114网址导航Security of TCP/IP Address Resolution Protocol(ARP)
May the source be with you, but remember the KISS-)
Security of TCP/IP Address Resolution Protocol (ARP)
The ARP protocol was defined in
, written by David Plummer in 1982.& The arp protocol is a broadcast protocol: it receives
a destination IP address and sends out a broadcast request for all machines to see. The request asks
the question:
If you are IP address N, please respond with your Ethernet (MAC) address.
Each host on the network monitors the network for these requests, when it's address is requested
it sends a
arp reply. The arp reply specifies the 48 bit Ethernet address to use for that IP
The ARP functions between the Internet and Network Interface layers of the TCP/IP model:
TCP/IP Layers
Application Layer
Transport Layer
Internet Layer
&----------------------------------------- ARP
Network Interface Layer
The necessary for ARP arise from the fact that in IP v4 packet header there is insufficient space
to store MAC address along with the host part of the IP address (IPX that means that for those protocols there is generally less need for protocols like
ARP although it still can serve MAC addresses to them, if necessary).
Therefore in order for two hosts to communicate, the logical IPv4 address must be resolved to the
physical address of the network adapter card (MAC address). This is done by ARP. The idea is somewhat
similar to DNS (in this case IP serves as pseudo DNS name that is resolved to MAC address).
The main problem with arp is that this a very old protocol created in 1982 and naturally it was created
without any regards for security.&
ARP is necessary because& the sending host must complete the contents of the target address
field in the Ethernet frame. The Ethernet frame must include the source and destination MAC addresses:
Destination Ethernet Address. Provided via ARP
SourceEthernet Address. The source Ethernet address is retrieved from the kernel which obtains
it from nonvolatile random access memory (NVRAM) on boot.&
Cyclic Redundancy Check
There can be two cases of MAC address resolution:
If the final destination (receiving system) of the message being sent is on the same LAN as
the sending system, only one MAC address resolution is required. This makes ARP somewhat similar
to DNS: like in DNS address can be cashed or you need to get it somehow by sending a request.
If, on the other hand, the final destination of the message is on a different LAN, multiple address
resolutions will be required, one for each hop through a router until the final destination is
ARP is a broadcast protocol that works strictly within local network segment (does not cross
the router boundary). If the final destination (receiving system) of the message being sent is on the
same LAN as the sending system, only one ARP address resolution is required. If the final destination
is on a different network, an address resolution might be required on each network that the message
traverses on the path to its final destination.
For example, assume that the& system alpha must communicate with the system beta
and it does not know the system beta MAC address.
In order to determine the MAC address of the system& the system alpha& sends an
ARP request to the local subnet& using& the Ethernet broadcast address (ff:ff:ff:ff:ff:ff).
The ARP request includes the IP address of the system beta.&
The broadcast is seen by all systems on the segment including, says, &beta and gamma.
The gamma and beta recognize that the ARP request contains the IP address
and MAC address of the alpha and add this information to their ARP tables, if it is not already
present. This type of entry is known as an unsolicited entry because the information
was not explicitly requested.
The system beta identifies its own IP address in the ARP request and sends an ARP reply
to the& system alpha. The ARP response includes the Ethernet address of the& system
beta, and it is sent using the unicast Ethernet address of the system alpha.
The system alpha receives the ARP reply and stores the information about beta
in its ARP table. This type of entry is a solicited entry because the system alpha
requested this information
It would be silly to continually send an ARP request broadcast each time a host needs to find the
physical address of another host it accesses often. For this reason, all implementations of TCP/IP include
an ARP cache to hold IP address to MAC address mappings.
