SCP and SFTP

Related articles

• SSHFS
• SFTP chroot
• Pure-FTPd

The Secure copy (SCP) is a protocol to transfer files via a Secure Shell connection. The SSH file transfer protocol (SFTP) is a related protocol, also relying on a secure shell back-end. Both protocols allow secure file transfers, encrypting passwords and transferred data. The SFTP protocol, however, features additional capabilities like, for example, resuming broken transfers or remote file manipulation like deletion.

Secure file transfer protocol (SFTP)

Install and configure OpenSSH. Once running, SFTP is available by default.

Access files with the sftp program or SSHFS. Many standard FTP programs should work as well.

Secure file transfer protocol (SFTP) with a chroot jail

Sysadmins can jail a subset of users to a chroot jail using openssh thus restricting their access to a particular directory tree. This can be useful to simply share some files without granting full system access or shell access. Users with this type of setup may use SFTP clients such as filezilla to put/get files in the chroot jail.

Setup the filesystem

Create a jail directory:

# mkdir -p /var/lib/jail

Optionally, bind mount the filesystem to be shared to this directory. In this example, /mnt/data/share is to be used. It is owned by root and has octal permissions of 755.

# mount -o bind /mnt/data/share /var/lib/jail

Tip: Consider adding an entry to /etc/fstab to make the bind mount survive a reboot.

Create an unprivileged user

Create the share user and setup a good password:

# useradd -g sshusers -d /var/lib/jail foo
# passwd foo

Setup OpenSSH

Add the following to the end of /etc/ssh/sshd_config to enable the share and to enforce the restrictions:

/etc/ssh/sshd_config

...
 Match group sshusers
  ChrootDirectory %h
  X11Forwarding no
  AllowTcpForwarding no
  PasswordAuthentication yes
  ForceCommand internal-sftp

Restart sshd.service to re-read the configuration file. See SFTP chroot to configure the keys correctly when using chroot or it will get permission denied.

Test that in fact, the restrictions are enforced by attempting an ssh connection via the shell. The ssh server should return a polite notice of the setup:

$ ssh [email protected]

[email protected]'s password:
This service allows sftp connections only.
Connection to someserver.com closed.

Secure copy protocol (SCP)

Install, configure and start OpenSSH. It contains the scp utility to transfer files.

More features are available by installing additional packages, for example rssh^AUR or scponly described below.

Note: Since OpenSSH 9.0 the scp utility uses the SFTP protocol by default. The -O option must be used to use the legacy SCP protocol.

Warning: The scp protocol is outdated, inflexible and not readily fixed. Its authors recommend the use of more modern protocols like sftp and rsync for file transfer instead.[1]

General Usage

Linux to Linux

Copy file from a remote host to local host SCP example:

$ scp username@from_host:file.txt /local/directory/

Copy file from local host to a remote host SCP example:

$ scp file.txt username@to_host:/remote/directory/

Copy directory from a remote host to local host SCP example:

$ scp -r username@from_host:/remote/directory/  /local/directory/

Copy directory from local host to a remote host SCP example:

$ scp -r /local/directory/ username@to_host:/remote/directory/

Copy file from remote host to remote host SCP example:

$ scp username@from_host:/remote/directory/file.txt username@to_host:/remote/directory/

Linux to Windows

Use a Windows program such as WinSCP

Scponly

Scponly is a limited shell for allowing users scp/sftp access and only scp/sftp access. Additionally, one can setup scponly to chroot the user into a particular directory increasing the level of security.

install scponly.

For existing users, simply set the user's shell to scponly:

# usermod -s /usr/bin/scponly username

Adding a chroot jail

The package comes with a script to create a chroot. To use it, run:

# /usr/share/doc/scponly/setup_chroot.sh

• Provide answers
• Check that /path/to/chroot has root:root owner and r-x for others
• Change the shell for selected user to /usr/bin/scponlyc
• sftp-server may require some libnss modules such as libnss_files. Copy them to chroot's /lib path.

Uploads to Chroot jail root dir

For security reasons the directory set as the chroot directory must be owned by root with only root having write access to it otherwise sftp/ssh connections will be denied. This of course means regular users cannot upload files to the root directory. In order to get around this while not compromising security you can create a folder inside the chroot directory which the regular user or group has write access to, e.g:

# cd /var/lib/jail
# mkdir uploads
# chown :sshusers uploads
# chmod 730 uploads

Note: This will only allow users of group "sshusers" to upload to (but not list the contents of) the "uploads" directory. Use chmod 770 to allow sshusers to view contents.

Some applications utilizing SFTP do not allow input of sub-directories when performing operations (e.g. uploading files), and will attempt to upload files to the chroot base directory (which will be denied). In order to force these applications to use a specific sub-directory you can append the following to the "ForceCommand" option:

/etc/ssh/sshd_config

...
 Match group sshusers
  ...
  ForceCommand internal-sftp -d /uploads

Users on connect will then have their start directory change to the specified sub-directory (remember to restart the sshd server).