted with the public key can be decrypted only with the private key (not with the public key or any other number), and anything encrypted with the private key can be decrypted only with the public key. If the private key is kept secret and the public key is distributed to the whole world, then any message that can be decrypted by the public key must have been encrypted with the private key, proving the identity of the sender ( authentication ); any message that is encrypted with the public key can only be decrypted by the private key, ensuring secrecy ( authorization ).
In the case of SSH, the ssh-keygen command generates a public/private key pair, placing the private key in ~/.ssh/id_dsa and the public key in ~/.ssh/id_dsa.pub . When the public key is copied to the remote machine and placed in ~/.ssh/authorized_keys , an access request encrypted with the private key can be authenticated using the public key. If the public key is protected with a passphrase, you will be prompted for it each time you connect to a remote machine; to reduce this burden, the ssh-agent program can store your passphrase for you. The ssh-add command prompts you for your passphrase(s) and hands them over to ssh-agent (which is run automatically when the GUI starts up).
SSH is very susceptible to man-in-the-middle attacks, where a system between the client and server intercepts communication and presents itself as the client to the server, and the server to the client. However, this type of attack is a lot harder to set up than it would first appear and is rarely encountered. The caching of the host key (presented onscreen in summary format as the fingerprint ) guards against this after the first contact between the client and server systems has been made.
4.10.3.1. ...compressing data?
The -C option (note the capital letter!) causes ssh to compress data with gzip before encryption. This can significantly improve performance in some cases:
$ ssh -C [email protected]
4.10.3.2. ...connecting to a Fedora system from a Windows system using SSH?
To do this, you need a Windows SSH client. There are several available, but for most purposes I'd recommend the free (libre et gratuit) program Putty, downloadable from http://www.chiark.greenend.org.uk/~sgtatham/putty/ . Of course, you won't be able to use graphical applications unless you've also installed an X server on your windows systembut that's not impossible (see http://x.cygwin.com/ for one possibility).
4.10.3.3. ...connecting to a Fedora system from a Mac OS X system?
Mac OS X and most other Unix/Linux/BSD-based systems generally have an SSH client installed.
4.10.3.4. ...connecting to my home system from another location?
If you're using a broadband connection with a router or gateway, you'll have to configure the router to pass incoming connections on the SSH port to your Fedora system. Use the router's Applications and Gaming, Port Forwarding, or Servers configuration options to forward TCP/IP port 22 to your Fedora system. Then you can connect to the Fedora system by specifying the address of the gateway system in the ssh client arguments.
For example, if the external IP address of the gateway is 1.2.3.4, and the LAN IP address of your Fedora system is 10.0.0.1, configure the router to pass incoming connections on TCP/IP port 22 to 10.0.0.1, and then use the gateway IP address in the client arguments:
$ ssh [email protected]
You may also need to configure Fedora's firewall to permit SSH connections.
4.10.3.5. ...using public/private keys without a passphrase?
Just leave the passphrase blank when running ssh-keygen . This is convenient because you won't need to use ssh-agent and ssh-add , and can always log in to remote systems without the passphrase. However, it's also dangerous because any attacker who obtains access to your local account will automatically gain access to your remote accounts as well.
4.10.4. Where Can I Learn More?
? The OpenSSH web site: http://openssh.org/
? The manpages for ssh , sshd , ssh_config , sshd_config , ssh-agent , ssh-add , and ssh-keygen
