Provided by: dhcp-probe_1.3.1-1_amd64 
NAME
dhcp_probe - locate DCHP and BootP servers on a directly-attached network
SYNOPSIS
dhcp_probe [ -c config_file ] [ -d debuglevel ] [ -f ] [ -h ] [ -l log_file ] [ -o capture_file ] [ -p
pid_file ] [ -Q vlan_id ] [ -s capture_bufsize ] [ -T ] [ -v ] [ -w cwd ] interface_name
DESCRIPTION
dhcp_probe attempts to discover DHCP and BootP servers on a directly-attached Ethernet network. A
network administrator can use this tool to locate unauthorized DHCP and BootP servers.
The program must be run with root privilege.
The program periodically broadcasts a number of DHCP and BootP request packets out a single physical
interface. Several different kinds of request packets are sent, as a DHCP or BootP server may only
respond to certain requests, depending on the server's configuration. Essentially, dhcp_probe mimics a
BootP or DHCP client in a variety of possible states, attempting to provoke responses from servers.
After sending each request packet, dhcp_probe listens for responses. After filtering out responses that
do not appear to be in response to the probe, and responses from known DHCP and BootP servers (identified
by their IP source addresses and optionally by their Ethernet source addresses), it logs any responses
from unknown servers.
Optionally, responses from unknown servers may also be written to a packet capture file.
Optionally, an external program may be called each time a response from an unknown server is received.
dhcp_probe may not be able to locate all DHCP and BootP servers; see LIMITATIONS below.
As DHCP broadcasts do not ordinarily cross IP routers, dhcp_probe will locate only servers that are
attached to the same physical network as the interface specified on the command line. Although BootP
Relay Agents running on this network may help the broadcasts cross IP routers, these agents typically are
configured to convert the broadcasts to unicasts directed to only the well-known DHCP or BootP servers
located on other physical networks. As a result, BootP Relay Agents will allow your the servers to
receive the requests issued by dhcp_probe, but will not cause remote unknown servers to hear these
requests. Therefore, if you have multiple physical networks, you may wish to run dhcp_probe on each of
these networks to discover unknown DHCP and BootP servers on each of them.
dhcp_probe functions on a single Ethernet interface specified on the command line; it does not listen on
multiple interfaces. However, if the host has multiple physical or logical interfaces, you may run an
instance of dhcp_probe on each physical or logical interface. If your physical interface supports
802.1Q, you can use that to create a logical (untagged) interface on each VLAN, then run an instance of
dhcp_probe on each logical (untagged) interface.
dhcp_probe is intended for use by a network administrator. Before running dhcp_probe on any network
other than one for which you are responsible, contact that network's administrator to learn if it is
acceptable for you to run this software on that network. Running this software on a network where you
don't have permission to do so may violate that network's acceptable use policy.
AVAILABILITY
dhcp_probe is a product of the Network Systems Group at Princeton University's Office of Information
Technology, and is available from https://www.net.princeton.edu/software/dhcp_probe/
Presently the product builds and runs on Solaris 10 on SPARC with gcc. The program relies on the pcap(3)
and libnet(3) libraries.
OPTIONS
-c config_file
Specifies the configuration file. If not specified, this defaults to /etc/dhcp_probe.cf. The
configuration file is read at startup, and is re-read whenever a SIGHUP is received. See
dhcp_probe.cf(5).
-d debuglevel
Sets the debuglevel variable that controls the amount of debugging messages generated. If not
specified, this defaults to 0 (no debugging). For a summary of the types of messages produced at
each debug level, see DEBUG LEVELS below.
-f Specifies that the program should not fork, but instead remain in the foreground. Only use this
when you are starting the program manually for testing purposes. When in the foreground, any
messages produced by the program are written to stderr instead of to syslog(3) or any log_file you
might specify with the -l option.
-h Display a brief usage summary, then exit.
-l log_file
Log messages to the specified file instead of to syslog(3). (This option is ignored if you also
specify the -f option, as that directs messages to stderr.) The log file is opened shortly after
the program starts. It is closed and re-opened when the program receives a SIGUSR1 signal.
-o capture_file
When a response packet is received from an unexpected server, write the packet to the specified
file. The file is opened and truncated shortly after the program starts. It is closed and re-
opened (and truncated) when the program receives a SIGUSR2 signal. The file is a pcap(3)
savefile, and may be read with any program that understands the pcap savefile format; e.g.
tcpdump(1).
