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Difference between revisions of "Package:OpenSSH"

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= Introduction =
== 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.
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 =
== Default Installation ==
Funtoo uses the OpenSSH daemon (sshd) to provide the SSH service by default. sshd is a member of [[OpenRC_(Funtoo)|OpenRC]]'s default runlevel.
Funtoo uses the OpenSSH daemon (sshd) to provide the SSH service by default. sshd is a member of [[OpenRC_(Funtoo)|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.
By default login is allowed for all users via the ssh daemon on port 22 with any valid username and password combination.


= Service configuration =
== Service configuration ==
There are 2 means of configuring <code>sshd</code>. The first is required, the second is optional.
There are 2 means of configuring <code>sshd</code>. The first is required, the second is optional.


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# <code>sshd</code> may be configured to use PAM.<br/>Permission may be granted or denied via PAM, allowing you to store usernames etc. using text files.
# <code>sshd</code> may be configured to use PAM.<br/>Permission may be granted or denied via PAM, allowing you to store usernames etc. using text files.


= Protocol version selection =
== Protocol version selection ==
The '''''default''''' protocol version is SSH-2. SSH-1 requires explicit activation. To select a protocol version, use the <code>Protocol</code> directive.
The '''''default''''' protocol version is SSH-2. SSH-1 requires explicit activation. To select a protocol version, use the <code>Protocol</code> directive.


e.g. <code>Protocol 2</code>
e.g. <code>Protocol 2</code>


= Cipher selection =
== Cipher selection ==
The <code>Ciphers</code> directive specifies the ciphers allowed for protocol version 2.
The <code>Ciphers</code> directive specifies the ciphers allowed for protocol version 2.


= User Authentication =
== User Authentication ==
== Single authentication method ==
=== Single authentication method ===
# Password authentication<br/>This is enabled by '''''default''''', it is configured using the <code>PasswordAuthentication</code> directive. Valid parameters are <code>yes</code> or <code>no</code>.<br/>When <code>PasswordAuthentication yes</code> is configured, the state of the <code>PermitEmptyPasswords</code> directive is evaluated.
# Password authentication<br/>This is enabled by '''''default''''', it is configured using the <code>PasswordAuthentication</code> directive. Valid parameters are <code>yes</code> or <code>no</code>.<br/>When <code>PasswordAuthentication yes</code> is configured, the state of the <code>PermitEmptyPasswords</code> directive is evaluated.
# Public key authentication
# Public key authentication
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# Host-based authentication
# Host-based authentication
== Requiring multiple authentication factors ==
=== 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 <code>AuthenticationMethods</code> 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.
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 <code>AuthenticationMethods</code> 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.


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e.g. <code>AuthenticationMethods "password,publickey password,keyboard-interactive"</code>
e.g. <code>AuthenticationMethods "password,publickey password,keyboard-interactive"</code>
== Password authentication using <code>sshd_config</code> ==
=== Password authentication using <code>sshd_config</code> ===
The following 4 directives are listed in order of evaluation by OpenSSH. They are configured directly; within <code>sshd_config</code>. Only user or group _names_ are valid, numerical IDs are not recognized. If the pattern takes the form <code>USER@HOST</code> then access is restricted to the <code>USER</code> when originating from the <code>HOST</code>.
The following 4 directives are listed in order of evaluation by OpenSSH. They are configured directly; within <code>sshd_config</code>. Only user or group _names_ are valid, numerical IDs are not recognized. If the pattern takes the form <code>USER@HOST</code> then access is restricted to the <code>USER</code> when originating from the <code>HOST</code>.


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


== Public key authentication ==
=== Public key authentication ===
<code>AuthorizedKeysFile</code>
<code>AuthorizedKeysFile</code>
<code>AuthorizedKeysCommand</code>
<code>AuthorizedKeysCommand</code>
<code>AuthorizedKeysCommandUser</code>
<code>AuthorizedKeysCommandUser</code>


===Host based authentication ===


== Host based authentication ==
== Access control ==
 
=== Controlling root access ===
= Access control =
== Controlling root access ==
Access by the root user can be controlled using the <code>PermitRootLogin</code> directive.
Access by the root user can be controlled using the <code>PermitRootLogin</code> directive.
=== Permit empty passwords ===
==== Permit empty passwords ====
Access to accounts with empty (i.e. blank) passwords can be controlled using the <code>PermitEmptyPasswords</code> directive.
Access to accounts with empty (i.e. blank) passwords can be controlled using the <code>PermitEmptyPasswords</code> directive.


ChallengeResponseAuthentication
ChallengeResponseAuthentication
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UsePAM
UsePAM


= X11 Forwarding =
== X11 Forwarding ==


By default X11 forwarding is disabled in OpenSSHd,
By default X11 forwarding is disabled in OpenSSHd,
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X11UseLocalhost yes
X11UseLocalhost yes
</pre>
</pre>


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
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 ==
=== Intrusion Prevention ===
ssh is a commonly attacked service. {{package|app-admin/sshguard}} monitors logs, and black list remote users who have repeatedly failed to login.
ssh is a commonly attacked service. {{package|app-admin/sshguard}} monitors logs, and black list remote users who have repeatedly failed to login.


Revision as of 02:46, December 28, 2014

OpenSSH

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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

Dialogs will be presented, you can press enter several times to accept defaults.

~/.ssh/id_rsa.pub will be generated. Copy or append the contents of this file to the servers ~/.ssh/authorized_keys

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.


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