ARP responses are cached in the ARP table to have the information available if it is required again
in the near future. The ARP table, cached in memory, stores requested (solicited) &Ethernet
addresses for up to 20 minutes. This table is read each time a destination Ethernet address is required
to prepare an Ethernet frame for transmission. If an Ethernet address does not appear in the ARP table,
an ARP request is sent. A host that replies to an ARP request also updates its ARP table with the IP
and Ethernet addresses of the requesting host. There are two major types of entries in the cache:
solicited entries are those where an Ethernet address was specifically asked for by a host,
unsolicited entries are a result of storing information about a host that was performing
an ARP request.
static entries (discussed later, must be added manually using arp -scommand)
published entries -- a special type of static entries for which the host answers
ARP entries on behave of another host.
In Solaris you need to use the command:
ndd /dev/ip ip_ire_arp_interval
to display the length of time that ARP entries are cached. The default is 1200000 microseconds (20
min).& That means that Solaris cache& solicited ARP entries for 20 minutes.
Unsolicited ARP entries cache for only 5 minutes.& Use the command:
ndd /dev/ip arp_cleanup_interval
If the entry has not been used for 15 minutes, it is deleted from the cache if the cache is at maximum
capacity. If the cache is at maximum capacity and a new entry comes in, the first found entry
will be yanked and replaced with the new entry.
In Solaris ARP cache parameters are stored in PROTOCOL.INI. The parameter, arptblsize,
under the [tcp_xif] section controls the size of the ARP cache. The default is (tcpconnections*2)
+ 6. The range is 6 to 512 entries.
There are seven major operation that you can perform with arp cache:
You can examine all entries in the ARP table with the command:
You can examine a specific ARP table entry with the command:
# arp hostname
You can also add a static (until reboot) ARP table entry using the command:
# arp -s hostname ethernet_address
You can populate an ARP table manually in situations in which the destination device cannot respond
to ARP requests.& To add a published ARP table entry, perform the command:
# arp -s hostname ethernet_address
You can create so called &published& ARP entry when you want a host to answer an ARP request on
behalf of another host. This is a useful option for heterogeneous environments and some SLIP or PPP
configurations in which some hosts cannot respond to ARP requests for themselves.
To add ARP entries from a file, perform the command:
# arp -f filename
Entries in the file should be in the form:
hostname ethernet_address [pub]
To delete an ARP table entry, perform the command:
# arp -d hostname
where hostname is the name of the host or its decimal-dot notated IP address.
A switched network does not lend itself to sniffing as easily as a non-switched network. It does
not broadcast most frames. But the development of switched networks was driven by the need for more
bandwidth, not for the need of more secure networks. Still it is much more secure that non-switched
network and there is no reason those days to use anything but switched networks. Although sniffing is
more difficult, it is not impossible and several methods are available to sniff switched networks:
. One of the basic operations of the Ethernet protocol
revolves around ARP (Address Resolution Protocol) requests and replies. In general, when Node A wants
to communicate with Node C on the network, it sends an ARP request. Node C will send an ARP reply which
will include the MAC address. Even in a switched environment, this initial ARP request is sent in a
broadcast manner. It is possible for Node B to craft and send an unsolicited, fake ARP reply to Node
A. This fake ARP reply will specify that Node B has the MAC address of Node C. Node A will unwittingly
send the traffic to Node B since it professes to have the intended MAC address. There are tools that
care cable of doing this automatically sending fake ARP replies to classes of machines (i.e., NFS servers,
HTTP servers, etc). One such tool is dsniff5 and it works well to sniff for specific types of traffic.
Other tools listen for the general ARP request and send the fake ARP reply at that time. The parasite4
program falls into this category and it serves well to sniff the entire network. For this type of attack
to work, we need the ability to forward on the frames we receive to their intended host. This is most
commonly achieved through some type of IP forwarding, either at the kernel or application level.
. Since switches are responsible for setting up the
virtual circuits from one node to another, they must keep a translation table that tracks which addresses
(specifically, which MAC addresses) are on which physical port. The amount of memory for this translation
table is limited. This fact sometimes allows the switch to be exploited by flooding the translation
table. Primitive switches, not knowing how to handle the excess data, will 'fail open'. That is, it
will revert to a hub and will broadcast all network frames to all ports. At this point generic network
sniffers will work.
