Difference between revisions of "SFTP chroot"

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(Configuration: accuracy disputed)
(Fix typo's, better write bind example, added configuration on Installing)
 
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[[Category:File Transfer Protocol]]
 
[[Category:File Transfer Protocol]]
[[Category:Security]]
+
[[Category:Secure Shell]]
OpenSSH 4.9+ includes a built-in chroot for sftp, but requires a few tweaks to the normal install.
+
[[ja:SFTP chroot]]
 +
{{Related articles start}}
 +
{{Related|SSHFS}}
 +
{{Related articles end}}
  
==Installation==
+
[[OpenSSH]] 4.9+ includes a built-in chroot for sftp, but requires a few tweaks to the normal install.
This package is available in the core repository. To install it, run
+
 
# pacman -S openssh
+
== Installation ==
 +
 
 +
[[Install]] and configure [[OpenSSH]]. Once running, make sure {{ic|sftp-server}} has been set correctly:
 +
 
 +
{{hc|/etc/ssh/sshd_config|
 +
Subsystem sftp /usr/lib/ssh/sftp-server
 +
}}
 +
 
 +
Access files with ''sftp'' or [[SSHFS]]. Many standard [[List_of_applications#File_transfer_clients|FTP clients]] should work as well.
  
 
==Configuration==
 
==Configuration==
In /etc/ssh/sshd_config, modify the Subsystem line for sftp:
 
  Subsystem      sftp    internal-sftp
 
  
{{Accuracy|{{ic|sshd_config(5)}} says: "ChrootDirectory ... All components of the pathname must be root-owned directories that are not writable by any other user or group. ..."}}
+
===Setup the filesystem===
 +
 
 +
Bind mount the live [[filesystem]] to be shared to this directory. In this example, {{ic|/mnt/data/share}} is to be used, owned by [[user]] {{ic|root}} and has octal [[permissions]] of {{ic|755}}:
 +
 
 +
# chown root:root /mnt/data/share
 +
# chmod 755 /mnt/data/share
 +
# mkdir -p /srv/ssh/jail
 +
# mount -o bind /mnt/data/share /srv/ssh/jail
 +
 
 +
Add entries to [[fstab]] to make the bind mount survive on a reboot:
 +
/mnt/data/share /srv/ssh/jail  none  bind  0  0
 +
 
 +
{{Note|Readers may select a file access scheme on their own. For example, optionally create a subdirectory for an incoming (writable) space and/or a read-only space. This need not be done directly under {{ic
 +
|/srv/ssh/jail}} - it can be accomplished on the live partition which will be mounted via a bind mount as well.
 +
}}
 +
 
 +
=== Create an unprivileged user ===
 +
{{Note|You do not need to create a group, it is possible to use {{ic|Match User}} instead of {{ic|Match Group}}.}}
 +
 
 +
Create the {{ic|sftponly}} [[group]]:
 +
 
 +
# groupadd sftponly
 +
 
 +
Create a [[user]] that uses `sftponly` as main group:
 +
 
 +
# useradd -g sftponly -d ''/srv/ssh/jail'' ''username''
  
At the end of the file, add something similar to the following for a group:
+
Set a (complex) password - to prevent {{ic|account is locked}} error:
  Match Group sftpusers
 
    ChrootDirectory /home/%u
 
    ForceCommand internal-sftp
 
    AllowTcpForwarding no
 
    X11Forwarding no
 
  
Or for a user:
+
# passwd ''username''
  Match User username
 
    ChrootDirectory /home/%u
 
    ForceCommand internal-sftp
 
  
The /home represents root of the users home directory.
+
You many to deny [[shell]] login access for the user:
  
'''Fixing path for authorized_keys'''
+
# usermod -s /sbin/nologin ''username''
  
With the standard-path of ''AuthorizedKeysFile'' the Pubkey-Authorization will fail on chrooted-users, to fix this we have to add an '%h' to the path.
+
=== Configure OpenSSH ===
  AuthorizedKeysFile      %h/.ssh/authorized_keys
+
{{Note|You may want to use {{ic|Match User}} instead of {{ic|Match Group}} as been given in the previous step.}}
  
