UVA ✎ linux_ssh_access [Computing Support]

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==== Linux Server Access ====

All Linux servers run "secure shell" - 'ssh'.  Anyone with a CS account may log into these servers. 

On Grounds, you can simply 'ssh' to CS servers typically by using a Terminal application like HyperTerm (Windows) or Terminal (Mac).

Off Grounds, you are not able to 'ssh' directly into CS servers.  However connections to ''%%portal.cs.virginia.edu%%'' are allowed from off Grounds.

You must use your CS domain userid (identical to your UVA userid) and password to 'ssh' to portal. For example:

''%%ssh -l abc1de portal.cs.virginia.edu%%''. Alternatively, ''%%ssh abc1de@portal.cs.virginia.edu%%''

Note that if you are opening a terminal application on your Mac or PC, your username on the Mac or PC may be different from your CS/UVA userid. So be sure to include your userid on the 'ssh' command line.

If you'd like to use a graphical windowing interface to department servers, see: [[nx_lab|NX Linux Remote Desktop Cluster]]

=== Access from off Grounds ===

**Use one of these options to access CS servers from outside of UVA.**

=== Option 1: VPN access ===


If you are outside of the UVA network (off grounds) then you can first start a VPN session to UVA using the [[https://virginia.service-now.com/its?id=itsweb_kb_article&sys_id=f24e5cdfdb3acb804f32fb671d9619d0|UVA VPN]]. Once the VPN is established, you can 'ssh' to CS servers directly.

=== Option 2: Access via portal.cs.virginia.edu ===

You can ssh directly into ''%%portal.cs.virginia.edu%%'' without having to use the UVA VPN.  Once you are logged into the ''%%portal%%'' cluster, you can then ssh to other CS servers.

<code>
[abc1de@outside-uva ~]$ ssh -l abc1de portal.cs.virginia.edu
abc1de@portal.cs.virginia.edu's password: 
Last login: Mon Jul 29 14:12:10 2019
abc1de@portal04 ~ $ hostname
portal04                                               <-- We are logged into portal cluster
abc1de@portal04 ~ $ ssh gpusrv01                         <-- We can now access gpusrv01
abc1de@gpusrv01's password:
....
abc1de@gpusrv01 ~ $
</code>

=== Jumphost Option ===

The ssh client has an option ''%%-J%%'' to specify a host to use as a "jumphost" that lets you access other servers directly, in one step.  This combines two steps (for example, ssh into portal.cs.virginia.edu and then ssh to gpusrv01) into one single command.  From the //man// page:

<code>
     -J destination
             Connect to the target host by first making a ssh connection to
             the jump host described by destination and then establishing a
             TCP forwarding to the ultimate destination from there.  Multiple
             jump hops may be specified separated by comma characters.  This
             is a shortcut to specify a ProxyJump configuration directive.
             Note that configuration directives supplied on the command-line
             generally apply to the destination host and not any specified
             jump hosts.  Use ~/.ssh/config to specify configuration for jump
             hosts.
</code>

Here is how we use this option to "jump" from portal.cs to another CS server.  Let's repeat the example of logging in to ''%%gpusrv01%%''

<code>
[abc1de@outside-uva ~]$ ssh -l abc1de gpusrv01 -J portal.cs.virginia.edu
abc1de@portal04.cs.virginia.edu's password:                            <-- first asked to authenticate to portal
abc1de@gpusrv01's password:                                              <-- immediately able to log into gpusrv01
....
abc1de@gpusrv01 ~ $
</code>

=== Server Domain Names ===

Computer Science hosts its own DNS server with authority over the ''%%cs.virginia.edu%%'' domain space.  Any server in CS will have a fully qualified domain name (fqdn) of ''%%hostname.cs.virginia.edu%%''.

=== Short Names ===

If you are inside of the Computer Science network then use the hostname of a server instead of its fully qualified name.  For example, if you are logged into a CS server, you can ping another server by its hostname alone.

<code>
abc1de@portal01:~$ ping portal03
PING portal03.cs.virginia.edu (128.143.67.43) 56(84) bytes of data.
64 bytes from portal03.cs.virginia.edu (128.143.67.43): icmp_seq=1 ttl=64 time=0.149 ms
64 bytes from portal03.cs.virginia.edu (128.143.67.43): icmp_seq=2 ttl=64 time=0.123 ms
</code>

This will not work from outside of the CS network unless you modify your DNS search path to contain ''%%cs.virginia.edu%%''.

=== Login to other servers ===

=== From Linux/Mac OS ===

To log into a server from another computer running Linux/Unix/MacOS, run the following from a shell:

<code>
abc1de@host ~ $ ssh username@gpusrv04.cs.virginia.edu
abc1de@gpusrv04's password:                              <- Enter Password
...
[abc1de@gpusrv04 ~]$
</code>

In Mac OS the Terminal app can be found in the Utilities folder under Applications.

=== From Windows ===

For information about SSH clients for Windows, see the article [[windows_ssh|SSH from Windows]]

=== Servers ===

For a listing of generally available servers in CS, see the article [[compute_resources|General Purpose Nodes]]

=== Login Restrictions (Info for Faculty) ===

We want to give all of our users fair and equal access to whatever computing resources we have to offer. We do not restrict login access to certain servers.  However, there are a number of servers that still have access restrictions in place.  This article is to show users with ''%%sudo%%'' privileges how to edit ''%%/etc/security/time.conf%%'' to allow user logins.

There are several configuration files located in ''%%/etc/security%%'' on Linux servers.  In this directory, we can use ''%%time.conf%%'' to restrict ssh login to a specific set of user accounts.

By default, access rules in ''%%time.conf%%'' are not used unless a //PAM module// (pluggable authentication module) is configured to read them.  This is done by adding a line to the ''%%sshd%%'' //PAM// module file.  

Add the following line to the file to the end ''%%/etc/pam.d/sshd%%'':

<code>
account             required                pam_time.so
</code>

Now that //PAM// is configured to read ''%%time.conf%%'' we can now put in a rule.  Here is an example rule from ''%%time.conf%%'':

<code>
sshd;*;!root&abc1de&fgh1ij&klm1no;!Al0000-2400
</code>

This line is formatted such that the users listed are separated by ampersand ''%%&%%'' characters.  This entry will allow the users ''%%root%%'', ''%%abc1de%%'', ''%%fgh1ij%%'' and ''%%klm1no%%'' are allowed access.  **Be sure to always include yourself and root in this rule.  Failure to do so may result in everyone becoming locked out.**

If we wanted to add the user ''%%pqr1st%%'' to this rule above, we would insert the string ''%%&pqr1st%%'' like this:

<code>
sshd;*;!root&abc1de&fgh1ij&klm1no&pqr1st;!Al0000-2400
</code>

Changes to this file take effect immediately, no services need to be restarted.  When editing this file, be sure that you keep at least one active ssh connection until you have tested your changes.  This will prevent becoming locked out if any errors are made!