How to enable LDAP auth for SFTP logins only

Question

I currently use ActiveDirectory for authentication on my Ubuntu 10.04 servers using a tool called Centrify Express. Centrify has a custom .so file that is configured in /etc/pam.d/common-auth.

I'd like to do normal LDAP authentication (bypassing centrify) for just SFTP users (not SSH logins). How would I configure /etc/pam.d/sshd to try LDAP authentication for SFTP logins, but not SSH logins? The idea is that my SFTP OU would allow users to download, and upload files, but they wouldn't be able to get a shell.

Solution

If I understand your question correctly, you are looking to provide only sftp service (that is, no interactive logins via ssh).

You can force connections to use the sftp service by adding this to your sshd_config file:

ForceCommand internal-sftp

This is documented in the sshd_config man page:

ForceCommand
  Forces the execution of the command specified by ForceCommand, ignoring any
  command supplied by the client and ~/.ssh/rc if present.  The command is
  invoked by using the user’s login shell with the -c option.  This applies to
  shell, command, or subsys- tem execution.  It is most useful inside a Match
  block.  The command originally supplied by the client is available in the
  SSH_ORIGINAL_COMMAND environment variable.  Specifying a command of
  “internal-sftp” will force the use of an in-process sftp server that requires
  no support files when used with ChrootDirectory.

You can modify /etc/pam.d/sshd to configure sshd to use the normal pam_ldap module rather than the pam module provided by Centrify.

With these configuration changes in place, sftp connections will use normal LDAP authentication, and interactive shells will not be available.

Comments

[user2751502]

What is it about Centrify that makes plain LDAP a more suitable alternative for sftp users?

[GregB]

Centrify Express (the free version) doesn't allow us to control the UID, and GID of users/groups. When a user connects, they are given a random UID near the top of the range. This makes managing permissions across servers difficult.