wiki:doc/OpenbsdChrootedTor

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Material from https://wiki.torbsd.org/doku.php?id=en:openbsd-chroot

Running Tor in a chroot under OpenBSD

We have taken the instructions on the Tor Project wiki as a starting point, and have adapted them to suit OpenBSD. We assume basic Unix shell skills and some familiarity with using OpenBSD; the FAQs and man pages are highly recommended.

For reference, these instructions apply to OpenBSD-current as of 2017-10-03. The precise meaning of “current” is given in the FAQ on OpenBSD flavors.

Preliminaries

Filesystem

From the mount(8) man page:

`

nodev Do not interpret character or block special devices on
         the file system. This option is useful for a server
         that has file systems containing special devices for
        architectures other than its own.

` The installer marks fstab entries for partitions created during installation as nodev by default; only the root filesystem, where /dev lives, is not. This presents a problem for Tor in a chroot, because Tor needs to use three device special files: /dev/random, /dev/urandom and /dev/null. The usual way of making device files available in a chroot is to use the mknod(8) command in the chroot, but if the chroot lives on a filesystem mounted nodev (likely under OpenBSD) this will not work as expected when Tor tries to use the device.

A discussion of disk partitioning and configuration is beyond the scope of this document. The FAQ on Disk Partitioning in the Installer has links to the relevant documentation. If you wish to make a small partition available for use as a chroot area this is where to start.

For our purposes here we will take another approach: create a small filesystem stored in a regular user file and mount it without the nodev option. The vnconfig(8) command is the key. Let's start by being careful and looking at what, if any files have been attached to vnd(4) devices:

$ doas vnconfig -l
vnd0: covering /home/me/something/file.img on sd0k, inode 4783121
vnd1: not in use
vnd2: not in use
vnd3: not in use

A couple important points:

  • by default there are four vnd devices configured;
  • if one is in use and you vnconfig it again it is overwritten, so you need to know which ones are not in use;
  • nothing in the system will use a vnd device by default.

In the above example vnd0 is already in use. This means we should use vnd1 for the chroot; we'll use vnd1 for the rest of this document to stay consistent, but the point is: use an unused vnd device for the chroot filesystem.

First, prepare a file to hold the disk image and attach it as vnd1:

$ dd if=/dev/zero of=chroot.img bs=1m count=50 $ doas vnconfig vnd1 chroot.img

Next, use disklabel(8) to add a partition of the right type that takes up all of the virtual disk; for simplicity we use the interactive editor (-E option); in the transcript below user input is entered after the > prompt:

$ doas disklabel -E vnd1
Label editor (enter '?' for help at any prompt)
> a a
offset: [0]
size: [102400]
FS type: [4.2BSD]
>w
>q
No label changes.

Finally, create a filesystem on the partition we just created (/dev/vnd1a):

$ doas newfs /dev/rvnd1a

At this point you can mount /dev/vnd1a somewhere in your filesystem and use it for the chroot; we will mount it under /home/tor/chroot below. Once you are done using the chroot (no Tor process running anymore), you can do the following to clean up:

$ doas umount /home/tor/chroot
$ doas vnconfig -u vnd1

Ports Tree

We assume you have the ports tree unpacked under /usr/ports. Read the anoncvs documentation and follow its instructions for getting a copy of the ports tree.

For the rest of this guide we assume you have done this and that your user ID is in the wsrc group and has R/W access to /usr/ports (as covered in the wsrc FAQ).

We will use the ports infrastructure to relieve us of worrying about the mechanics of grabbing and verifying the code and chasing down dependencies.

Basic Chroot Setup

There are several elements that must be taken into account:

  • shared libraries;
  • the password database;
  • device nodes;
  • directory structure and ownership;
  • Tor software and configuration.

First off, set an environment variable to point at the base directory of the chroot. We use this everywhere in subsequent commands:

$ TORCHROOT=/home/tor/chroot

Note that this only sets this variable in your current shell. If you switch terminals or shells then you'll have to set it again.

Add a “tor” user and home directory:

$ doas useradd -d /home/tor -s /sbin/nologin tor
$ doas mkdir /home/tor
$ doas chown tor:tor /home/tor

Mount our little filesystem under /home/tor/chroot:

$ doas mkdir /home/tor/chroot
$ doas chown tor:tor /home/tor/chroot
$ mount /dev/vnd1a /home/tor/chroot

Compile Tor and Install in Chroot

We'll now use the ports tree to grab the Tor source code, verify it is the right stuff, unpack it, patch it (if necessary) and build it:

$ cd /usr/ports/net/tor
$ env SUDO=doas make

This will download, verify, extract and patch the tor source code and leave it in a port-specific directory under the ports build area, /usr/ports/pobj. It will also ensure that everything needed to build the port is installed. We set the SUDO environment variable in the make invocation in case any ports on which Tor depends on must be installed prior to building; this tells the ports system to become root at the appropriate times to install pacakges that are created as a by-product of chasing down these dependencies. As of this writing there is only one dependency (libevent) for Tor but as that changes these instructions should remain valid.

Note that we just used the ports tree to make sure we're starting with code that is known to build and work under OpenBSD, we're not building a package from the port as is normally done.