Restart sshd:
+
{{hc|/etc/ssh/sshd_config|<nowiki>
# systemctl restart sshd.service
+
Subsystem sftp /usr/lib/ssh/sftp-server
  
===Adding new chrooted users===
+
Match Group sftponly
If using the group method above, ensure all sftp users are put in the appropriate group, i.e.:
+
  ChrootDirectory %h
   usermod -g sftpusers
+
  ForceCommand internal-sftp
 +
   AllowTcpForwarding no
 +
  X11Forwarding no
 +
  PasswordAuthentication no
 +
</nowiki>}}
  
Also, set their shell to /sbin/nologin to prevent a normal ssh login:
+
[[Restart]] {{ic|sshd.service}} to confirm the changes.
  usermod -s /sbin/nologin
 
  
You also need to add /sbin/nologin to /etc/shells, or the sftp-users won't be able to login.
+
==== Fixing path for authorized_keys ====
 +
{{Tip|Use the [[SSH_keys#Key_ignored_by_the_server|debug mode]] of OpenSSH on the client and server in case of {{ic|(pre)auth}} error(s).}}
 +
With the standard path of ''AuthorizedKeysFile'', the [[SSH keys]] authentication will fail for chrooted-users. To fix this, [[append]] a root-owned directory on ''AuthorizedKeysFile'' to {{ic|/etc/openssh/sshd_config}} e.g. {{ic|/etc/ssh/authorized_keys}}, as example:
  
{{Warning|Some daemon users erroneously specify 'nologin' as their shell as one way of ensuring that these users cannot log-in. If 'nologin' is added as a valid shell, users should make sure that these daemon user accounts are sufficiently locked. Alternatively, the affected user accounts can be changed to use /usr/bin/false as shell to ensure that the selected shell is considered invalid by PAM}}
+
{{hc|/etc/ssh/sshd_config|
 +
AuthorizedKeysFile ''/etc/ssh/authorized_keys/%u'' .ssh/authorized_keys
 +
PermitRootLogin no
 +
PasswordAuthentication no
 +
PermitEmptyPasswords no
 +
Subsystem sftp /usr/lib/ssh/sftp-server
 +
}}
  
Note that since this is only for sftp, a proper chroot environment with a shell and /dev/* doesn't need to be created.
+
Create ''authorized_keys'' folder, generate a [[SSH_keys#Choosing_the_key_location_and_passphrase|SSH-key]] on the client, [[SSH_keys#Manual_method|copy]] the contents of the key to {{ic|/etc/ssh/authorized_keys}} (or any other preferred method) of the server and [[SSH_keys#Key_ignored_by_the_server|set correct permissions]]:
  
Their chroot will be the same as their home directory. The permissions are not the same as a normal home, though. Their home directory must be owned as root and not writable by another user or group. This includes the path leading to the directory. My recommendation is to use /usr/local/chroot as a root and build the home directories under that.
+
# mkdir /etc/ssh/authorized_keys
 +
# chown root:root /etc/ssh/authorized_keys
 +
# chmod 755 /etc/ssh/authorized_keys
 +
# echo 'ssh-rsa <key> <username@host>' >> ''/etc/ssh/authorized_keys/username''
 +
# chmod 644 /etc/ssh/authorized_keys/''username''
  
==Logging==
+
[[Restart]] {{ic|sshd.service}}.
  
'''1)'''
+
==Tips and tricks==
 +
=== Write permissions ===
 +
The [[#Setup the filesystem|bind]] path needs to be fully owned by {{ic|root}}, however files and/or subdirectories don't have to be.
 +
In the following example the [[user]] ''www-demo'' uses {{ic|/srv/ssh/www/demo}} as the jail-directory:
 +
# mkdir /srv/ssh/www/demo/public_html
 +
# chown www-demo:sftponly /srv/ssh/www/demo/public_html
 +
# chmod 775 /srv/ssh/www/demo/public_html
  
The user will not be able to access {{ic|/dev/log}}. This can be seen by running {{ic|strace}} on the process once the user connects and attempts to download a file. Create the sub-dircetory {{ic|dev}} in the {{ic|ChrootDirectory}}, for example:
+
The user should now be able to create files/subdirectories inside this directory. See [[File permissions and attributes]] for more information.
  sudo mkdir /usr/local/chroot/theuser/dev
+
 
  sudo chmod 755 /usr/local/chroot/theuser/dev
+
=== Logging ===
 +
{{Accuracy|Is this possible with systemd-journal? Are there no security concerns?}}
 +
 