-p pid_file
Specifies the file that will contain the program's processid. If not specified, this defaults to
/var/run/dhcp_probe.pid. The pid_file is written shortly after the program starts, and is removed
when the program exits under its own control.
-Q vlan_id
Specifies that when the program constructs a packet to send, it should add an 802.1Q tag with the
specified vlan_id. Valid values range from 0 to 4095. If not specified, the program does not add
an 802.1Q tag to the packets it constructs. (The operating system or Ethernet driver may still do
so.)
-s capture_bufsize
Specifies the size of the buffer that will be used to capture all the responses (Ethernet frames)
to a single request packet; responses which do not fit are silently dropped. The value is
specified in bytes, and must fit into your host's range for an int; values outside that range may
result in unpredictable results. If not specified, this defaults to 30280, which is enough for
twenty maximum-size Ethernet frames (1514*20). Typical responses are Ethernet frames ranging from
342-590 bytes, so the default capture buffer size should hold over 50 of them.
-T Enables the 'socket receive timeout' feature. On some platforms, dhcp_probe may ignore the
response_wait_time (described in dhcp_probe.cf(5)), instead waiting forever for a response after
it sends a probe packet. As per pcap(3), this is because the read timeout we pass to
pcap_open_live() is not supported on all platforms. If you encounter this issue, try enabling our
'socket receive timeout' feature; it might help. Enabling this feature causes the program to also
set a socket receive timeout on the socket underlying the pcap capture; we set this timeout to the
response_wait_time. On some platforms, the program's socket receive timeout feature does not
work; instead the program will report that it cannot set the receive timeout, and will exit.
-v Display the program's version number, then exit.
-w cwd Specifies the working directory; shortly after starting the program changes its current working
directory to this. If not specified, this defaults to /.
interface_name
Specifies the name of the interface the program should use; this argument is required. This must
be an Ethernet interface which is up. Typically this interface must also have an IP address
assigned; specifying do_not_lookup_enet_and_ip_addresses in dhcp_probe.cf(5) may allow the program
to operate on an interface which lacks an IP address.
OPERATION
After initialization, the program enters its main event loop, in which it remains until you signal the
program to exit with a SIGINT, SIGTERM, or SIGQUIT.
The main event loop (a.k.a. the "probe cycle") consists of the following actions, repeated until the
program receives a request to quit:
1. Handle any signals that have been received.
2. Install a pcap(3) filter to listen for UDP packets destined to the BootP client port (UDP
port 68).
3. Broadcast a DHCP or BootP request packet out the specified interface.
4. Listen for response_wait_time milliseconds for any responses received by the pcap(3) filter.
(The response_wait_time defaults to 5000 milliseconds (5 seconds), and may be changed in the
dhcp_probe.cf(5) file.)
Any responses that contains a bootp_chaddr field not equal to the chaddr used in the probe is
ignored, as are any that have incorrect bootp_htype or bootp_hlen fields. These are not
responses to our probe.
Any responses from known DHCP and BootP servers are ignored. The IP source address for
responses from each known server is declared using a legal_server statement in the
dhcp_probe.cf(5) file. Any response with an IP source address that does not appear in a
legal_server statement is treated as an unknown server.
The Ethernet source address for responses from each known server is also optionaly declared
using a legal_server_ethersrc statement in the dhcp_probe.cf(5) file. If at least one
legal_server_ethersrc is specified, then any response with an Ethernet source address that
does not appear in a legal_server_ethersrc statement is treated as an unknown server. If no
legal_server_ethersrc statements appear, then the response's Ethernet source address is not
checked. (The legal_server_ethersrc statement is considered experimental in versions 1.3.0 -
1.3.1, as it has received only limited testing.)
