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DHCP is built directly on UDP and IP which are as inherently insecure. Furthermore, DHCP is generally intended to make maintenance of remote and/or diskless hosts easier. While perhaps not impossible, configuring such hosts with passwords or keys may be difficult and inconvenient. Therefore, DHCP in its current form is quite insecure.
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Unauthorized DHCP servers may be easily set up. Such servers can then send false and potentially disruptive information to clients such as incorrect or duplicate IP addresses, incorrect routing information (including spoof routers, etc.), incorrect domain nameserver addresses (such as spoof nameservers), and so on. Clearly, once this seed information is in place, an attacker can further compromise affected systems.
Malicious DHCP clients could masquerade as legitimate clients and retrieve information intended for those legitimate clients. Where dynamic allocation of resources is used, a malicious client could claim all resources for itself, thereby denying resources to legitimate clients.
The discussion below is based on an excellent paper Network Penetration.com/Flaws within the Dynamic Host Configuration Protocol by Ste Jones
DHCP is used to automatically configure machines with an IP address so that the hosts do not have to be statically assigned IP addresses. For a clients NIC to be assigned an IP address the DHCP packet exchange is as follows
Once this sequence of packets occurs a client has been assigned an IP address and probably a default gateway and DNS server. Numerous options can be set by the DHCP server, for a full list consult the RFC documentation.
If attackers are able to compromise a DHCP server on the network, they might disrupt network services, preventing DHCP clients from connecting to network resources. By gaining control of a DHCP server, attackers can configure DHCP clients with fraudulent TCP/IP configuration information, including an invalid default gateway or DNS server configuration. There are several avenues of attack of this protocol some trivial and some less so:
Some DHCP servers issue ARP requests or ICMP pings to detect for IP addresses that may be reclaimed by the server. This is done as operating systems / interfaces do not release there assigned IP address when shutdown. Basic testing of the denial of service code successfully defeat the ARP method of reclaiming IP addresses (ICMP method was not tested) as the number and speed of requests for IP addresses was significantly higher than the number of ARP requests issued by the DHCP server (when running multiple copies of the source code in a script). The source code could be extended to sniff for ARP requests / ICMP ping requests and reply accordingly thus defeating the servers method of reclaiming addresses.
A Windows 2000 machine running DHCP with active directory, sends
a packet at boot up to check that it is the only DHCP server on the
network, if it is the only DHCP server then it is authorized and allowed
to act as a DHCP server. Further investigation is required to see if
this can be reversed to deny a win2k DHCP server from starting.
The source code grabs an IP address from the DHCP server using the same method as the denial of service but instead of stealing all the IP addresses only one IP address is stolen. A rogue DHCP server is then started and listens for a client to send a discovery packet to the broadcast address. The rogue server and the valid server then both send a offer packet (the rogue server issues the IP address stolen at the start of the attack, this is to ensure that no IP address conflicts occur) and depending on which reaches the client first, determines which server the client uses. If the client uses the valid DHCP server then the man in the middle attack will fail.... If the client uses the rogue DHCP server the man in the middle attack will succeed.
Some problems with the man in the middle attack
Man in the middle attack a machine on a small network (noisy method)
1) Denial of service a machine on the network, try using jolt2.
2) Before the machine reboots, steal the IP address it was allocated
so that it has to request a new address
3) Start a rogue DHCP server, and hope that it wins the fight to assign
a clients IP address.
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Rogue Detect sends DHCPDISCOVER packets to the network and listens for DHCP servers to respond. It can run continuously as a daemon, or do a single probe and quit. It's written in Perl and can be easily extended to take any action you can think of in response to an unknown DHCP server (send pages, disable them, etc).
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DHCP Server Detection
The library and simple script included are made possible by Oregon State University Open Minds. Open Doors. Open Source. http://oregonstate.edu
Quick and Easy
Requirements:
- Carp
- Net::RawIP
- Data::Dumper
- FileHandle
- POSIX (to do fork() stuff)
- Net::SMTP (to send email)
- Sys::Syslog (optional)
I've only tested this with the one box I wrote it on at this point, its Debian Linux (woody-though with some sid/unstable packages installed)
I don't see any reason it wouldn't work elsewhere though.
INSTALL
- Unpack the DHCPDetect.pm and dhcpdetector.pl files somewhere
- Install required perl libraries (www.cpan.org) (in debian try 'perl -MCPAN -eshell')
- Edit dhcpdetector.pl – add your DHCP server's HWA, and the HWA of the iface used to scan and edit the defaults to your taste.
- set dhcpdetect.pl executable (chmod u+x dhcpdetect.pl)
- run dhcpdetect as root: ./dhcpdetect.pl
dhcpdetect has two modes available: continuous scan mode, and single scan mode (default).