 +
The user will not be able to access {{ic|/dev/log}}. This can be seen by running {{ic|strace}} on the process once the user connects and attempts to download a file.  
 +
 
 +
==== Create sub directory ====
 +
Create the sub-directory {{ic|dev}} in the {{ic|ChrootDirectory}}, for example:
 +
# mkdir /usr/local/chroot/user/dev
 +
# chmod 755 /usr/local/chroot/user/dev
  
 
{{ic|syslog-ng}} will create the device {{ic|/usr/local/chroot/theuser/dev/log}} once configured.
 
{{ic|syslog-ng}} will create the device {{ic|/usr/local/chroot/theuser/dev/log}} once configured.
  
'''2)'''
+
==== Syslog-ng configuration ====
 
 
 
Add to {{ic|/etc/syslog-ng/syslog-ng.conf}} a new source for the log and add the configuration, for example change the section:
 
Add to {{ic|/etc/syslog-ng/syslog-ng.conf}} a new source for the log and add the configuration, for example change the section:
 
{{bc|<nowiki>source src {
 
{{bc|<nowiki>source src {
Line 86: Line 140:
 
}}
 
}}
  
(Optional) If you'd like to similarly log SSH messages to it's own file:
+
(Optional) If you would like to similarly log SSH messages to its own file:
  
 
{{bc|<nowiki>#sshd configuration
 
{{bc|<nowiki>#sshd configuration
Line 93: Line 147:
 
log { source(src); filter(f_ssh); destination(ssh); };</nowiki>
 
log { source(src); filter(f_ssh); destination(ssh); };</nowiki>
 
}}
 
}}
(From [[Syslog-ng#Move_log_to_another_file]])
+
(From [[Syslog-ng#Move log to another file]])
  
'''3)'''
+
==== OpenSSH configuration ====
  
 
Edit {{ic|/etc/ssh/sshd_config}} to replace all instances of {{ic|internal-sftp}} with {{ic|internal-sftp -f AUTH -l VERBOSE}}
 
Edit {{ic|/etc/ssh/sshd_config}} to replace all instances of {{ic|internal-sftp}} with {{ic|internal-sftp -f AUTH -l VERBOSE}}
  
'''4)'''
+
==== Restart service ====
  
Restart logging and SSH:
+
[[Restart]] service {{ic|syslog-ng}} and {{ic|sshd}}.
  systemctl restart syslog-ng.service
 
  systemctl restart sshd.service
 
  
 
{{ic|/usr/local/chroot/theuser/dev/log}} should now exist.
 
{{ic|/usr/local/chroot/theuser/dev/log}} should now exist.
  
==Testing your chroot==
+
== Alternatives to SFTP ==
# ssh username@localhost
 
 
 
should refuse the connection or fail on login. The response varies, possibly due to the version of OpenSSH used.
 
 
 
# sftp username@localhost
 
  
should place you in the chroot'd environment.
+
=== Secure copy protocol (SCP) ===
 +
Installing {{Pkg|openssh}} provides the ''scp'' command to transfer files. SCP may be faster than using SFTP [https://superuser.com/questions/134901/whats-the-difference-between-scp-and-sftp].
  
==Troubleshooting==
+
[[Install]] {{Aur|rssh}} or {{Pkg|scponly}} as alternative shell solutions.
Error while trying to connect
 
Write failed: Broken pipe                                                                                             
 
Couldn't read packet: Connection reset by peer
 
If you also find similar message in /var/log/auth.log
 
sshd[12399]: fatal: bad ownership or modes for chroot directory component "/path/of/chroot/directory/" 
 
  
This is a {{ic|ChrootDirectory}} ownership problem.  sshd will reject SFTP connections to accounts that are set to chroot into any directory that has ownership/permissions that sshd considers insecure.  sshd's strict ownership/permissions requirements dictate that every directory in the chroot path must be owned by root and only writable by the owner.  So, for example, if the chroot environment is /home must be owned by root.
+
==== Scponly ====
See below for possible alternatives.
 
