Symlinks and chroot()

Contributor: Rod Whitworth <rodw at witworx dot com>

There have been many questions on the ProFTPD user mailing list about why symlinked directories are not visible to chrooted 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
  > 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
  > 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.

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

    * 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.

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.