- Single Scan mode (default) will send a single DHCPDISCOVER packet to the network and wait a configurable number of seconds for unauthorized servers.
- Continuous mode will continue to run, sending DHCPDISCOVER packets every
minutes, sending out alerts etc. Single Scan mode is good for running from the commandline periotically or from a cron script, while Continuous mode is good for letting the detector run indefinately.
Syntax:
dhcpdetect.pl --continuous [--freq=x] [--iface=interface] [--nodaemon] [--debug=x] OR dhcpdetect.pl [--waitfor=x] [--iface=interface] [--debug=x] --continuous Cause the program to run in a loop, sending DHCPDISCOVER occationally and listning for replies --nocontinuous (default) Send only one DHCPDISCOVER and wait only long enough for a response. Then quit. (for commandline or crontab use) --freq=x In continuous mode only: how long between DHCPDISCOVER packets --waitfor=x In normal (non-continuous) mode only: how long to wait after sending a DHCPDISCOVER for replies. --iface=interface Network interface to use (eg eth0). Unfortunately only one can be done at a time (sorry) run multiple copies for now. --daemon/--nodaemon in continuous mode only: wether or not the program should fork and go into the background. Default is yes. --debug=x Debug level. 1-5 supported. 3 is good for info. higher levels give more information.Bugs
- Continuous mode could have memory leaks. I havnt' checked (yet)
Notes
Sending a DHCPDISCOVER packet causes any DHCPSERVERS listning to allocate an IP address for a few seconds, while they wait for the detector to ACK their offer. Since we never do send an ACK, the IP is not allocated to us. Hence, it should be ok to run this on the network.. but do so at your OWN RISK!!
This package is nice in that you do not have to have a clear view of the network to run it (ie, it works behind a switch). You DO have to be within broadcast range, which usually means on the same subnet as the DHCP server. In some cases scaning port 68 (67?) on every machine may be the better answer to finding dhcp servers, but with this program, as apposed to a passive one like snort, you do not have to be able to see traffic not destined for you.
Our resnet has 19 subnets, so I use a linux box with 19 aliased interfaces in addition to its onboard network.In linux they are eth1, eth2.0132, eth3.0133, etc… so I must run 19 copies of dhcpdetect.pl, one for each interface.
If you have questions please contact us at https://roguedetect.bountysource.com Alex Schumann, Karl Vollmer
Dec 21, 2006
I have finally found a tool that works for RHEL and Debian (and probably all Linuxes). It's a Perl module and accompanying script which can optionally be run as a demon and be setup to email you each time it finds a rogue server. It's called RogueDetect and is GPL. I have tested version 0.4Alpha2. I've also used this code as a basis for a Nagios plugin the code for which I'll be releasing over the next few days on this blog.
Good post bart, nice to see that windows has some decent commands relating to standard internet protocols. In the longer term, you could have a permanent method of spotting rogue dhcp servers. I found a rule for snort:
#
# DHCP Servers
#
alert udp !$DHCP_SERVERS 67 -> 255.255.255.255 any (msg: "possible rogue DHCP Server"; sid:1000001;)I know you use nagios, I dunno if theres a plugin for that yet.
I am by no means shy about pointing out problems with Windows on this blog but I do try to be objective in my criticism. Today I have the rare pleasure of sharing a Windows command-line tool that I haven't been able to find an equal to on RedHat Enterprise Linux (RHEL) 4. A few weeks ago a rogue DHCP server caused us some serious problems at work so I was asked to find a simple tool for detecting rogue servers. In theory there are loads of tools for this on Linux but after having tried everything Google was able to find for me (an entire day's work) I'd still drawn a blank. Everything I tried insisted that our main DHCP server did not exist. What's moretcpdumpshowed responses from the DHCP server arriving at the machine but all the tools insisted they got no reply regardless. In my search I did come across dhcploc.exe, a small MicroSoft command-line tool that ships with most versions of Windows. Granted, it's not part of the default install but it is on the CD none the less and more importantly it works!Technorati Tags: Windows, RedHat, RHEL, DHCP, Rogue DHCP Servers
To install
dhcploc.exeyou have to install the Support Tools which you have to look for on your Windows CD. Once you have this installed you can set it looking for rogue servers with the following command:dhcploc
dhcploc.exewill then run continually and display all DHCP responses it sees. It will beep each time a rogue server is seen. You can also get it to only show you rogue traffic with the-pflag.It's rare that Windows has more powerful command-line tools than Linux but when it comes to detecting rogue DHCP servers Windows had it nailed with this simple tool that just works! I just hope this tool still exists in Vista!
