ndhc/README

ndhc + ifchd, Copyright (C) 2004-2014 Nicholas J. Kain.
See LICENSE for licensing information.  In short: Two-clause / New BSD.

Requirements:

Linux kernel
GNU Make (tested: 3.82) or CMake (tested: 2.8)
libcap (available via ftp.kernel.org)

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

ndhc consists of a set of daemons that cooperate in order to provide
privilege-separated 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.

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.

RFC3927's IPv4 Link Local Addressing is not supported.  I have found v4 LLAs
to be more of an annoyance than a help.  v6 LLAs work much better in practice.

FEATURES
--------

Privilege-separated.  Neither ifchd or ndhc runs as full root, and capabilities
are divided between the programs.  Both programs run in a chroot.

Robust.  ndhc performs no runtime heap allocations -- malloc() is never called
(and neither is brk(), mmap(), etc), and ndhc never performs recursive calls
and only stack-allocates fixed-length types, so stack depth is bounded, too.
ifchd lightly uses malloc(), but no heap allocations have long lifetimes, and
are bounded from being large.

Active defense of IP address and IP collision avoidance.  ndhc fully implements
RFC5227.  It is capable of both a normal level of tenacity in defense, where
it will eventually back off and request a new lease if a peer won't relent
in the case of a conflict, and of relentlessly defending a lease forever.  In
either mode, it rate-limits defense messages, so it can't be tricked into
flooding by a hostile peer or DHCP server, either.

Small.  Both ndhc and ifchd avoid unnecessary outside dependencies and are
written in plain C.  The only library used is libcap, as the raw raw kernel API
for capabilities is not guaranteed to stay stable.

Fast.  ndhc filters input using the BPF/LPF mechanism so that uninteresting
packets are dropped by the operating system before ndhc even sees the data.
ndhc also only listens to DHCP traffic when it's necessary.

Flexible.  ndhc can request particular IPs, send user-specified client IDs,
write a file that contains the current lease IP, write PID files, etc.  One
ifchd session can service multiple ndhc sessions.

Aware of the hardware link status.  If you disconnect an interface on which
ndhc is providing dhcp service, it will be aware.  When the link status
returns, ndhc will fingerprint the reconnected network and make sure that it
corresponds to the one on which it has a lease.  If the new network is
different, it will forget about the old lease and request a new one.

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
-------------

DHCP clients aren't naturally very portable.  It's necessary to perform a lot
of tasks that are platform-specific.  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.

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 did not really fit my requirements well, 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.

DHCP PROTOCOL QUIRKS
--------------------

Send a packet that has an options field set to:
'DHCP-OPTION-OVERLOAD:3'
Then in the file and sname fields:
'DHCP-OPTION-OVERLOAD:3'
I suspect some bad dhcp programs will hang given this input.

DHCP explicitly specifies that there is no minimum lease time and also
specifies that the minimum default rebinding time is leasetime*0.875 and
the minimum default renewing time is leasetime*0.500.  All times are relative
to the instant when the lease is bound and are specified in seconds.  Taken
together, this means that a client strictly implementing the RFC should
accept a lease that either is perpetually rebinding (lease == 1s) or instantly
expires (lease == 0s).  ndhc ignores the RFC and specifies a minimum lease
time of one minute.

Renew and rebind times are optionally specified and may take on any value.
This means that a malicious server could demand a rebind time before a renew
time, or make these times ridiculously short, or specify both times past
that of the lease duration.  ndhc avoids all of this nonsense by simply
ignoring these options and using the default values specified by the RFC.

There are other quirks, but these are just several interesting ones that
immediately occur to me while I'm writing this document.