Package:OpenSSH

OpenSSH

   Tip

We welcome improvements to this page. To edit this page, Create a Funtoo account. Then log in and then click here to edit this page. See our editing guidelines to becoming a wiki-editing pro.

Introduction

SSH is a cryptographically confidential network protocol for data transmission between 2 networked computers. There are 2 protocol versions; SSH-1 and SSH-2.

Default Installation

Funtoo uses the OpenSSH daemon (sshd) to provide the SSH service by default. sshd is a member of OpenRC's default runlevel.

By default login is allowed for all users via the ssh daemon on port 22 with any valid username and password combination.

Service configuration

There are 2 means of configuring sshd. The first is required, the second is optional.

1. sshd reads its configuration data from /etc/ssh/sshd_config by default.
2. sshd may be configured to use PAM.
   Permission may be granted or denied via PAM, allowing you to store usernames etc. using text files.

Protocol version selection

The default protocol version is SSH-2. SSH-1 requires explicit activation. To select a protocol version, use the Protocol directive.

e.g. Protocol 2

Cipher selection

The Ciphers directive specifies the ciphers allowed for protocol version 2.

User Authentication

Single authentication method

1. Password authentication
   This is enabled by default, it is configured using the PasswordAuthentication directive. Valid parameters are yes or no.
   When PasswordAuthentication yes is configured, the state of the PermitEmptyPasswords directive is evaluated.
2. Public key authentication

This is enabled with combinations of AuthorizedKeysFile, AuthorizedKeysCommand and AuthorizedKeysCommandUser.

Passwordless Authentication

Client

On your client, run

root # ssh-keygen -t rsa

This command will generate a public and private key, stored at ~/.ssh/id_rsa.pub and ~/.ssh/id_rsa respectively. The private key should not be shared with anyone. The pubilc key can be freely shared, and can only be used to grant you access to remote systems by use of your private key. To grant yourself access to a remote account, append the contents of ~/.ssh/id_rsa.pub to the file ~/.ssh/authorized_keys on a remote system. Note that ~/.ssh/authorized_keys must not be readable by anyone but the user for OpenSSH to process the file -- do this by running chmod -R go-rwx ~/.ssh on the remote system.

Providing a passphrase for your private key will cause it to be stored in an encrypted format, using this passphrase. Before you can use your private key, you will be prompted by OpenSSH to type in this passphrase. This is similar to typing in a password, but with the use of this authentication method, your password is not sent over the network at all.

The program ssh-agent exists to allow decrypted private keys to be cached in memory for easy access, so you do not need to type in your passphrase every time. It can still be quite complex to use, so it's recommended to use a front-end for ssh-agent -- see the Keychain page to learn about such a tool. Keychain is also part of Funtoo.

Server

Create a user, or select which user the client will be accessing the server as, then place clients id_rsa.pub file into the users ~/.ssh/authorized_keys

Single Machine Testing

root # ssh-keygen -t rsa

Press enter several times to accept default settings.

root # cp ~/.ssh/id_rsa.pub ~/.ssh/authorized_keys

root # ssh localhost

1. Host-based authentication

Requiring multiple authentication factors

These options are only available for SSH-2. The default is not to require multiple authentication. To identify to the daemon that you wish to require more than one authentication, you must use the AuthenticationMethods directive. This directive is followed by one or more comma separated lists of authentication method names. Lists are separated with a space. Successful authentication requires completion of every method in at least one of these lists.

1. password
2. publickey
3. keyboard-interactive

e.g. AuthenticationMethods "password,publickey password,keyboard-interactive"

Password authentication using sshd_config

The following 4 directives are listed in order of evaluation by OpenSSH. They are configured directly; within sshd_config. Only user or group _names_ are valid, numerical IDs are not recognized. If the pattern takes the form USER@HOST then access is restricted to the USER when originating from the HOST.

DenyUsers PATTERN PATTERN ...

Login is forbidden for users whose username matches one of the patterns

AllowUsers PATTERN PATTERN ...

Login is permitted to users whose username matches one of the patterns

DenyGroups PATTERN PATTERN ...

Login is forbidden for users whose primary group or supplementary group list matches one of the patterns

AllowGroups PATTERN PATTERN ...

Login is permitted to users whose primary group or supplementary group list matches one of the patterns

Public key authentication

AuthorizedKeysFile AuthorizedKeysCommand AuthorizedKeysCommandUser

Host based authentication

Access control

Controlling root access

Access by the root user can be controlled using the PermitRootLogin directive.

Permit empty passwords

Access to accounts with empty (i.e. blank) passwords can be controlled using the PermitEmptyPasswords directive.

ChallengeResponseAuthentication Ciphers

GSSAPIAuthenticaion GSSAPICleanupCredentials GSSAPIStrictAcceptorCheck HostBasedAuthentication HostBasedUsesNameFromPacketOnly HostCertificate HostKey HostKeyAgent LoginGraceTime MAC MaxAuthTries MaxSessions MaxStartups PasswordAuthentication PermitEmptyPasswords PubkeyAuthentication RevokedKeys RhostsRSAAuthentication RSAAuthentication TrustedUserCAKeys UseLogin UsePAM

X11 Forwarding

By default X11 forwarding is disabled in OpenSSHd,

If you would like to forward X11 from your Funtoo box to a remote system you must first edit your /etc/ssh/sshd_config file

change

#X11Forwarding no
#X11DisplayOffset 10
#X11UseLocalhost yes

to

X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost yes

X forwarding will now be enabled from that machine, so if you connect from your remote with 'ssh -X <user>@<ipaddress>' X sessions will be forwarded

Intrusion Prevention

ssh is a commonly attacked service. app-admin/sshguard monitors logs, and black list remote users who have repeatedly failed to login.