. It's not difficult to imagine that, since
all frames on the network are routed based on their MAC address, that the ability to impersonate another
host would work to our advantage. That's just what MAC duplicating does. You reconfigure Node B to
have the same MAC address as the machine whose traffic you're trying to sniff. This differs from ARP
Spoofing because, in ARP Spoofing, we are 'confusing' the host by poisoning it's ARP cache. In a MAC
Duplicating attack, we actually confuse the switch itself into thinking two ports have the same MAC
address. Since the data will be forwarded to both ports, no IP forwarding is necessary.
There are several methods to protect again these attacks. Some of these methods are applicable to
both the non-switched and switched environments. Some advanced switches has built-in protection from
flooding. One solution is so called Sticky ARP like in Cisco 7600 Series Router
Sticky ARP prevents MAC address spoofing by ensuring that ARP entries (IP address, MAC
address, and source VLAN) do not get overridden. The router maintains ARP entries in order
to forward traffic to end devices or other routers. ARP entries are usually updated periodically or
modified when ARP broadcasts are received. During an attack, ARP broadcasts are sent using a spoofed
MAC address (with a legitimate IP address) so that the router learns the legitimate IP address with
the spoofed MAC address and begins to forward traffic to that MAC address. With sticky ARP enabled,
the router learns the ARP entries and does not accept modifications received through ARP
broadcasts.
If you attempt to override the sticky ARP configuration, you will receive an error message. For
a complete description of the system error messages, refer to the Cisco 7600 Series Router Cisco
IOS System Message Guide at this URL:
The ARP table, cached in memory, stores requested Ethernet addresses for up to 20 minutes. If an
Ethernet address does not appear in the ARP table, an ARP request is sent. Solicited ARP entries cache for 20 minutes, while unsolicited ARP entries cache for 5 minutes.
Most important commands
arp -a -- views cache
arp hostname & -- examines a specific ARP table entry
arp -s hostname ethernet_address& -- adds a static (until reboot) ARP table
arp -s hostname ethernet_address pub -- adds a published ARP table entry
arp -f filename -- adds ARP entries from a file. Entries in the file should be in the form:
&&&&&&&&&&&&&&&&&&
hostname ethernet_address [pub]
arp -d hostname -- deletes an ARP table entry
Displays and modifies the IP-to-Physical address translation tables used by
address resolution protocol (ARP).
ARP -s inet_addr eth_addr [if_addr]
ARP -d inet_addr [if_addr]
ARP -a [inet_addr] [-N if_addr]
-a Displays current ARP entries by interrogating the current protocol data. If inet_addris
specified, the IP and Physical addresses for only the specified computer are displayed. If more than
one network interface uses ARP, entries for each ARP table are displayed.
-g Same as -a.
inet_addr Specifies an internet address.
-N if_addr Displays the ARP entries for the network interface specified by if_addr.
-d Deletes the host specified by inet_addr. inet_addr may be wildcarded with * to delete
all hosts.
-s Adds the host and associates the Internet address inet_addr with the Physical
address eth_addr. The Physical address is given as 6 hexadecimal bytes separated by hyphens. The entry
is permanent.
eth_addr Specifies a physical address.
if_addrIf present, this specifies the Internet address of the interface whose address
translation table should be modified. If not present, the first applicable interface will be used.
arp -s 157.55.85.212 00-aa-00-62-c6-09 # Adds a static entry.
# Displays the entire arp table.
157.55.85.212
# Displays the arp table entry for 157.55.85.212
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Archived content - No warranty is made as to technical accuracy. Content may contain URLs that were
valid when originally published, but now link to sites or pages that no longer exist.
Chapter 3 from Firewalls, 24 seven, published by Sybex, Inc.
If you are reading this book, you presumably have a good understanding of how computers work and
you have a working knowledge of how to use Internet tools such as Web browsers, Telnet, and e-mail.