Next, we change working directories to the root of the source tree:

$ cd make show=WRKSRC
$ doas make install prefix=$TORCHROOT exec_prefix=$TORCHROOT sysconfdir=$TORCHROOT/etc

The make show=FOO idiom is useful in the ports tree generally; it shows you the full expansion of a make(1) variable used in the port's Makefile. The WRKSRC variable will contain the name of the working source tree directory, which is where we want to go. There is comprehensive documentation on this and the other variables used in ports Makefiles in the bsd.port.mk(5) man page.

Shared Libraries

We must deal with shared libraries both for the tor binary and for a system utility that we will copy into the chroot, pwd_mkdb(8). We also need ldconfig(8) in the chroot:

$ doas mkdir $TORCHROOT/sbin
$ doas cp /sbin/ldconfig $TORCHROOT/sbin
$ doas mkdir -p $TORCHROOT/usr/sbin
$ doas cp /usr/sbin/pwd_mkdb $TORCHROOT/usr/sbin

We must copy all shared libraries that both pwd_mkdb and tor need into the right places under the chroot. We define a shell function called shlibpaths to make the commands easier to type:

$ shlibpaths () {

ldd $1 | sed -e 1,3d | awk '{print substr($7,2)}'

}
$ tar -C / -cf - shlibpaths $TORCHROOT/bin/tor | doas tar -C $TORCHROOT -xf -
$ tar -C / -cf - shlibpaths $TORCHROOT/usr/sbin/pwd_mkdb | doas tar -C $TORCHROOT -xf -

The shlibpaths function works like so:

ldd(1) spits out the raw list of shared libraries;
sed(1) removes the first three lines of output, which are noise for our purposes;
awk(1) pulls out the seventh whitespace-separated column and prints all but the first character, making the path to the shared library relative to root.

The -C option to tar(1) tells tar to change directories to the given place before doing anything else. In both cases the tar command at the head of the pipe packs up the shared libraries in question and the tar command at the tail of the pipe unpacks them under the chroot.

Now all the shared libraries that tor and pwd_mkdb depend on are in $TORCHROOT/usr/lib and $TORCHROOT/usr/local/lib. We must now run ldconfig in the chroot to set up the /var/run/ld.so.hints file:

$ doas mkdir -p $TORCHROOT/var/run $ doas chroot $TORCHROOT /sbin/ldconfig /usr/lib /usr/local/lib

Now that we have pwd_mkdb usable in the chroot, set up a minimal password database:

$ doas sh -c "grep tor /etc/passwd > $TORCHROOT/etc/passwd"
$ doas sh -c "grep
tor /etc/group > $TORCHROOT/etc/group"
$ doas sh -c "grep tor /etc/master.passwd > $TORCHROOT/etc/master.passwd"
$ doas sh -c "grep
_shadow /etc/group >> $TORCHROOT/etc/group"
$ doas chroot $TORCHROOT /usr/sbin/pwd_mkdb /etc/master.passwd

Set up device nodes that Tor needs:

$ doas mkdir $TORCHROOT/dev
$ doas mknod -m 644 $TORCHROOT/dev/random c 45 0
$ doas mknod -m 644 $TORCHROOT/dev/urandom c 45 2
$ doas mknod -m 666 $TORCHROOT/dev/null c 2 2

At this point the software and basic system configuration in the chroot is complete.

Configure Tor

Create a minimal Tor configuration in the chroot:

$ doas sh -c "cat > $TORCHROOT/etc/tor/torrc"
User tor
DataDirectory /var/lib/tor
GeoIPFile /share/tor/geoip
PidFile /var/run/tor/tor.pid
Log notice file /var/log/tor/log
^D

Create the run-time directories needed by tor and make sure they are owned by the tor user and have the right permissions:

$ doas mkdir -p $TORCHROOT/var/run/tor
$ doas mkdir -p $TORCHROOT/var/lib/tor
$ doas chmod 700 $TORCHROOT/var/lib/tor
$ doas mkdir -p $TORCHROOT/var/log/tor
$ doas chown tor:tor $TORCHROOT/var/run/tor
$ doas chown tor:tor $TORCHROOT/var/lib/tor
$ doas chown tor:tor $TORCHROOT/var/log/tor

Start Tor

Finally, you should be able to start Tor in the chroot:

$ doas chroot $TORCHROOT /bin/tor

This should produce output that looks something like:

Oct 04 16:46:55.116 [notice] Tor 0.3.0.10 (git-c33db290a9d8d0f9) running on OpenBSD with Libevent 2.0.22-stable, OpenSSL LibreSSL 2.6.3 and Zlib 1.2.3.
Oct 04 16:46:55.116 [notice] Tor can't help you if you use it wrong! Learn how to be safe at https://www.torproject.org/download/download#warning
Oct 04 16:46:55.126 [notice] Read configuration file "/etc/tor/torrc".
Oct 04 16:46:55.192 [notice] Opening Socks listener on 127.0.0.1:9050

It will also produce a verbose log of its operation in $TORCHROOT/var/log/tor/log.

You can interrupt Tor when run this way by pressing C. If you want Tor to fork into the background and run as a daemon, add one more line to your torrc file:

$ doas sh -c "echo RunAsDaemon 1 >> $TORCHROOT/etc/tor/torrc"

If you start Tor with the above command after adding that line it will start silently and continue running. To shut it down use the PidFile we configured above:

$ doas sh -c "kill cat $TORCHROOT/var/run/tor/tor.pid "

At this point your chroot'ed Tor installation is working and listening for SOCKS connections on port 9050.

Last modified 6 months ago Last modified on Mar 1, 2018, 4:38:47 AM