Update 21 December 2006
I have finally found a tool that works for RHEL and Debian (and probably all Linuxes). It's a Perl module and accompanying script which can optionally be run as a demon and be setup to email you each time it finds a rogue server. It's called RogueDetect and is GPL. I have tested version 0.4Alpha2. I've also used this code as a basis for a Nagios plugin the code for which I'll be releasing over the next few days on this blog.
Finally, there are a few tools you can use to detect the presence of rogue DHCP servers on your network so you can get rid of them. These include both tools from Microsoft and from third-party sources, and we'll end by examining two of these tools.
Dhcploc.exe
Dhcploc.exe is a command-line tool that is part of the Windows Support Tools found in the \Support\Tools folder on your Windows XP product CD and it can be used to display all DHCP servers that are active on the local subnet. Dhcploc.exe has been around since Windows NT 4.0 and it works by sending out DHCPREQUEST messages and displaying the IP addresses of the DHCP servers that responded with DHCPACK. You can find the syntax of this tool in the Help file that is installed when you install the Support Tools on your machine.
dhcp_probe
The Network Systems Group at Princeton University's Office of Information Technology has developed a tool called dhcp_probe that tries to discover DHCP and BOOTP servers on a directly-attached Ethernet network. The existing build runs on Solaris 8 on SPARC with gcc and there is patch for it to work on Linux also, so if you have either of those platforms you can try using this tool to detect any rogue DHCP servers that might be running on your network. You can find the Network Systems Group's tools page here and can download dhcp_probe directly from here.
Roaming DHCP - for registered laptops on the SLAC network
Roaming DHCP allows the laptop user to connect his registered laptop to network taps on different subnets of the SLAC internal network. There is no DHCP service for desktops. There is no wireless networking on the SLAC internal network.The MAC address of the laptop must first be registered with the DHCP server. After the laptop is registered and connected to the SLAC network the DHCP server will dynamically assign it a SLAC IP address and other network configuration information.
As long as the laptop is connected to the network the lease will not expire and the laptop will keep the assigned IP address. If the laptop disconnects from the network the lease will expire after the lease time is up and the IP address will be freed up to be used by another DHCP client. The laptops lease will have to be renewed the next time the laptop is connected to the network. The lease time is 1 hour.
If a registered MAC address is not used for 9 months it will be removed from DHCP and will need to be registered again.
Policy and Security Guidelines to be followed
The group desktop administrator should determine that the laptop meets current security requirements set by SLAC Computing Security before registering the laptop for DHCP. In order to keep your DHCP registration, your machine must keep up-to-date with these security requirements, or else your DHCP registration will be revoked.For Windows:
Before requesting DHCP you are required to read and implement the Windows at SLAC guidelines, including the Guidelines for Connecting Computers to SLAC Internal Network . Check with your Desktop Administrator for help.For Linux:
Before requesting DHCP you are required to read and implement the Linux at SLAC guidelines. For help check with [email protected] .How to get your laptop registered for Roaming DHCP
Contact a DHCP Czar who works in your group or your area and provide him with the DHCP Registration Information listed below. He can register it for you.All other requests should be sent to [email protected] .
Copy and paste the DHCP Registration Information and your responses to your email:
DHCP Registration Information (all information is required):
* = Name must be in the SLAC Phone Book
- MAC Address e.g. 00:0d:9d:c9:1c:48 :
- PC Number e.g. PC12345 or "Not SLAC Property" :
- OS Version e.g. WXP SP2, Linux RH10 :
- User's Name * :
- User's Home Institution e.g. SLAC, INFN, TRIESTE :
- Contact / Supervisor * e.g. user's supervisor, manager, or sponsor :
- System Admin * the person who will administer this laptop :
- Security The laptop user and the System Admin have read,
understood, and implemented the Security Guidelines
and the System Admin has determined that the laptop
meets current security patch levels. reply yes or no :Visitor DHCP - for unregistered laptops on the visitor network (for SLAC visitors)
Visitor DHCP allows SLAC visitors a quick and easy connection to the Internet to read email, connect to hosts at home institutions, and browse the web. This connection is on the visitor network, located outside the SLAC internal network and firewall, and restricts access to many SLAC network services.No registration is needed. Users simply need to find a network tap configured for the visitor network and connect their laptop to it. Currently there are visitor taps located in the Auditorium, many conference rooms, and other locations at SLAC. Check with your Desktop Administrator for specific locations.
The visitor network also supports wireless networking. See the SLAC wireless web page for more information.
Temporary DHCP - for system administrators for new machine configuration
Temporary DHCP is used by authorized system administrators, who are also DHCP Czars, to allow them to configure new machines on the network. The lease period for temporary DHCP expires at midnight of the day it's acquired.