For each response from an unknown server:
a) If the reponse packet contains a non-zero yiaddr field, and one or more
lease_network_of_concern statements were specified, determine if the yiaddr value falls
within any of the "Lease Networks of Concern".
a) Log a message showing the response packet's source IP and Ethernet addresses. If the
response packet's yiaddr is non-zero and falls within a "Lease Networks of Concern", the
log message also reports that.
b) If the -o option was specified, the packet is also written to a packet capture file.
c) If an alert_program_name was specified in the dhcp_probe.cf(5) file, that program is
executed, with the following arguments in order: the name of the calling program (e.g.
dhcp_probe), the name of the interface on which the unexpected response packet was
received, the IP source address of the packet, and the Ethernet source address of the
packet. (We do not wait for the alert_program_name to complete; it runs in a child
process.)
d) If an alert_program_name2 was specified in the dhcp_probe.cf(5) file, that program is
executed, with the following required options:
-p the name of the calling program (e.g. dhcp_probe)
-I the name of the interface on which the unexpected response packet was received
-i the IP source address of the packet
-m and the Ethernet source address of the packet
If the response packet's yiaddr is non-zero and falls within a "Lease Networks of
Concern", the following optional options are also passed:
-y the non-zero yiaddr value
(We do not wait for the alert_program_name2 to complete; it runs in a child process.)
5. Remove the pcap(3) filter installed earlier.
6. If any signals have arrived requesting that we quit, exit gracefully.
7. Repeat steps 2-6 for each flavor of DHCP and BootP request packet the program supports
(see PACKET FLAVORS below).
8. Handle any signals that have been received.
9. Sleep for cycle_time seconds. (The cycle_time defaults to 300 seconds, and and may be
changed in the dhcp_probe.cf(5) file.)
The pcap(3) filter the program installs normally does not specify that the interface should be placed
into promiscuous mode (although it is possible the interface is already in promiscuous mode for some
other reason). However, if in the dhcp_probe.cf(5) file you specify a chaddr or ether_src value other
than the interface's actual hardware address, then the pcap filter will specify that the interface should
be placed into promiscuous mode.
Although the filter used with pcap(3) specifies only UDP packets destined to port bootpc should be
collected, on systems where bpf isn't part of the kernel, pcap(3) must implement bpf as part of the
application. This can increase the number of packets that must be passed from the kernel to user space
to be filtered. The program attempts to minimize the side-effects of this by removing the pcap(3) filter
when it isn't actually listening for responses. In particular, the filter is not installed during the
time the program sleeps between each probe cycle (the cycle_time).
If you do specify an alert_program_name, take care that the program you specify is safe for a privileged
user to run; it is executed with the same (i.e. root) privileges as the calling program.
PACKET FLAVORS
No single request packet is likely to provoke a response from every possible BootP and DHCP server. Some
servers may only response to either BootP, or DHCP, but not both. Some servers may be configured to only
respond to a small set of known clients. Some DHCP servers will only provide leases to a small set of
known clients, but may be willing to respond (negatively) to unknown clients that request a lease renewal
on an inappropriate IP address. Therefore, dhcp_probe actually sends not one, but five different flavor
request packets, in the hopes of provoking responses from a wider variety of unknown servers.
The packet flavors are:
BOOTPREQUEST
This packet is typical of a BootP client requesting an IP address.
It will typically provoke a BOOTPREPLY from a BootP server willing to respond to any BootP client.
(BootP servers configured to only respond to a set of known clients may not respond.)
DHCPDISOVER (INIT)
This packet is typical of a DHCP client in the INIT state.
The options field contains a DHCP Message Type specifying DHCPDISCOVER.
The options field contains a DHCP Client Identifier, which is computed by prepending 0x'01' to the
value of chaddr. (The value chaddr is specified in the dhcp_probe.cf(5) file, otherwise it
defaults to the interface's Ethernet address.)
This packet will typically provoke a DHCPOFFER from a DHCP server willing to respond to any DHCP
client. (DHCP servers configured to only offer leases to a set of known clients may not respond.)
DHCPREQUEST (SELECTING):
This packet is typical of a DHCP client in the SELECTING state; i.e. a client which has previously
issued a DHCPDISCOVER, then received a DHCPOFFER from some DHCP server.
The options field contains a DHCP Message Type specifying DHCPREQUEST.
The options field contains a DHCP Client Identifier, which is computed by prepending 0x'01' to the
value of chaddr. (The value chaddr is specified in the dhcp_probe.cf(5) file, otherwise it
defaults to the interface's Ethernet address.)
The options field contains a DHCP Server Identifier specifying server_id, which should be an IP
address that does not correspond to any valid DHCP Server Identifier on your network. (The value
server_id is specified in the dhcp_probe.cf(5) file, otherwise it defaults to 10.254.254.254.)
