How to execute a script on sftp connection?

Question

I run an ssh server on my machine and I restrict access to certain users to sftp only with internal-sftp & ChrootDirectory. What I need is a way to execute a script before chrooting users. Actually, the goal is to mount an encrypted filesystem on client connection and unmount it on close. Thx in advance.

Answer 1

If you don't want to hack the openssh code you have to use the external sftp server. If you do it is a simple matter of putting a wrapper around it. For example: in sshd_config

Subsystem       sftp    /usr/local/bin/sftp-server

In /usr/local/bin/sftp-server:

#!/bin/sh
mount_special_fs
chroot /my/secret/stuff /usr/libexec/openssh/sftp-server
umount_special_fs

It might be possible to put a wrapper around sshd and launch the wrapper from inetd but launching sshd from inted is discouraged because it is to slow to start up.

Answer 2

You can also use pam_script, a pam module to execute script from a pam authentication module. You can add it in your pam sshd configuration. URL: http://linux.die.net/man/5/pam_script