The ChrootDirectory feature in OpenSSH (4.9 and up) provides an easy way to set up a restricted SFTP service without needing to go through the more complicated work of chroot(2) configuration and maintenance:
Unfortunately, setting up a chroot(2) environment is complicated, fragile and annoying to maintain. The most frequent reason our users have given when asking for chroot support in sshd is so they can set up file servers that limit semi-trusted users to be able to access certain files only. Because of this, we have made this particular case very easy to configure.
In a previous commit, markus@ implemented an "in-process" sftp server in sshd, basically linking sftp-server(8) into sshd(8). When the in-process sftp server is used, sshd does not need any special chroot configuration (no /dev nodes, no libraries, no statically-linked sftp-server) so the chroot setup and maintenance burden is eliminated.
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