Softpanorama (slightly skeptical) Open Source Software Educational Society

May the source be with you, but remember the KISS principle ;-)

DHCP Security

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.

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

1. The client sends a DHCP discover packet, indicating that a clients interface requires a IP address from a DHCP server. The clients interface may ask for its previous IP address from the server, this may cause problems with the man in the middle attack (explained later).
2. The server sends a DHCP offer packet, informing the client of what IP address is on offer. The IP address being offered may or may not be the one requested (if the interface asked for a specific IP address with the discover packet) depending on how busy the network is. If the network is busy then the IP address requested with the discover packet may have already been re-assigned to a different interface, thus a different IP address will be offered.
3. The client sends a DHCP request, informing the DHCP server that the clients NIC wishes to be assigned the IP address sent by the servers offer.
4. The server sends a DHCP ack, acknowledging that the NIC has sent a request for a specific IP address. At this point the clients interface assigns / binds the IP address from the DHCP servers offer packet in step two.

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.

Classification of attacks on DHCP

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:

• Denial of service attack By spoofing the clients packet exchange a DHCP server will happily give all the available leases to spoofed MAC address thus causing a denial of service. Any machine wishing to join the network after the attack would not be allocated an IP address as the whole of the DHCP range will have been either allocated to valid interfaces (ie. interfaces already joined to the network before the attack took place) or spoofed MAC addresses (from the attack). Any interface already joined to the network would not notice the effect of the attack as they have already been assigned an IP address, but interfaces without a IP address would not be able to join the network as the DHCP server will have no available IP addresses.

  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.
   

• Rogue DHCP server By setting up a rogue DHCP server, a hacker could create a veritable playground for himself. The DHCP protocol can aid a hacker to redirect traffic through their machine (man in the middle attack) or send users to false web pages (via a rogue DNS server). This could occur as a DHCP server can set various options such as what IP address to use for the default gateway and also what DNS servers to use.
   
• Man in the middle attack By starting a rogue DHCP server, the real DHCP server and the rogue server will fight to assign a interface an IP address. If a rogue server wins then the interface could be assigned a different default gateway. By assigning a different default gateway (i.e. a hackers machine) all outgoing packets would be sent via the hackers machine thus sniffable. The machine acting as the default gateway would need to rewrite the MAC layer to enable the packets to be forwarded to the correct destination (ie the correct default gateway).

  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
  

  • One problem with the DHCP man in the middle attack is that it may not work on a small network. The attack may not work if the NIC's request for its old IP address is fulfilled. If the normal DHCP server can fulfil the request for the specified IP address, the NIC will be assigned the previous IP address and not the one from the rogue server. The only way a rogue server can assign IP addresses is if the requested IP address is not available on the normal DHCP server (i.e. The address has been re-allocated to another interface). The rogue server would not be able to fulfill the NIC's initial request as the rogue servers address range is based on stolen addresses from the normal DHCP server, and is unlikely to contain the IP address requested. If a rogue server issues a requested IP address to any NIC that wanted it, problems would occur on the network as multiple machines may have the same IP address.
  • Another problem with the attack is that it would only be a one way attack as the default gateway assigned by the rogue DHCP server is not the real default gateway. The fake gateway would need to sniff the packets and rewrite the MAC layer to enable the packets to be sent to the correct default gateway. The problem would occur with packets being sent from the correct default gateway back to the attacked machine as the packets would not pass through the rogue gateway, thus can not be sniffed. This basically means that all outgoing traffic can be sniffed and all incoming traffic can't.
  • A full man in the middle attack can be established using programs such as Dsniff and Ettercap which both utilize ARP poisoning to establish the man in the middle attack.
     

• Exploiting DHCP to trick users into using a fake DNS server As mentioned above a DHCP server can tell a interface which DNS server to use, so by specifying a hackers machine running a fake DNS server could make getting usernames, passwords, credit card numbers relatively easily. The fake DNS server would point for example www.hotmail.com to the hackers IP address.... so as long as the hacker has a convincing copy of hotmail's front page the username and password could be easily stolen.
   

• Stealing a machines identity Many servers that use DHCP get re-assigned the same IP address every time they request an IP address. A list of MAC addresses (maintained by the administrator) is used to re-assign the same IP address to a specific MAC address. By spoofing the MAC address of a specific machine and requesting the corresponding IP address a machines identity can be stolen. For this to occur the target machine needs to DoSed, and the packet exchange (to steal the identity) to take place before the machine is rebooted. If successful the target machines IP address will be given to the hacker and with a bit of ARP trickery (reply to ARP requests) that state should be maintained.

  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.

Defense Recommendations

• Rogue Detect is a Perl script which 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).
• 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.

Notes: Those pages are written by people for whom English is not a native language. Some amount of grammar and spelling errors should be expected. This is a Spartan WHYFF (We Help You For Free) site. It cannot replace the best teachers and the best books. The site contain some obsolete pages as it develops like a living tree... Some links on older pages are broken. Please try to use Google, Open directory, etc. to find a replacement link (see HOWTO search the WEB for details). We would appreciate if you can mail us a correct link. Search Amazon by keywords:     Open directory Research Index

Old News ;-)

[Jan 5, 2007] freshmeat.net Project details for Rogue Detect

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).

[Jan 5, 2007] Bounty Source - Rogue Detect - Wiki

[Dec 21,  2006] Problems Detecting Rogue DHCP Servers on Linux  UPDATE

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.

[Dec 20, 2006] Problems Detecting Rogue DHCP Servers on Linux

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 more tcpdump showed 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.exe you 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.exe will 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 -p flag.

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.

[Oct 27, 2006 ] DHCP Server Security (Part 2) Detecting Rogue DHCP Servers

[Nov 22, 2006] DHCP at SLAC

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 unix-admin@slac.stanford.edu .

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 dhcp-admin@slac.stanford.edu .

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.

[Oct 30, 2006] TechRepublic/DHCP – An administrator's delight or a security issue.

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

[Dec 03, 2003] Macworld News Apple offers DHCP security workaround

Recommended Links

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

In this excerpt from Securing Windows Server 2003, author Mike Danseglio explores the core network services of DHCP and DNS.

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.

Related Reading Securing Windows Server 2003 By Mike Danseglio Table of Contents Index Sample Chapter Read Online--Safari Search this book on Safari: Only This Book All of Safari Code Fragments only

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.

DHCP Server Security (Part 1)

Part 1 of this two-part article looks at the different types of threats faced by DHCP servers and countermeasures for mitigating these threats. www.windowsecurity.com/articles/DHCP-Security-Part1.html - 31k - Cached

DHCP Server Security (Part 2)

In Part 1 of this two-part article we looked at the different types of threats faced by DHCP servers and outlined some general countermeasures for ... www.windowsecurity.com/articles/DHCP-Security-Part2.html - 33k - Cached

DHCP and security

P-107 Security Vulnerability in Solaris 8 DHCP Administration Utilities

Reference

See also Solaris DHCP

RFC 2131 Dynamic Host Configuration Protocol

RFC 2132 DHCP Options and BOOTP Vendor Extensions

Last Modified: June 02, 2008