ProFTPD: Symlinks and chroot()

## `DefaultRoot`, Symlinks and `chroot()`

Restricting Users' Directories
One of the most common questions for new users of ProFTPD is "How do I restrict my users to only certain directories?" or, phrased another way, "How can I put my users in a chroot jail?" As a common question, it definitely has a place in the FAQ. Many users, I fear, do not read the FAQ carefully, and so miss that section. The answer is ProFTPD's `DefaultRoot` configuration directive, which accomplishes this functionality by using the `chroot(2)` function.

This configuration directive may appear in the `<VirtualHost>`, `<Global>`, and the "server config" (meaning not in any `<VirtualHost>` or `<Global>` sections) configuration contexts. The most common configuration requested is to restrict users to their home directories, which can be done simply by adding the following line to your `proftpd.conf`:

      DefaultRoot ~

The `~` (tilde) is a Unix-ism that is expanded to the logging-in user's home directory. For slightly more complex setups, administrators may want to restrict only a subset of their users into home directories (or some other directory), but leave some privileged users unrestricted. For example, say you have your privileged users all as members of a group called `ftp-special`. The `DefaultRoot`'s optional second parameter, a group-expression, can then be used, like so:

      DefaultRoot ~ !ftp-special

This says to `chroot()` every user who is not a member of group `ftp-special` to their respective home directory, and:

      DefaultRoot /path/to/dir group1,group2

will `chroot()` users who are members of both `group1` and `group2` into `/path/to/dir`. More complex group-expressions can be used as needed.

Note that the execute bit (`--x`) must be on in order to `chroot()` a user into that directory. This bit is also needed for a user to be able to `chdir` into that directory.

Symlinks
There have been many questions on the ProFTPD user mailing list about why symlinked directories are not visible to `chroot`ed users (this includes `<Anonymous>` users as well as users restricted using `DefaultRoot`. This document is intended to clarify the issues and discuss some ways of achieving what is commonly desired.

These issues are not specific to ProFTPD, but rather to the workings of a Unix system. First, a brief review of how links work, and why `chroot(2)` poses such a problem. Then a look at ways around the issue.

How Links Work
There are two types of links in Unix: hard and symbolic.

A hard link is a file that is, for all intents and purposes, the file to which it is linked. The difference between a hardlink and the linked file is one of placement in the filesystem. Editing the hardlink edits the linked file. One limitation of hard links is that linked files cannot reside on different filesystems. This means that if `/var` and `/home` are two different mount points in `/etc/fstab` (or `/etc/vfstab`), then a file in `/var/tmp` cannot be hardlinked with a file in `/home`:

      # pwd
      /var/tmp
      # ln /home/tj/tmp/tmpfile tmplink
      ln: cannot create hard link `tmplink' to `/home/tj/tmp/tmpfile': Invalid cross-device link

A symbolic link (also referred to as a "symlink") is a file whose contents contain the name of the file to which the symbolic link points. For example:

      lrwxrwxrwx   1 root     root           11 Mar  2  2000 rmt -> /sbin/rmt

The file `rmt` contains the nine characters `/sbin/rmt`. The reason symbolic links fail when `chroot(2)` is used to change the position of the root (`/`)of the filesystem is that, once `/` is moved, the pointed-to file path changes. If, for example, if `chroot(2)` is used to change the filesystem root to `/ftp`, then the symlink above would be actually be pointing to `/ftp/sbin/rmt`. Chances that that link, if `chroot(2)` is used, now points to a path that does not exist. Symbolic links that point to nonexistent files are known as dangling symbolic links. Note that symbolic links to files underneath the new root, such as symlinks to a file in the same directory:

      # pwd
      /var/ftp
      # ls -l
      -rw-r--r--   1 root     root            0 Jan 16 11:50 tmpfile
      lrwxrwxrwx   1 root     root            7 Jan 16 11:50 tmplink -> tmpfile

will be unaffected; only paths that point outside/above the new root will be affected.

Link Creation Tricks
Knowing the above, it is now possible to demonstrate how some symlinks can work within a chrooted session, depending on how you create them.

Here is an example to demonstrate this point. Assume the following directory structure:

      /path/to/ftp/
      /path/to/ftp/folders/
      /path/to/ftp/folders/incoming/
      /path/to/ftp/incoming --> /path/to/ftp/folders/incoming/ (symlink)

And assume that you have the following in your `proftpd.conf`:

      DefaultRoot /path/to/ftp/

If the symlink is created using:

      # ln -s /path/to/ftp/folders/incoming /path/to/ftp/incoming

then that symlink will not work in the chrooted session; the path you gave ("/path/to/ftp/folders/incoming") is an absolute path, and will not exist after the chroot.

Instead, if you create the symlink using:

      # cd /path/to/ftp
      # ln -s folders/incoming incoming

then that symlink should work within the chroot; the key is to use relative paths which do not go above your `DefaultRoot` directory.

If your symlink does need to go above the `DefaultRoot` directory, then you need to use one of the other tricks described below.

Filesystem Tricks
A typical scenario is one where "`DefaultRoot ~`" is used to restrict users to their home directories, and where the administrator would like to have a shared upload directory, say `/var/ftp/incoming`, in each user's home directory. Symbolic links would normally be used to provide an arrangement like this. As mentioned above, though, when `chroot(2)` is used (which is what the `DefaultRoot` directive does), symlinks that point outside the new root (the user's home directory in this case) will not work. To get around this apparent limitation, it is possible on modern operating systems to mount directories at several locations in the filesystem.