You might not, however, know just how your computer connects to other computers and downloads Web pages,
exchanges e-mail, or establishes a Telnet session. This chapter will give you a better idea of what
is going on behind the scenes.
Routing IP
Reverse Address Resolution Using RARP and BootP. Reverse ARP (RARP) was defined
in RFC 903. If a router does not know the IP address of one of its Ethernet ...
[Mar 16 2005]
Been playing alot with Cain and Abel recently and after realising how powerful this tool is for sniffing
traffic I want to now focus on counter measures against an attack this tool can perform - ARP poisoning.
So can anyone recommend the best tools / techniques to protect against this?EDIT: Just found this
is already been covered on GovSec in the past. Im currently reading this paper that was recommended:
However, i would like to hear from peoples experience on protecting against ARP poisoning and tools
they have used.
I don't know what Microsoft has changed to the ARP cache behaviour...but ARP spoofing attacks are
still possible!
You can easily reproduce that (you need at least three machines - one could also be a router) -
Download and start
Click &Configure& and select the appropriate network interface
Activate the Sniffer and go to the &Sniffer& Tab
Click the &+& Icon - Cain will now scan the subnet for all attached devices (in my case my laptop
with XP SP2 installed and the cisco router which is my default gateway)
Go to the &APR Tab& (ARP Poisoning & Routing)
Click the &+& Icon
Now select the machine(s) which communication you wan to redirect to your machine (in my case
i selected the laptop on the left and the router on the right)
Activate APR
That's it - now all the traffic between the router and the laptop passes my machine (even in a switched
network). Cain can recognize and collect various passwords directly from the wire including ftp, http
forms auth, telnet, pop3, sql server...you can see the collected passwords on the &Passwords& tab.
Those passwords that need cracking can be sent with right-click &send to cracker& to a cracking module
(e.g. ntlm, kerberos pre-authentication...).
To get more insight into the packets just fire up
on your machine and you'll get the
full story.
So whatever they did, those SP2 changes don't increase the protection from these attacks. maybe
they make it harder to start an attack from SP2 - and then again this is as stupid as removing raw
sockets...
In fact i already saw operating systems with working anti ARP spoofing measures, e.g. the Cisco IOS.
If they see a ARP broadcast on the wire where someone pretends to be them they immediately send a bunch
of correct ARP packets out on the network.
an excellent explanation of how ARP spoofing works can be found
btw - the only protection against ARP spoofing at the moment (on Windows) is to add static ARP entries
in the cache, e.g. for your standard gateway with
arp -s IPAddress MACAdress
...but who does that?
OptimumPath's wireless router is heavy on security and contains a great deal of functionality
for both wireless and wired infrastructure networks. The RTC-2000 provides cost savings resulting
from the integration of a wide variety of network functions, making it a component that offers significant
value when deploying enterprise and public wireless LAN solutions.
Since one of the RTC-2000's strong suits is protection against address resolution protocol (ARP)
attacks, it's worth a few paragraphs first to define ARP and explain related issues. ARP is an important
protocol that a sending station (network card) uses to discover the physical address of a destination
Before a station can send a packet to another station, the sending station must obtain the destination's
physical address, which is the same as its Medium Access Control (MAC) address ().
The sending station first broadcasts an ARP request that announces the IP address of the destination
station. The station having the corresponding IP address will then respond with its MAC address.
A noteworthy problem with the ARP process is that it offers a significant security issue resulting
from ARP spoofing. All a hacker needs to do to spoof a user is to independently send an ARP response
from a rogue network device that maps the IP address of a legitimate network device, such as a wireless
access point or router, to the MAC address of the rogue device.
As a result, legitimate stations on the network will automatically update their ARP tables and
send future packets to the rogue device rather than the legitimate access point or router. With this
Man-in-the-Middle attack, a hacker can easily manipulate user sessions flowing over encrypted links
and access sensitive, password-protected information. Because firewalls are always open to ARP, attacks
can stem from outside the facility -- something that should definitely cause IT managers to lose
For some credible details on ARP security issues, refer to a
(not light reading).
