ndhc, Copyright (C) 2004-2017 Nicholas J. Kain.
See LICENSE for licensing information. In short: Two-clause / New BSD.
Requirements:
Linux kernel
GNU Make or CMake
Ragel
ncmlib
INTRODUCTION
------------
ndhc is a multi-process, privilege-separated DHCP client. Each subprocess runs
with the minimal necessary privileges in order to perform its task. Currently,
ndhc consists of three subprocesses: the ndhc-master, ndhc-ifch, and
ndhc-sockd.
ndhc-master communicates with DHCP servers and handles the vagaries of the DHCP
client protocol. It runs as a non-root user inside a chroot. ndhc runs as a
normal user with no special privileges and is restricted to a chroot that
contains nothing more than a domain socket filesystem object (if using syslog),
a urandom device node, and a null device node.
ndhc-ifch handles interface change requests. It listens on a unix socket for
such requests. ndhc-ifch runs as a non-root user inside a chroot, and retains
only the power to configure network interfaces. ndhc-ifch automatically forks
from ndhc-master to perform its job.
ndhc-sockd plays a similar role to ndhc-ifch, but it instead has the ability to
bind to a low port, the ability to open a raw socket, and the ability to
communicate on broadcast channels. ndhc communicates with ndhc-sockd
over a unix socket, and the file descriptors that ndhc-sockd creates are
passed back to ndhc over the unix socket.
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. ndhc does not run as root after initial startup, and
capabilities are divided between the subprocesses. All processes run in a
chroot.
Robust. ndhc performs no runtime heap allocations -- malloc() (more
specifically, brk(), mmap(), etc) is never called after initialization (libc
behavior during initialization time will vary), and ndhc never performs
recursive calls and only stack-allocates fixed-length types, so stack depth is
bounded, too.
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 avoids unnecessary outside dependencies and is written in
plain C.
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.
Self-contained. ndhc does not exec other processes, or rely on the shell.
Further, ndhc relies on no external libraries aside from the system libc.
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
-----
0) Make sure that ncmlib is present in the ndhc source directory:
$ ls
CMakeLists.txt LICENSE Makefile ncmlib README src
1) Compile and install ndhc.
a) make
b) Install the build/ndhc executable in a normal place. I would suggest
/usr/sbin or /usr/local/sbin.
1alt) Compile and install ndhc.
a) Create a build directory:
mkdir build && cd build
b) Create the makefiles:
cmake ..
c) Build ndhc:
make
d) Install the ndhc/ndhc executable in a normal place. I would suggest
/usr/sbin or /usr/local/sbin.
2) Time to create the jail in which ndhc will run.
a) Become root and create new group "ndhc".
$ su -
# umask 077
# groupadd ndhc
b) Create new users "dhcpifch" and "dhcp". The primary group of these
users should be "ndhc".
# useradd -d /var/lib/ndhc -s /sbin/nologin -g ndhc dhcpsockd
# useradd -d /var/lib/ndhc -s /sbin/nologin -g ndhc dhcpifch
# useradd -d /var/lib/ndhc -s /sbin/nologin -g ndhc dhcp
c) Create the state directory where DUIDs and IAIDs will be stored.
# mkdir /etc/ndhc
# chown root.root /etc/ndhc
# chmod 0755 /etc/ndhc
d) 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 dhcp.ndhc var
# chmod -R a+rx var
# chmod g+w var/run
e) 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
f) (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; ndhc is ready to be used. An example
of invoking ndhc:
# ndhc -i wan0 -u dhcp -U dhcpifch -D dhcpsockd -C /var/lib/ndhc
4o) If you encounter problems, I suggest running ndhc in the foreground and
examining the printed output.
5) I suggest running ndhc under some sort of process supervision. This will
allow for reliable functioning in the case of unforseen or
unrecoverable errors.
BEHAVIOR NOTES
--------------
ndhc 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
(-H) flags. See ndhc --help.
STATE STORAGE NOTES
-------------------
ndhc requires a read/writable directory to store the DUID/IAID states. By
default this directory is /etc/ndhc. It exists outside the chroot. The DUID
will be stored in a single file, DUID. The IAIDs exist per-interface and are
stored in files with names similar to IAID-xx:xx:xx:xx:xx:xx, where the xx
values are replaced by the Ethernet hardware address of the interface.
If it is impossible to read or store the DUIDs or IAIDs, ndhc will
fail at start time before it performs any network activity or forks any
subprocesses.
If the host system lacks volatile storage, then a clientid should manually
be specified using the -I or --clientid command arguments.
GRSECURITY NOTES
----------------
Make sure that CONFIG_GRKERNSEC_CHROOT_CAPS is disabled. Otherwise, ndhc will
lose its capabilities (in particular, the ability to reconfigure interfaces)
when it chroots.
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
uses many Linux-specific features. The following list is not intended to
be exhaustive:
1) ndhc 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.
2) ndhc configures network interfaces and routes. Interface and route
configuration is entirely non-portable.
3) ndhc uses netlink sockets for fetching data, setting data, and hardware link
state change notification events.
4) 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.
5) ndhc uses epoll() and signalfd(). These are Linux-specific.
6) Numerous socket options are used, and the AF_PACKET socket family is used
for raw sockets and ARP. These are largely Linux-specific, too.
7) ndhc can optionally use seccomp-filter to allow only a set of whitelisted
syscalls. This functionality is Linux-specific.