To have an exact duplicate of the `/var/ftp/incoming directory` available in `/home/bob/incoming` and `/home/dave/incoming`, use one of these commands:

- Linux (as of the 2.4.0 kernel):

        mount --bind /var/ftp/incoming /home/bob/incoming
        mount --bind /var/ftp/incoming /home/dave/incoming

  or, alternatively:

        mount -o bind /var/ftp/incoming /home/bob/incoming
        mount -o bind /var/ftp/incoming /home/dave/incoming

- BSD (as of 4.4BSD):

        mount_null /var/ftp/incoming /home/bob/incoming
        mount_null /var/ftp/incoming /home/dave/incoming

- Solaris:

        mount -F lofs /var/ftp/incoming /home/bob/incoming
        mount -F lofs /var/ftp/incoming /home/dave/incoming

The same technique can be used for `<Anonymous>` directories, which also operate in a `chroot()`ed environment. Also, it should be possible to mount specific files this way, in addition to directories, should you need to (a directory is just another file in Unix).

As usual, more information can be found by consulting the man pages for the appropriate command for your platform. The commands for other flavors of Unix will be added as needed.

In order to have these tricks persist, to survive a system reboot, the `/etc/fstab` (or `/etc/vfstab`) file may need to have these mounts added. Consult your local `fstab(5)` (or `vfstab(4)` for Solaris) man pages for more information.

Chroots and Remote Filesystems
If the chroot directories for your users happen to reside on an NFS partition, then you need to make sure that root privileges are not blocked (e.g. often referred to as "root squash") by the NFS mount. Otherwise, the chroot will fail.

Frequently Asked Questions

Question: I am using the `DefaultRoot` directive, but my logins are failing. The debug logging shows the following:

      USER user: Login successful.
      Preparing to chroot to directory '/home/users/user'
      user chroot("/home/users/user"): Permission denied
      error: unable to set default root directory

I am starting `proftpd` with root privileges, so why is the `chroot()` failing with "Permission denied"?
Answer: The above situation can happen in cases where even root privileges are insufficient. There have been reports of this happening for security-hardened systems (e.g. SELinux, OpenWall, etc); it can also happen when NFS is involved.

If the `DefaultRoot` directory in question is mounted via NFS, make sure that the NFS configuration mounts the directory with root privileges. The `chroot(2)` system call requires root privileges; a no-root-privs mounted NFS directory does not allow the `chroot(2)` to succeed.

Similarly, instead of "Permission denied", you might see "No such file or directory":

      user chroot("~"): No such file or directory

The reasons for this error are explained here.

Question: Is it possible to configure `DefaultRoot` for all users except some special users, which will have a different root directory?
Answer: Yes, this is possible. ProFTPD supports having multiple `DefaultRoot` directives in the proftpd.conf at the same time; `proftpd` checks all of them in the order they appear. The first one which matches the logging-in user is applied.

To illustrate, here's an example. Keep in mind that the optional parameters to the `DefaultRoot` directive are group names, not user names.

      DefaultRoot /path/to/admin/dir admin-group
      DefaultRoot /path/to/special/dir special-group
      DefaultRoot ~  # everyone else

If the logging-in user is a member of group 'admin-group', then `proftpd` will chroot to the `/path/to/admin/dir` directory. If the logging-in user is not a member of group 'admin-group' but is a member of group 'special-group', then `/path/to/special/dir` is used for the chroot. And if the user is not a member of either of these groups, then the normal home directory is used for the chroot. It's always a good idea of have a "applies to everyone" `DefaultRoot` directive in your proftpd.conf, at the end of the list of `DefaultRoot`s, as a catch-all.

Question: Does `DefaultRoot` work properly if the path/home directory is a symlink?
Answer: Yes.

Note that some sites consider this a security risk; if that home directory can be deleted by remote users, and replaced with a symlink of their own creation (e.g. via SSH or some other webapp), this can be a problem. To help mitigate situations like this, you can use the `AllowChrootSymlinks` directive:

      # Do not follow symlinks when chrooting
      AllowChrootSymlinks off

As stated in the documentation, using `AllowChrootSymlinks` does not prevent this problem entirely; it simply means that `proftpd` cannot be used to get around the restrictions.

Question: I have configured `DefaultRoot` in my proftpd.conf, but my clients still see the root directory. Is it a bug?
Answer. Usually not.

First, make sure that you have restarted `proftpd`, so that the config changes you made (e.g. adding/modifying your `DefaultRoot` settings) are picked up by the running daemon.

Second, make sure that you have cleared any client cache. Many FTP clients (especially browsers) will cache the directory listings that they have obtained from an FTP server. Thus once you have restarted `proftpd` and you still see the root filesystem displayed by your client, you need to make sure that that client is actually getting that listing from the FTP server, rather than showing you a stale/cached copy. The command-line `ftp(1)` client is good for testing this situation, as it is very simplistic and does not cache such things.

Last, double-check the proftpd debug logging. It could be that `proftpd` is not using the configuration like you assume it is. Maybe a different config file is being used than the one you edited, or maybe the `DefaultRoot` directive is not in a `<Global>` section and you are using `<VirtualHost>` sections, etc.

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