Advanced Connection Options
From FarmShare
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Public Key Authentication
Public key authentication is not supported on FarmShare systems.
GSSAPI Authentication
FarmShare systems do support password-less authentication using GSSAPI if you have a valid Kerberos ticket for the stanford.edu realm.
OpenSSH
On macOS and Linux systems you can enable GSSAPI by adding the following lines to ~/.ssh/config on your local system.
GSSAPIKeyExchange yes GSSAPIAuthentication yes
Recent versions of macOS no longer support the GSSAPIKeyExchange option; you can either omit this option or, if you prefer to keep a standard configuration, add the additional option IgnoreUnknown GSSAPIKeyExchange to your ~/.ssh/config file.
In some cases GSSAPI authentication may be enabled by default, but ssh will not forward your Kerberos ticket to the remote system. This can be inconvenient, especially in the legacy FarmShare environment, or when you expect to access AFS on rice. You can enable forwarding by adding GSSAPIDelegateCredentials yes to ~/.ssh/config, but you should do so only for trusted systems; to restrict the option, add it to a Host block:
Host rice rice.stanford.edu rice?? rice??.stanford.edu cardinal cardinal.stanford.edu cardinal? cardinal?.stanford.edu GSSAPIDelegateCredentials yes
See the man page for ssh_config for more information on GSSAPI options.
PuTTY
PuTTY supports GSSAPI authentication by default; to enable forwarding, select Connection → SSH → Auth → GSSAPI → Allow GSSAPI credential delegation.
SecureCRT
SecureCRT supports GSSAPI authentication, but it is disabled by default. To enable GSSAPI, open the Session Options dialog and select Connection → SSH2 → Authentication → GSSAPI and Connection → SSH2 → Authentication → Key exchange → Kerberos (Group Exchange). SecureCRT attempts authentication and key exchange methods in the order listed, so these methods should be moved to the top of their respective stacks. Delegation is enabled by default when GSSAPI authentication is selected.
Two-step Authentication
You can avoid some of the inconvenience of two-step authentication using ssh multiplexing. This feature creates a master session on initial connection to a particular host; subsequent sessions reuse the existing connection as a tunnel, so no further authentication is required. The master session can be configured to remain open even after you have closed the initial connection using the ControlPersist option. On macOS and Linux systems you can add the following lines to ~/.ssh/config file on your local system to enable multiplexing.
ControlMaster auto ControlPath ~/.ssh/%C ControlPersist yes
Note that while PuTTY and SecureCRT support multiplexing, they do not support ControlPersist, so the feature is of less utility for this purpose on Windows systems.
If you'd like to learn more, the OpenSSH Cookbook has a good introduction to multiplexing.
Keep-alive
A connection that is left open but idle might be closed after some time. Many SSH clients have a keep-alive feature that can be used to prevent idle disconnections.
OpenSSH
Add the following line to ~/.ssh/config on your local system.
ServerAliveInterval 60
PuTTY
Select Connection → Sending of null packets to keep session active → Seconds between keepalives (0 to turn off) 60 and Connection → Low-level TCP connection options → Enable TCP keepalives (SO_KEEPALIVE option).
SecureCRT
In the Session Options dialog, select Terminal → Anti-idle → Send protocol NO-OP every 60 seconds.
Sharing Host Keys
FarmShare consists of many individual nodes, grouped by function: login (or interactive use), compute, and GPU. Each group of systems shares an SSH host key, and you can configure OpenSSH to cache each of these keys once, and use it whenever you connect to a particular system in a given group, using the HostKeyAlias option. This can save some time when connecting for the first time to any particular system in a group if you've previously connected to another system in that same group.
Host rice rice.stanford.edu rice?? rice??.stanford.edu HostKeyAlias rice.stanford.edu Host wheat?? wheat??.stanford.edu HostKeyAlias wheat.stanford.edu Host oat?? oat??.stanford.edu HostKeyAlias oat.stanford.edu Host cardinal cardinal.stanford.edu cardinal? cardinal?.stanford.edu HostKeyAlias cardinal.stanford.edu
Mosh
Mosh is also supported for interactive terminal sessions. Mosh connections are more responsive under high-latency conditions, and can persist when your network changes or while your computer sleeps. The native Mosh client is available on macOS and Linux; on Windows you can use MobaXterm or the Mosh extension for Chrome.
Suggested OpenSSH Configuration
The options described above are here collected into a configuration that should be safe and convenient for most users connecting from macOS and Linux systems.
# ~/.ssh/config IgnoreUnknown GSSAPIKeyExchange # FarmShare 2 Host rice rice?? cardinal cardinal? HostName %h.stanford.edu Host rice rice.stanford.edu rice?? rice??.stanford.edu HostKeyAlias rice.stanford.edu Host wheat?? wheat??.stanford.edu HostKeyAlias wheat.stanford.edu Host oat?? oat??.stanford.edu HostKeyAlias oat.stanford.edu Host cardinal cardinal.stanford.edu cardinal? cardinal?.stanford.edu HostKeyAlias cardinal.stanford.edu Host rice rice.stanford.edu rice?? rice??.stanford.edu cardinal cardinal.stanford.edu cardinal? cardinal?.stanford.edu ControlMaster auto ControlPersist yes GSSAPIDelegateCredentials yes # All hosts Host * ControlPath ~/.ssh/%r@%h:%p GSSAPIKeyExchange yes GSSAPIAuthentication yes ServerAliveInterval 60
