[FreeNX-kNX] preventing data transfers over SSH, yet still allow NX sessions.

Wed Feb 6 06:49:35 UTC 2013

Hello,

I have searched something to do that and i find !!

I use lshell to prevent all actions over ssh and in a shell, but allow 
NX sessions.

If you want more informations, you're welcome.

Sincerely,

Jean

Jean Milot  - jmilot at dotriver.eu - www.dotriver.eu<http://www.dotriver.eu/>
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Le 06/02/2013 00:51, Mark Christian a écrit :
>> From: freenx-knx-bounces at kde.org [mailto:freenx-knx-bounces at kde.org] On
>> Behalf Of chris at ccburton.com
>> Sent: Saturday, February 02, 2013 1:17 AM
>> To: User Support for FreeNX Server and kNX Client
>> Subject: Re: [FreeNX-kNX] preventing data transfers over SSH, yet still
>> allow NX sessions.
>>
>>
>> freenx-knx-bounces at kde.org wrote on 01/02/2013 17:41:20:
>>
>>> I was wondering if it is possible to configure sshd_config, possibly
>>> using the ForceCommand keyword, to prevent arbitrary command
>>> execution/data transfers on the same host which is providing the NX
>>> sessions.  For example I can configure sshd_config with:
>>>
>>> ForceCommand /bin/bash
>>>
>>> ..which subsequently prevents, scp, rsync over ssh, and even
>>
>> That might stop ssh xfer but doesn't stop
>>
>>   cat  secretfile|mail -s "Innocent stuff" tome at mydomain
>>
>> ftp myhost
>>
>> etc
>>
>>
>>> something like "ssh remoteHost 'cat /etc/passwd'", but still allows
>>> interactive ssh sessions with a bash shell.
>>
>>
>> You can still cat /etc/passwd or secretfile from within an nx session.
>>
>> OK, if secretfile is large then you need ftp myhost etc
>>
>>
>>
>>
>>
>>
>> I'm not quite sure what you are geting at with this question,
>>          but I can see two things I think you might mean.
>>
>>
>>
>>
>>
>>
>> If you are asking about the "nx" username then :-
>>
>> ( do you know how it all works ?? )
>>
>> because your FreeNX ssh session consists of :-
>>
>> first a remote dsa key-file based ssh connection as user nx
>> then within that "tunnel"
>> a localhost ssh password authenticated connection as your user
>> then
>> kde gome etc is launched as the user . .
>>
>> So
>> the remote ssh connection is always dsa-key as username "nx"
>>
>> User nx :-
>>
>> 1/        has its shell set (in /etc/passwd) to
>>                   /usr/bin/nxserver
>>          which is        a restricted shell
>>                  (i.e. not listed in /etc/shells)
>> so
>>          your  ForceCommand idea won't work
>>           cos it uses the user shell to run the command
>>
>> but also NOTE
>>                  su -s /bin/bash nx
>>          from a server command line also won't work
>>                   due to the restricted shell
>>
>>
>> User nx :-
>> 2/          doesn't have a password set in /etc/shadow
>>
>> so also
>>          su nx won't work (unless you have root access
>>                  already)
>>
>>
>> User nx :-
>> 3/        has its authorized_keys entry set with:-
>>                  no-port-forwarding,no-X11-forwarding,
>>                  no-agent-forwarding,command="/usr/bin/nxserver"
>> so
>>          whatever "command" you run with ssh will be ignored
>>                  i.e  it will always run nxserver anyway even if you run
>>                  ssh   nx at freenx-server /bin/bash
>>                  ssh   nx at freenx-server -s /bin/bash
>>
>>
>>
>> Also ( did you actually mean this ?? )
> No, not really, see my response to your suggestion to run 2 ssh daemons.
>
>> Even if you did manage to use
>>           ForceCommand /bin/bash
>>   as
>>           username "nx"
>> and
>> got a shell on the FreeNX server, you could almost certainly
>> transfer anything file which user nx can open by tftp, ftp,
>> email etc back to your workstation
>>
>>
>>
>>
>> BUT on the other hand
>>
>>
>>
>>
>>
>> if you DON'T mean within the "nx" username
>>
>> i.e.
>> Your users "user1" etc are  ssh-ing the freenx server using
>>           password authentication and transfering data
>> then
>>
>> to prevent users ssh-ing as themselves you need two sshd
>> daemons running simultanaously:-
>>
> This is exactly what I was looking for.  Thank you for your patience and managing to decipher my cryptic requirements ;)
>> ssh daemon 1
>>   - allows incoming connections for admins and username "nx"
>> configured in /etc/ssh/sshd_config_external
>>
>>
>>                  Port 22
>>                  ListenAddress 0.0.0.0
>>                  AllowUsers nx the-administrator
>>                  RSAAuthentication yes
>>                  PubkeyAuthentication yes
>>                  PasswordAuthentication no
>>                  # Well probably no password tho' allowusers
>>                           may be enough for you . .
>>
>>
>> ssh daemon 2
>>   - allow localhost ssh for the password authenticated login as the user
>> configured in /etc/ssh/sshd_config_internal
>>
>>
>>                  Port 900
>>                  ListenAddress 127.0.0.1
>>                  AllowGroups freenx-users
>>                  PasswordAuthentication yes
>>
>> ** AND **
>>
>> set
>>                  SSHD_PORT=900
>> in
>>                   /etc/nxserver/node.conf
>>
>>
>> OR you could leave the
>>           PasswordAuthentication daemon 1 as Port 22
>> and
>>          have the dsa key daemon 2 on Port 900
>>                   for external connections
>>          provided  you update the nxclients and
>>                   any Firewalls along the way
>>
>>
>>> Does anyone have any ideas on how I can provide NX sessions to a
>>> remoteHost, yet prevent any data transfers to/from that sameHost
>>> over ssh?
>> If the issue is that your users are sshing in as themselves
>> the the two ssh daemon config as above will stop that . . .
>>
>>
>> NOTE
>> You can't easily prevent users xfering data unless you switch off
>> all outgoing connections (including email) too
> I plan to run a firewalling blocking outgoing ports from the NX host.
>
>> Username nx is fairly safe even tho the users
>>   "in the know" can copy the client.id_dsa.key
>> from within the nxclient and connect with ssh
>>
>>
>> User nx shouldn't have been given access to any data at
>> all tho'
>> particularly if you have kept the default key which is
>> the same on every machine.
>>
>>
>>
>>>   Using the example above can I ForceCommand the NX
>>> tunneling bits, and if so what are they?
>>
>> Not with username nx
>>
>>>   Or can NX be configured not to use ssh?
>> No
>>
>>> Thank you for your time.
>>>
>>> Mark Christian
>>
>>
>> So what actually did you mean ?????
>>
>>
>
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