The options field contains a DHCP Requested IP Address specifying client_ip_address, which should
be an IP address that does not correspond to any valid IP address on your network. (The value
client_ip_address is specified in the dhcp_probe.cf(5) file, otherwise it defaults to
172.31.254.254.)
This packet occassionally provokes a response from a broken DHCP server that fails to respect the
DHCP Server Identifier option.
DHCPREQUEST (INIT-REBOOT):
This packet is typical of a DHCP client in the INIT-REBOOT state; i.e. a client which has obtained
a DHCP lease in the past, is bringing up its IP stack, and hopes to obtain (or extend) a DHCP
lease on the same IP address as in the past.
The options field contains a DHCP Message Type specifying DHCPREQUEST.
The options field contains a DHCP Client Identifier, which is computed by prepending 0x'01' to the
value of chaddr. (The value chaddr is specified in the dhcp_probe.cf(5) file, otherwise it
defaults to the interface's Ethernet address.)
The options field contains a DHCP Requested IP Address specifying client_ip_address, which should
be an IP address that does not correspond to any valid IP address on your network; ideally it
should be one that is topologically inappropriate for your network. (The value client_ip_address
is specified in the dhcp_probe.cf(5) file, otherwise it defaults to 172.31.254.254.)
If the Requested IP Address option is topologically inappropriate for your network, this packet
may provoke a DHCPNAK from any DHCP server that believes it is authoritative for the network's IP
topology.
DHCPREQUEST (REBINDING)
This packet is typical of a DHCP client in the REBINDING state; i.e. a client which has obtained a
DHCP lease which is between its DHCP T2 and expiration time.
The options field contains a DHCP Message Type specifying DHCPREQUEST.
The options field contains a DHCP Client Identifier, which is computed by prepending 0x'01' to the
value of chaddr. (The value chaddr is specified in the dhcp_probe.cf(5) file, otherwise it
defaults to the interface's Ethernet address.)
The ciaddr field contains client_ip_address, which should be an IP address that does not
correspond to any valid IP address on your network; ideally it should be one that is topologically
inappropriate for your network. (The value client_ip_address is specified in the dhcp_probe.cf(5)
file, otherwise it defaults to 172.31.254.254.)
If the value of ciaddr is topologically inappropriate for your network, this packet will provoke a
DHCPNAK from any DHCP server that believes it is authoritative for the network's IP topology.
All the request packets sent by the program share the following common characteristics:
Ethernet Header
destination: ff:ff:ff:ff:ff:ff
source: ether_src from dhcp_probe.cf(5), else interface hardware address
type: ETHERTYPE_IP (0x0800)
IP Header
version: 4
header length: 5
tos: 0
total length: 328 (20-byte IP header + 8-byte UDP header + 300-byte BootP/DHCP payload)
identifier: 1
flags: 0
fragment offset: 0
ttl: 60
protocol: IPPROTO_UDP (17)
header checksum: (computed)
source address: 0.0.0.0
destination address: 255.255.255.255
options: (none)
UDP Header
source port: PORT_BOOTPC (68)
dest port: PORT_BOOTPS (67)
checksum: (computed)
BootP/DHCP Payload
op: BOOTREQUEST (1)
htype: HTYPE_ETHER (1)
hlen: HLEN_ETHER (6)
hops: 0
xid: 1
secs: 0
flags: 0
ciaddr: 0.0.0.0 (except for DHCPREQUEST (REBINDING) packets it is client_ip_address from
dhcp_probe.cf(5), else 172.31.254.254)
siaddr: 0.0.0.0
giaddr: 0.0.0.0
chaddr: chaddr from dhcp_probe.cf(5), else interface hardware address
sname: (all 0's)
file: (all 0's)
options: RFC1048 cookie (0x63825363), possibly followed by DHCP options, followed by END option
(0xFF), followed by PAD options (0x00) to bring the field to 64 bytes
MULTIPLE INTERFACES
Although dhcp_probe only supports monitoring a single physical or logical interface, you may run an
instance of the program on each physical or logical interface; each monitors a different network.
When running multiple copies of dhcp_probe, be sure to specify a different pid_file for each instance.
If you specify a log_file and/or a capture_file, be sure to specify a different one for each instance.