  
The reason for this is to [http://lists.mindrot.org/pipermail/openssh-unix-dev/2009-May/027651.html prevent a user from escalating their privileges] and becoming root, escaping the chroot environment.
+
[[install]] {{Pkg|scponly}}.  
  
If chroot environment is in user's home directory, make sure user have access to it's home directory, or user would not be able to access it's publickey, produce following error
+
For existing users, simply set the user's shell to scponly:
Permission denied (publickey).
 
  
== Write access to chroot dir ==
+
# usermod -s /usr/bin/scponly ''username''
  
As above, if a user is able to write to the chroot directory then it is possible for them to escalate their privileges to root and escape the chroot.  One way around this is to give the user two home directories - one "real" home they can write to, and one SFTP home that is locked down to keep sshd happy and your system secure.  By using {{ic|mount --bind}} you can make the real home directory appear as a subdirectory inside the SFTP home directory, allowing them full access to their real home directory.
+
See [https://github.com/scponly/scponly/wiki the Scponly Wiki] for more details.
  
This can also be used to achieve other goals.  For example, a user's home directory can be locked down per the sshd chroot rules, and bind mounts used to provide users access to other directories:
+
==== Adding a chroot jail ====
  
# mkdir /home/user/web
+
The package comes with a script to create a chroot. To use it, run:
# mount --bind /srv/web/example.com /home/user/web
 
  
Now the user can log in with SFTP, they are chrooted to {{ic|/home/user}}, but they see a folder called "web" they can access to manipulate files on a web site (assuming they have correct permissions in {{ic|/srv/web/example.com}}.
+
# /usr/share/doc/scponly/setup_chroot.sh
 +
* Provide answers
 +
* Check that {{ic|/path/to/chroot}} has {{ic|root:root}} owner and {{ic|r-x}} for others
 +
* Change the shell for selected user to {{ic|/usr/bin/scponlyc}}
 +
* sftp-server may require some libnss modules such as libnss_files. Copy them to chroot's {{ic|/lib}} path.
  
==Links & References==
+
== See also ==
 
*[http://www.minstrel.org.uk/papers/sftp/ http://www.minstrel.org.uk/papers/sftp/builtin/]
 
*[http://www.minstrel.org.uk/papers/sftp/ http://www.minstrel.org.uk/papers/sftp/builtin/]
 
*[http://www.openbsd.org/cgi-bin/man.cgi?query=sshd_config http://www.openbsd.org/cgi-bin/man.cgi?query=sshd_config]
 
*[http://www.openbsd.org/cgi-bin/man.cgi?query=sshd_config http://www.openbsd.org/cgi-bin/man.cgi?query=sshd_config]

Latest revision as of 15:38, 16 August 2017

Related articles

OpenSSH 4.9+ includes a built-in chroot for sftp, but requires a few tweaks to the normal install.

Installation

Install and configure OpenSSH. Once running, make sure sftp-server has been set correctly:

/etc/ssh/sshd_config
Subsystem sftp /usr/lib/ssh/sftp-server

Access files with sftp or SSHFS. Many standard FTP clients should work as well.

Configuration

Setup the filesystem

Bind mount the live filesystem to be shared to this directory. In this example, /mnt/data/share is to be used, owned by user root and has octal permissions of 755:

# chown root:root /mnt/data/share
# chmod 755 /mnt/data/share
# mkdir -p /srv/ssh/jail
# mount -o bind /mnt/data/share /srv/ssh/jail

Add entries to fstab to make the bind mount survive on a reboot:

/mnt/data/share /srv/ssh/jail  none   bind   0   0
Note: Readers may select a file access scheme on their own. For example, optionally create a subdirectory for an incoming (writable) space and/or a read-only space. This need not be done directly under /srv/ssh/jail - it can be accomplished on the live partition which will be mounted via a bind mount as well.

Create an unprivileged user

Note: You do not need to create a group, it is possible to use Match User instead of Match Group.

