Caveats, issues, special cases and possible problems


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SSH side effects

Please note that if you remote account name is different from your RCF account name, you will need to use

% ssh -X

specifying explicitly username rather as the form

% ssh -X

will assume a username defaulting to your local machine (remote from the BNL ssh-daemon stand point) user name where you issue the ssh command. This has been a source of confusion for a few users. The first form by the way is preferred as always work and removes all ambiguities.

X11 Forwarding: -X or -Y ??

-X is used to automatically set the display environment to a secure channel (also called untrusted X11 forwarding) . In other words, it enables X11 forwarding without having to grant remote applications the right to manipulate your Xserver parameters. If you want ssh client to always act like with X11 forwarding, have the following line added in your /etc/ssh/ssh_config (or any /etc/ssh*/ssh*_config ).

ForwardX11 yes

-Y enables trusted X11 forwarding. So, what does trusted mean? It means that the X-client will be allowed to gain full access to your Xserver, including changing X11 properties (i.e. attributes and values which alters the look and feel of opened X windows or things such as mouse controls and position info, keyboard input reading and so on).  Starting with OpenSSH 3.8, you will need to set

ForwardX11Trusted yes 

in the client configuration  to allow remote nodes full access to your Xserver as it is NOT enabled by default.

When to use trusted, when to use untrusted

Recent OpenSSH version supports both untrusted (-X) and trusted (-Y) X11 Forwarding. As hinted above, the difference is what level of permissions the client application has on the Xserver running on the client machine.  Untrusted (-X) X11 Forwarding is more secure, but unfortunately several applications (especially older X-based applications) do not support running with less privileges and will eventually die and/or crash your entire Xserver session.

Dilema? A rule of thumb is that while using trusted (-Y) X11 Forwarding will have less applications problems for the near future, try first the most secured untrusted (-X) way and see what happens. If remote X applications fail with a errorssimilar to the below:

X Error of failed request: BadAtom (invalid Atom parameter)
  Major opcode of failed request: 18 (X_ChangeProperty)
  Atom id in failed request: 0x114
  Serial number of failed request: 370
  Current serial number in output stream: 372

you will have to use the trusted (-Y) connection.

Per client / server setup?

Instead of a system global configuration which will require your system administrator's assistance, you may create a config file in your user’s home directory (client side) under the .ssh directory with the following line $HOME/.ssh/config

ForwardX11Trusted yes 

But it gets better as the config file allows per host or per-domain configuration. For example, the below is valid

Host *.edu
	ForwardX11 no
	User jlauret

Host *
	ForwardX11 yes
    	Cipher blowfish
	User jeromel

     ForwardAgent yes
     Cipher 3des
     ForwardX11Trusted yes

    User exampleoptions
    Port 666
    Compression yes
    PasswordAuthentication no
    KeepAlive yes
    ForwardAgent yes
    ForwardX11 yes
    RhostsAuthentication no
    RhostsRSAAuthentication no
    RSAAuthentication yes
    TISAuthentication no
    PasswordAuthentication no
    FallBackToRsh no
    UseRsh no

As a side note, 3des is more secure thank blowfish but also 3x slower. If speed and security is important, use at least aes cypher.

Kerberos hand-shake, How to.

OK, now you are logged to the facility gatekeeper but any sub-sequent login would ask for your password again (and this would defeat security). But you can cure this problem by, on the gatekeeper, issue the following command (we assume $user is your user name)

% kinit -5 -d -l 7d $user

-l 7d is used to provide a long life K5 ticket (7 days long credentials). Note that you should afterward be granted an AFS token automatically upon login to the worker nodes on the facility. From the gatekeeper, the command

% rterm

would open a terminal from the least loaded node on the cluster where you are allowed to log.

Generic (group) accounts

Due to policy regulations, group or generic accounts login cannot be allowed at the facility unless the login is traceable to an individual. The way to log in is therefore to

  • Log to the gatekeeper using SSH keys under your PERSONAL account as described at SSH Keys and login to the SDCC
  • kinit -5 -4 -l 7d $gaccount
  • In case of wide use generic account, one more jump to a "special" node will be necessary. For starreco and starlib for example, this additional gatekeeper node is rcas6003. From there, login to the rest of the facility could be done using rterm as usual (at least in STAR)

Special nodes

This section is about standing on one foot, tapping on to of your head and chanting a mantra unless the moon is full (in such case, the procedure involves parsley and sacrificial offerings). OK, we are in the realm of the very very special tricks for very very special nodes:

  • The rmine nodes CANNOT be connected to anymore. However, one can use has a gatekeeper, using your desktop keys and then jump from there to the rmine nodes.
    Scope: Subject to special authorization.
  • The test node cannot be accessed using a Kerberos trick. Since there are two HOPs from your machine to aplay1, you need to use the ssh-agent. See instructions on the Using the SSH Agent help page.

K5 Caveats

  • If you log to gatekeeper GK1 for your personal account, you will need to chose another gatekeeepr GK2 for your group account login. This will allow not interference of Kerberos credentials.
  • Whenever you log in a gatekeeper and you know you had previously obtained Kerberos credentials on this gatekeeper, you should ensure the destruction of previous credential to avoid premature lifetime expiration. In other words, -l 7d will NOT give you a 7 days lifetime K5 ticket on a gatekeeper where previous credentials exists. To destroy previous credentials, be sure
    1. you do not have (still) opened windows using the credential. Check this by issuing a klist and observe the listing. Valid credentials used in opened session would look like this
      Valid starting     Expires            Service principal
      12/26/06 10:59:28  12/31/06 10:59:28  krbtgt/RHIC.BNL.GOV@RHIC.BNL.GOV
             renew until 01/02/07 10:59:25
      12/26/06 10:59:30  12/31/06 10:59:28  host/
             renew until 01/02/07 10:59:25
      12/26/06 11:11:48  12/31/06 10:59:28  host/
             renew until 01/02/07 10:59:25
      12/26/06 17:51:05  12/31/06 10:59:28  host/
             renew until 01/02/07 10:59:25
      12/26/06 18:34:03  12/31/06 10:59:28  host/
             renew until 01/02/07 10:59:25
      12/26/06 18:34:22  12/31/06 10:59:28  host/
             renew until 01/02/07 10:59:25
      12/28/06 17:53:29  12/31/06 10:59:28  host/
             renew until 01/02/07 10:59:25 
    2. If nothing appears to be relevant or existing, it is safe to issue the kdestroy command to wipe out all old credentials and then re-initiate a kinit.