ndhc/README

ifchd, copyright (c) 2004-2011 Nicholas Kain.  Licensed under GNU GPL.

Requirements:

Linux kernel (tested: 2.4, 2.6)
        * libcap is required (available via ftp.kernel.org)

C99-compliant C compiler (for C99 struct subobject init)
        * any modern GCC should be sufficient

CMake (tested: 2.8)

Tested with glibc.  dietlibc is not compatible.  I have not tested uclibc.

INTRODUCTION
------------

ndhc consists of a set of daemons that cooperate in order to provide
privilege-seperated dhcp client services.  Each daemon runs with the minimal
necessary privileges in order to perform its task.  Currently, ndhc consists of
two daemons: the eponymous ndhc and ifchd.

ndhc communicates with dhcp servers and handles the vagaries of the dhcp
client protocol.  It runs as a non-root user inside a chroot.  ndhc retains
only the minimum necessary set of privileges required to perform its duties.
These powers include the ability to bind to a low port, the ability to open a
raw socket, and the ability to communicate on broadcast channels.  ndhc holds
no other powers and is restricted to a chroot that contains nothing more than a
domain socket filesystem object and a urandom device node.

ifchd handles interface change requests.  It listens on a UNIX domain socket
for such requests, and denies any client that does not match an authorized gid,
uid, or pid.  ifchd runs as a non-root user inside a chroot, and retains only
the power to configure network interfaces.  ifchd is designed so that it has
the ability to service multiple client requests simultaneously; a single ifchd
is sufficient for multiple ndhc clients.  Only exotic setups should require
this functionality, but it does exist.

Note that ndhc does not support the entire DHCP client protocol.  Only the
minimum necessary featureset is implemented.  This behavior should be familiar
to anyone who has used software that purports to be be secure.

USAGE
-----

1) Compile and install ifchd and ndhc.
    a) Create a build directory:
       mkdir build && cd build
    b) Create the makefiles:
       cmake ..
    c) Build ifchd and ndhc:
       make
    d) Install the ifchd/ifchd and ndhc/ndhc executables in a normal place.  I
       would suggest /usr/sbin or /usr/local/sbin.

2) Time to create the jail in which ifchd and ndhc will run.
    a) Become root and create new group "ifchd".

        $ su -
        # umask 077
        # groupadd ifchd

    b) Create new users "ifchd" and "dhcp".  The primary group of these
       users should be "ifchd".

        # useradd -d /var/lib/ndhc -s /sbin/nologin -g ifchd ifchd
        # useradd -d /var/lib/ndhc -s /sbin/nologin -g ifchd dhcp

    b) Create the jail directory and set its ownership properly.

        # mkdir /var/lib/ndhc
        # chown root.root /var/lib/ndhc
        # chmod a+rx /var/lib/ndhc
        # cd /var/lib/ndhc
        # mkdir var
        # mkdir var/state
        # mkdir var/run
        # chown -R ifchd.ifchd var
        # chmod -R a+rx var

    c) Create a urandom device for ndhc to use within the jail.

        # mkdir dev
        # mknod dev/urandom c 1 9
        # mknod dev/null c 1 3
        # chown -R root.root dev
        # chmod a+rx dev
        # chmod a+r dev/urandom
        # chmod a+rw dev/null

        d) (optional) If you wish for logging to properly work, you
           will need to properly configure your logging daemon so that it
           opens a domain socket in the proper location within the jail.
           Since this varies per-daemon, I cannot provide a general
           configuration.

3)  At this point the jail is usable; ifchd and ndhc are ready to
    be used.  As an example of a sample configuration, here is my
    rc.dhcp:

--START--

#!/bin/sh
case "$1" in
        start)
                ifchd -i wan0 -p /var/run/ifchd.pid -u ifchd -g ifchd -U dhcp \
                      -G ifchd -c /var/lib/ndhc &> /dev/null
                ndhc -b -i wan0 -u dhcp -C /var/lib/ndhc &> /dev/null
                ;;
        stop)
                killall ndhc ifchd
                ;;
esac

--END--

    This script works fine with my personal machines, which are set up
    exactly as I have outlined above.  If you have not entirely followed my
    directions, the script will of course require modifications.

4o) If you encounter problems, I suggest running both ifchd and ndhc in the
   foreground, and perhaps compiling ndhc with extra debugging output
   (uncomment DEBUG=1 in the Makefile).


BEHAVIOR NOTES
--------------

ifchd does not enable updates of the local hostname and resolv.conf by default.
If you wish to enable these functions, use the --resolve (-r) and --hostname
(-o) flags.  See ifchd --help.

ifchd can be set such that it only allows clients to configure particular
network interfaces.  The --interface (-i) argument does the trick, and may
be used multiple times to allow multiple interfaces.

GRSECURITY NOTES
----------------

Make sure that CONFIG_GRKERNSEC_CHROOT_CAPS is disabled.  Otherwise, ifchd will
lose its capabilities (in particular, the ability to reconfigure interfaces)
when it chroots.


PORTING NOTES
-------------

There are seven major functions that ifchd depends upon that are not generally
portable.  First, it uses the SO_PEERCRED flag of getsockopt() to discriminate
authorized connections by uid, gid, and pid.  Similar functionality exists in
at least the BSDs; however, it has a different API.  Second, ifchd takes
advantage of Linux capabilities so that it does not need full root privileges.
Capabilities were a proposed POSIX feature that was not made part of the
official standard, so any implemention that may exist will be system-dependent.
Third and fourth, ifchd configures network interfaces and routes.  Interface
and route configuration is entirely non-portable, usually requiring calls to
the catch-all ioctl(), and will almost certainly require platform-dependent
code.  Fifth and sixth, both ifchd and ndhc use epoll() and signalfd(), which
are Linux-specific.  Seventh, ndhc uses netlink sockets extensively for
both fetching data and hardware link state change notification events.

Some standard C libraries include a native implementation of strlcpy() and
strlcat().  Such defines may conflict with my implementations in strl.c/strl.h.
It is up to the user whether the standard C library implementations should be
used.  Note that some machines implement strlcpy() and strlcat() with
nonstandard semantics (notably Solaris).  On these systems, using the
system-provided implementations may lead to security problems.  Such problems
are the fault of the vendor.  If you are unsure whether your system is correct
or not, I suggest using the implementation that I provide.