You may specify a different config_file for each instance. If you don't need to customize the settings
in that file for each instance, you may use the same configuration file for all instances.
If you have multiple logical interfaces on the same VLAN on the same physical interface, or multiple
logical IP networks on the same VLAN running on a single physical network, there is no need to run
multiple instances of dhcp_probe to monitor each logical interface or logical network. A single instance
of the program running on a physical interface or logical interface is sufficient to provoke any servers
on that one VLAN on that physical (or logical) network that might be willing to respond.
If your physical interface supports 802.1Q, you can use a single physical interface to monitor multiple
VLANs. Use your operating system to create a logical interface (on the same physical interface) for each
VLAN. When frames arrive on the physical interface containing the 802.1Q tag you specified for the
logical interface, your operating system is responsible for delivering those frames to the logical
interface with the 802.1Q tag removed. (Some Ethernet drivers will retain the 802.1Q tag but change the
VLAN ID to 0; we understand that format.) Run an instance of the program on each logical interface. If
your operating system and network driver does not automatically add an 802.1Q header to the packets we
transmit, you will also need to specify the -Q option to instruct the program to add an 802.1Q header to
the packets it constructs. (If the operating system or network driver automatically adds the 802.1Q
header to these packets, then specifying -Q will likely not do what you want. Selecting the wrong choice
often means that the packets we construct will be transmitted incorrectly, or may be silently discarded
by the operating system or the Ethernet switch.)
SIGNALS
The program will respond to a number of signals:
SIGUSR1
If logging to a file, close and re-open it. If the program is in the middle of a probe cycle,
handling the signal is deferred until the end of the cycle. (Has no effect if logging to
syslog(3) or if the -f option was specified.)
SIGUSR2
If capturing to a file, close and re-open it. If the program is in the middle of a probe cycle,
handling the signal is deferred until the end of the cycle. (Has no effect if the -o option was
not specified.)
Because re-opening the capture file causes the file to be truncated and a new pcap(3) header to be
written to it, if you want to save the prior contents of the capture file, move the existing
capture file aside before sending the signal.
SIGHUP Reread the configuration file. If the program is in the middle of a probe cycle, handling the
signal is deferred until the end of the cycle.
SIGTERM, SIGINT, SIGQUIT
Exit gracefully. If the program is in the middle of a probe cycle, handling the signal is
deferred until the program finishes sending and receiving responses for the current flavor request
packet.
LEASE NETWORKS OF CONCERN
Most rogue BootP/DHCP servers distribute private IP addresses to clients, or send DHCPNAK messages to
legitimate clients. Some even more disruptive rogue BootP/DHCP servers may distribute IP addresses that
fall within your own networks' IP ranges. The "Lease Networks of Concern" feature is intended to help
you identify these particularly disruptive servers.
You may activate the feature by specifying the lease_network_of_concern statement in your configuration
file. Use the statement multiple times to specify all your legitimate network ranges.
When a rogue BootP/DHCP server is detected, if the rogue's response packet contains a non-zero yiaddr
value, the value is compared to the "Lease Networks of Concern" you specified. If the value falls within
any of those network ranges, the message logged by dhcp_probe is extended to make note of this, and to
report the yiaddr value. Furthermore, if you are using the alert_program_name2 feature, the alert
program is called with an extra -y yiaddr option so that alert program can take any additional action
desired.
DEBUG LEVELS
The program produces increasingly detailed output as the debuglevel increases. Under normal
circumstances, you can run at debuglevel 0. Here's roughly what messages are added at each debuglevel.
0 Display the IP source (and Ethernet source) of each unexpected DHCP or BootP response packet.
Startup and shutdown notice.
Non-fatal errors in the configuration file.
Fatal errors.
1 At startup, show some information about the program's configuration.
2 Show each time we start and finish (re-)reading the configuration file.
Show each time we close and re-open the logfile or capture file.
Report on response packets that could not be parsed (e.g. truncated).
3 Each time we (re-)read the configuration file, echo the information we obtain from it.
7 For each parsable response packet, show the Ethernet source and destination, the IP source and
destination, and indicate when the IP source is a legal (known) server.
11 For each probe cycle, show when the cycle begins and ends, when we write a packet, and when we
begin and end listening for response packets.