Create the sftponly group:

# groupadd sftponly 

Create a user that uses `sftponly` as main group:

# useradd -g sftponly -d /srv/ssh/jail username

Set a (complex) password - to prevent account is locked error:

# passwd username

You many to deny shell login access for the user:

# usermod -s /sbin/nologin username

Configure OpenSSH

Note: You may want to use Match User instead of Match Group as been given in the previous step.
/etc/ssh/sshd_config
Subsystem sftp /usr/lib/ssh/sftp-server

Match Group sftponly
  ChrootDirectory %h
  ForceCommand internal-sftp
  AllowTcpForwarding no
  X11Forwarding no
  PasswordAuthentication no

Restart sshd.service to confirm the changes.

Fixing path for authorized_keys

Tip: Use the debug mode of OpenSSH on the client and server in case of (pre)auth error(s).

With the standard path of AuthorizedKeysFile, the SSH keys authentication will fail for chrooted-users. To fix this, append a root-owned directory on AuthorizedKeysFile to /etc/openssh/sshd_config e.g. /etc/ssh/authorized_keys, as example:

/etc/ssh/sshd_config
AuthorizedKeysFile /etc/ssh/authorized_keys/%u .ssh/authorized_keys
PermitRootLogin no
PasswordAuthentication no
PermitEmptyPasswords no
Subsystem sftp /usr/lib/ssh/sftp-server

Create authorized_keys folder, generate a SSH-key on the client, copy the contents of the key to /etc/ssh/authorized_keys (or any other preferred method) of the server and set correct permissions:

# mkdir /etc/ssh/authorized_keys
# chown root:root /etc/ssh/authorized_keys
# chmod 755 /etc/ssh/authorized_keys
# echo 'ssh-rsa <key> <username@host>' >> /etc/ssh/authorized_keys/username
# chmod 644 /etc/ssh/authorized_keys/username

Restart sshd.service.

Tips and tricks

Write permissions

The bind path needs to be fully owned by root, however files and/or subdirectories don't have to be. In the following example the user www-demo uses /srv/ssh/www/demo as the jail-directory:

# mkdir /srv/ssh/www/demo/public_html
# chown www-demo:sftponly /srv/ssh/www/demo/public_html
# chmod 775 /srv/ssh/www/demo/public_html

The user should now be able to create files/subdirectories inside this directory. See File permissions and attributes for more information.

Logging

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: Is this possible with systemd-journal? Are there no security concerns? (Discuss in Talk:SFTP chroot#)

The user will not be able to access /dev/log. This can be seen by running strace on the process once the user connects and attempts to download a file.

Create sub directory

Create the sub-directory dev in the ChrootDirectory, for example:

# mkdir /usr/local/chroot/user/dev
# chmod 755 /usr/local/chroot/user/dev

syslog-ng will create the device /usr/local/chroot/theuser/dev/log once configured.

Syslog-ng configuration

Add to /etc/syslog-ng/syslog-ng.conf a new source for the log and add the configuration, for example change the section:

source src {
  unix-dgram("/dev/log");
  internal();
  file("/proc/kmsg");
};

to:

source src {
  unix-dgram("/dev/log");
  internal();
  file("/proc/kmsg");
  unix-dgram("/usr/local/chroot/theuser/dev/log");
};

and append:

#sftp configuration
destination sftp { file("/var/log/sftp.log"); };
filter f_sftp { program("internal-sftp"); };
log { source(src); filter(f_sftp); destination(sftp); };

(Optional) If you would like to similarly log SSH messages to its own file:

#sshd configuration
destination ssh { file("/var/log/ssh.log"); };
filter f_ssh { program("sshd"); };
log { source(src); filter(f_ssh); destination(ssh); };

(From Syslog-ng#Move log to another file)

OpenSSH configuration

Edit /etc/ssh/sshd_config to replace all instances of internal-sftp with internal-sftp -f AUTH -l VERBOSE

Restart service

Restart service syslog-ng and sshd.

/usr/local/chroot/theuser/dev/log should now exist.

Alternatives to SFTP

Secure copy protocol (SCP)

Installing openssh provides the scp command to transfer files. SCP may be faster than using SFTP [1].

Install rsshAUR or scponly as alternative shell solutions.

Scponly

install scponly.

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

# usermod -s /usr/bin/scponly username

See the Scponly Wiki for more details.

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.

See also