Saturday, August 2, 2014

How to configure chroot SFTP in Linux

There are some scenario where system admin wants only few users should be  allowed to transfer files to Linux boxes not ssh. We can achieve this by setting up SFTP in chroot environment.
Background of SFTP & chroot :
SFTP stands for SSH File Transfer protocol or Secure File Transfer Protocol. SFTP provides file access, file transfer, and file management functionalities over any reliable data stream. When we configure SFTP in chroot environment , then only allowed users will be limited to their home directory , or we can say allowed users will be in jail like environment where they can’t even change their directory.
In article we will configure Chroot SFTP in RHEL 6.X & CentOS 6.X. We have one user ‘Jack’ , this users will be allowed to transfer files on linux box but no ssh access.
Step:1  Create a group
[root@localhost ~]# groupadd  sftp_users
Step:2 Assign the secondary group(sftp_users) to the user.
If the users doesn’t exist on system , use below command :
[root@localhost ~]# useradd  -G sftp_users  -s /sbin/nologin  jack
[root@localhost ~]# passwd jack
For already existing users , use below usermod command :
[root@localhost ~]# usermod –G sftp_users  -s /sbin/nologin  jack
Note : if you want to change the default home directory of users , then use ‘-d’ option in useradd and usermod  command and set the correct permissions.
Step:3 Now edit the config file “/etc/ssh/sshd_config”  
# vi /etc/ssh/sshd_config
#comment out the below line and add a line like below
#Subsystem sftp /usr/libexec/openssh/sftp-server
Subsystem sftp internal-sftp
# add Below lines  at the end of file
Match Group sftp_users
  X11Forwarding no
  AllowTcpForwarding no
  ChrootDirectory %h                    
  ForceCommand internal-sftp
Where :
Match Group sftp_users – This indicates that the following lines will be matched only for users who belong to group sftp_users
ChrootDirectory %h – This is the path(default user's home directory) that will be used for chroot after the user is authenticated. So, for Jack, this will be /home/jack.
ForceCommand internal-sftp – This forces the execution of the internal-sftp and ignores any command that are mentioned in the ~/.ssh/rc file.
Restart the ssh service
# service sshd restart
Step:4 Set the Permissions :
[root@localhost ~]# chmod 755 /home/jack
[root@localhost ~]# chown root /home/jack
[root@localhost ~]# chgrp -R sftp_users /home/jack
If You want that jack user should be allowed to upload files , then create a upload folder with the below permissions ,
[root@localhost jack]# mkdir /home/jack/upload
[root@localhost jack]# chown jack. /home/jack upload/
Step:5  Now try to access the system & do testing
Try to access the system via ssh
As You can see below jack user is logged in  via SFTP and can't change the directory becuase of chroot environment.
Now do the uploading and downloading testing as shown below:
sftp-upload-downloadAs we can see above , both uploading & downloading working fine for jack user.

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