- I can well understand with large networks the ability to configure new PC's and it on the network and have the convenience of DHCP issue a valid IP and DNS server as a very very useful tool.
However, how many of use consider the security repercussions of handing out access to you total network just by the action of plugging a PC in.
The problem gets worse as the lager you network the more likely you will be dependant on DHCP.
From my point of view if you have a small to moderate Network you should NEVER use DHCP to hand out the keys to you network to anyone who plugs in a PC and the only thing that separates the newly plugged in PC is a password and user name.
It is very very hard to keep track of all the wired access points in your office and so the threat compounds itself.
Security threats come i all different types and we should never forget the internal threat any possibly has by just plugging a PC or laptop into ANY RJ socket.
If you network not a never ending story in its size managing it without DHCP should be a major consideration.
If you cannot live without automatically assigning IP's then DONT hand out DNS servers for free. This can be a compromise for large expansive networks.
The other issue to consider is the use of different routes available on you network. I can not image being asked as a security consultant NOT to plan different routes depending on the level of security you need.
NOT every PC needs to have access to each other. All that is needed is your severs ability to see everyone. Defining different routes by not using DHCP on every one or using DHCP and defining different routes using differing subnet masks is an option here.
For a real world issue having Management on the same route as the rest of the office is a definite no no.
Payroll and accounting can also have a different route.
Remember if you have not removed the default hidden share of every XP workstation's root drive of every physical drive in each PC should not be underestimated or forgotten.
Static assignment of Private IP's is also helpful for every internal security audit. Sure once DHCP has assigned an IP to a PC it i more than likely subsequent assignment will be the same, however when it comes to internal security audits it wont stand up to any audit investigation.
Using static private addresses may take a little more time during setup, however dont give out the key to you network to anyone who asks for it unless it is totally not manageable – due to size and personally I still have security issues – no matter how big the network.
If you want to comment on this discussion, don't use the excuse you are too busy. The assignment needs only to be made once and I am not advocating suddenly changing a 300+ workstation Network.
Non emotive discussion is helpful and passionate opinions are respected, however we work in the technology industry where non-fact is not worthy of the time it takes to write it.
Please tell me how you have inviolately secured your network without DHCP and static private network assignment.
- Posted: 10/30/2006 @ 20:24
By Peter Cohen
It's been recently noted that Mac OS X can be susceptible to a security problem involving malicious DHCP servers. The chances of the security issue affecting Mac users en masse is miniscule, but Apple recently posted details to its online Knowledgebase explaining how to circumvent potential problems.
According to a report from Carrel.org, a malicious response from a Dynamic Host Configuration Protocol (DHCP) server can grant root access on various versions of Mac OS X. DHCP is commonly used as an easy way to provide users on a network with TCP/IP addresses.
"In many cases, your Mac is protected from this kind of exploit because the malicious DHCP server has to be part of your local network, or 'subnet,'" said Apple in its recent tech note. "If your computers are the only ones on your local network and you have a broadband connection (DSL or cable service) with a Network Address Translation device -- such as an AirPort Base Station -- this exploit is not possible."
Carrel.org disputes that claim. "If you have not secured your network (especially a wireless network) against malicious devices connecting to it, you can be exploited even if you are using NAT since the attack happens behind the NAT on your local subnet," said William Carrel.
That chain of events would require someone to set up or hack a DHCP server on an exposed wireless network -- one with no password, for example, or one whose password had been compromised.
Regardless, Apple has offered steps that users concerned with this issue can take to make sure their Macs can't be exploited in this way. Steps include making sure that LDAP-based directory services are turned off, or making sure that search parameters for directory services are changed.
Google matched content |
Network Penetration.com/Flaws within the Dynamic Host Configuration Protocol by Ste Jones
Microsoft/Security information for DHCP
WindowsDevCenter.com -- DHCP and DNS Securityby Mike Danseglio
[PDF] DHCP and the changing art of Network Security Why DHCP deserves ...Lots of elements are involved in a healthy network. Many are provided by hardware, such as the routing and switching of data. Others are provided by software and are frequently based on the network protocols in use. These services are often overlooked by security administrators and implementers. But attackers can use these services to launch, support, or continue an attack. In fact, denial-of-service attacks can be very effectively carried out by just attacking these services alone. This chapter explores two important services that exist on most networks but get very little security attention.
Microsoft has recognized the lack of security in DHCP and DNS. As a result, Windows Server 2003 has several security technologies that are not necessarily standards-based or fully compatible with other operating systems; however, depending on your computing environment and need for security, these technologies can prove beneficial.