AUTHOR
The program was written by Irwin Tillman of Princeton University's OIT Network Systems Group. It was
written to run on Solaris, relying on the generally-available pcap(3) and libnet(3) libraries.
FILES
/etc/dhcp_probe.cf
Configuration file read by the program. See dhcp_probe.cf(5). The name of this file can be
overridden by a command-line option.
/etc/dhcp_probe.pid
Contains the program's processid. The name of this file can be overridden by a command-line
option.
LIMITATIONS
dhcp_probe is not guaranteed to locate all unknown DHCP and BootP servers attached to a network. If a
BootP server is configured so it only responds to certain clients (e.g. those with certain hardware
addresses), it will not respond to the BOOTPREQUEST packet we sent. If a DHCP server is configured so it
only responds to certain clients (e.g. those with certain hardware addresses or DHCP Client Identifiers),
it will not respond to the packets we send that mimic DHCP clients in the INIT state. If a DHCP server
is configured so it does not send DHCPNAK packets to clients requesting topologically-inappropriate IP
addresses, it will not respond the packets we send that mimic DHCP clients in the INIT-REBOOT and
REBINDING states.
The upshot is that it is possible that dhcp_probe will be unable to provoke some BootP and DHCP servers
into responding at all.
Flushing out such servers can be extremely difficult. One approach is to capture all UDP/IP packet
destined to the BootP client port which cross your network; since most of these packets are unicast at
Layer 2, capturing is only effective if all such packets must pass by your capture device's Ethernet
interface (e.g. the capture device is located at a network choke point, or the network does not involve
any Layer 2 switching). Another approach is to do UDP port scanning for all devices listening on the
BootP server port, and assume that those which are listening on that port are running a BootP or DHCP
server.
Malicious BootP or DHCP servers that forge the IP source address (and possibly the Ethernet source
address) of their responses to match the values specified by legal_server and legal_server_ethersrc
statements will not be detected.
If the network contains any Ethernet switch which selectively filters (rather than floods) layer 2
broadcasts sent by DHCP/BootP clients, dhcp_probe will be unable to locate DHCP and BootP servers on the
far side of that Ethernet switch. Such switches prevent the probe packets from reaching rogue DHCP/BootP
servers. For example, some Cisco Nexus(R) switch/routers models configured to act as a network's IPv4
router and BootP Relay Agent may not flood such frames, instead delivering them only to the router's
embedded BootP Relay Agent.
BUGS
The packet capture buffer size is limited; if a single request packet provokes more responses than will
fit into the buffer, those that do not fit are silently dropped, without any diagnostic indicating that
the buffer was too small. You can adjust the size of the packet capture buffer size using the
-s capture_bufsize option.
We do not support non-Ethernet interfaces.
Because (re-)opening a packet capture file causes the file to be opened for writing (not appending), the
contents of any existing packet capture file of the same name is lost when the program starts or receives
a SIGUSR2 signal. If the file's previous contents should be preserved, move the old file aside before
starting the program or sending it a SIGUSR2 signal. (This "feature" exists because opening a pcap(3)
savefile always involves writing a pcap header record to the start of the file, so pcap always opens the
file using mode "w".)
Because pcap(3) opens the packet capture file with a simple fopen(3) without checking to see if the file
already exists, dhcp_probe may be tricked into overwriting or corrupting an existing file. As dhcp_probe
is run with root privileges, this is a serious concern. To avoid this problem, if you use the -o option,
ensure that the directory that will contain the capture file is writable only by root.
The packet capture file that is written is unparseable after the first packet. E.g. if read with
tcpdump(8), it reports: tcpdump: pcap_loop: truncated dump file.
On platforms where pcap(3) is unable to support the timeout argument to pcap_open_live, the program may
not reliably detect responses from DHCP and BootP servers, or may not function at all.
SEE ALSO
dhcp_probe.cf(5)
pcap(3) (a.k.a. libpcap, a packet capture library), available from http://www.tcpdump.org. (An older
version is available from ftp://ftp.ee.lbl.gov/libpcap.tar.Z.)
libnet(3) (a.k.a libwrite, a packet writing library), available from http://www.packetfactory.net/libnet
TRADEMARKS
Cisco Nexus(R) is a trademark of Cisco Systems.
Princeton Univ. Jan 18 2021 DHCP_PROBE(8)