ndhc/README

ndhc + ifchd, copyright (c) 2004-2011 Nicholas Kain.  Licensed under GNU GPL2.

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.  Notably, DHCP
options concatenation and IPv4 Link Local Addressing as defined in RFC3927 are
not and will not be supported.

On the other hand, ndhc fully implements RFC5227's address conflict detection
and defense.  Great care is taken to ensure that address conflicts will be
detected, and ndhc also has extensive support for address defense.  Care is
taken to prevent unintentional ARP flooding under any circumstance.

ndhc also monitors hardware link status via netlink events and reacts
appropriately when interface carrier status changes or an interface is
explicitly deconfigured.  This functionality can be useful on wired networks
when transient carrier downtimes occur (or cables are changed), but it is
particularly useful on wireless networks.

USAGE
-----

1) Compile and install ifchd and ndhc.
    a) gmake
    b) Install the build/ifchd and build/ndhc executables in a normal place.  I
       would suggest /usr/sbin or /usr/local/sbin.

1alt) 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
        # chmod g+w var/run

    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 examining the printed output.


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.

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

ndhc is rather platform-dependent, and it extensively uses Linux-specific
features.  Some of these features are also available on the BSDs.

1) Both ndhc and ifchd use 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.

2) 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.

3) ifchd configures network interfaces and routes.  Interface and route
configuration is entirely non-portable, usually requiring calls to the
catch-all ioctl(), or even more unusual mechanisms like netlink sockets.

4) ndhc uses netlink sockets extensively for both fetching data and hardware
link state change notification events.

5) ndhc uses the Berkeley Packet Filter / Linux Packet Filter interfaces to
drop unwanted packets in kernelspace.  This functionality is available on
most modern unix systems, but it is not standard.

6) ndhc uses epoll() and signalfd().  These are Linux-specific.

7) Numerous socket options are used, and the AF_PACKET socket family is used
for raw sockets and ARP.  These are largely Linux-specific, too.

8) ndhc uses strlcpy() and strlcat().  Native versions are provided.
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.

HISTORY
-------

I started writing ndhc back in 2004.  My ISP at the time required a dhcp
client for connection authentication, and I was not comfortable with any
of the existing clients, which all ran as root and had colorful security
histories.  DHCP is generally not a routed protocol, and lacks real
authentication mechanisms in real world deployments (some largely
abandoned RFCs for such behavior do exist), so no program existed to
fill the niche of a truly secure DHCP client.

My router/server at the time ran a custom Linux distro that was designed
for extreme security.  A root privileged DHCP client would be nearly the
only root-owned process running on the machine, so I was highly motivated
to develop an alternative.

ifchd was first written entirely from scratch.  It did not take long to write,
since it is by design rather simple, and I was already familiar with
the quirks of Linux capabilities.  That left me with the choice of adapting
an existing DHCP client or writing my own from scratch.

At the time, I just wanted something that would work, so my choice was to
adapt udhcpc to work with ifchd.  udhcpc was chosen since it was intended to
be used with resource-constrained or embedded systems, and was thus very
small.  ISC dhclient was another alternative, but it is an extremely large
program, and it would have been very hard to audit it for correctness.

udhcpc was not all that great of a choice, since it was designed to be small at
all costs, sacrificing correctness when necessary.  The code was hard to
follow, and had many quirks.  Bounds-checking was rare, type aliasing common,
and state transitions were convoluted.  Not all of the client was asynchronous,
and no precautions were taken against conflicting peers.  ARP was not used at
all.

However, it was small.  With a lot of work, I ripped out the script-calling
mechanisms and replaced them with ifchd requests.  Bounds-checking was
aggressively (and somewhat hamfistedly) retrofitted into the code.  It was
cleaned to a degree, and importantly it worked for connecting to my ISP.

Then I changed ISPs.  My new ISP used PPPoE, not dhcp.  Around the same time, I
also switched to using Gentoo rather than a hand-built distribution.  I didn't
have time to maintain the old custom setup, and it was very hard keeping up
with library vulnerabilties in eg, zlib or openssl, and ensuring that all
installed binaries, dynamic and static, were updated.  ndhc was abandoned for
many years.  It wasn't needed on my server, and it was "too much effort" to
deviate from the stock distro dhcp clients on other machines.

Then, around 2008, I changed ISPs again.  This time my new ISP used dhcp and
not PPPoE.  So, after a few months, I decided to dust off the old ndhc/ifchd
project and adapt it to my modern standards and machines.

ifchd was in good shape and required little work.  I ended up rewriting
ndhc.  The only parts that remained from the original were the parts that
I had already rewritten before, and some of those were rewritten, too.

The end result is a modern DHCP client is largely RFC-compliant, except where
the RFCs dictate behavior that would be problematic, overly complex, useless,
or exploitable.  DHCP is poorly specified, and real-world servers and clients
vary a lot from the RFCs, so these conditions are necessary for a useful
program.

Although ndhc's implementation and behavior are different, I have to credit
the idea of using netlink events to discover hardware link status transitions
to Stefan Rompf and his 'dhcpclient' program.  The Linux netlink events that
are used are otherwise rather obscure and poorly documented, and I wouldn't
have known about them otherwise.

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.