By providing a secure tunnel between each client and the router, the RTC-2000 completely protects
wireless networks from ARP attacks. OptimumPath's Secure ARP (SARP) provides the secure tunnel between
the client and the RTC and ignores all reverse ARP requests not associated with the tunnel.
For example without SARP enabled, we were able to use
to establish a man-in-the-middle
attack and hack into a user logging into an SSL-based Website account (yahoo e-mail). After activating
SARP, it was not possible to replicate this form of attack. With SARP running, you can certainly
rest at ease regarding ARP attacks.
As I've mentioned in a
rogue access points are a big security concern. Employees of a company may inadvertently connect
access points purchased from the local office supply store into the corporate network without coordinating
the action with IT support.
As a result, IT managers should deploy mechanisms that monitor for rogues before the security
hole a rogue provides lets a hacker or even casual snoopers onto the network. The ability to effectively
identify rogues, however, is missing from most wireless LAN routers on the market. The RTC-2000 shines
in this department by implementing a comprehensive suite of heuristics that identify the presence
of rogue access points.
The RTC-2000 is a complete solution, offering the right mix of functionality for most enterprise
and public wireless LAN systems. The integration of routing, authentication, bandwidth control, intrusion
detection, auditing, self provisioning, wall garden, virus filtering and spam protection among other
valuable tools into one unit results in lower overall costs compared to purchasing individual components.
This is extremely beneficial, especially for start-ups deploying
Some Downsides to Consider
is a major concern for some wireless LAN applications. The RTC-2000 offers superb wired side
DoS protection. The system uses a flexible and proactive stance against inbound and outbound datagrams.
This allows the system to block intrusion by unauthorized users. As with other wireless LAN routers
on the market, the RTC-2000, though, doesn't offer provisions to counter radio-side DoS attacks.
For example, someone could flood the network with 802.11 Clear-to-Send (CTS) frames and cause
other stations to indefinitely hold off transmitting data frames. OptimumPath engineers are aware
of these types of problems, though, and they're working on future upgrades to counter wireless DoS
This is nit picky, but the current version of the RTC-2000 offers somewhat of a basic graphical
user interface (GUI) for configuration. An average IT person can add the RTC-2000 to a typical network
using the GUI to significantly improve security, but the command line interface (CLI) is necessary
(and somewhat difficult to learn) to optimally tune the router. As a result, you might need services
from OptimumPath to assist with the initial installation and configuration of the router.
All-in-all, the RTC-2000 is a high end, secure wireless LAN router that has features that stand
out among the sea of other routers on the market. Definitely consider including this product in enterprise
and public wireless LAN solutions to lower risks and liabilities resulting from information flying
around on airwaves.
Jim Geier provides independent
to companies
developing and deploying wireless network solutions. He is the author of the book,
and offers
focusing on
wireless LANs.
RE: [suse-security] What to do against ARP-Poisoning?
From: Reckhard, Tobias (tobias.)
Date: Tue Mar 19 2002 - 22:31:42 PST
Next message:
Previous message:
Maybe in reply to:
Next in thread:
Messages sorted by:
& & And remember that switches aren't security enforcement devices and
& & shouldn't be expected to be.
& The 3com switches that we have (SuperStack II) certainly have
& some active
& security measures. Enabling &Port Security& on a port makes
& the switch
& remember the first MAC address it receives and locks that MAC
& address to that
& port until overridden by manual intervention. Although
& unfortunately they
& cannot perform any kind of ARP poison countermeasures.
AFAIK, some other switches even let you configure the valid MAC addresses
per port. However, there have been reports of switches being confused if
they're swamped with MAC addresses and resorting to broadcast mode.
Actually, that's perfectly normal spanning tree algorithm behaviour, that
algorithm being the basis of all switches. As such, it's buried deep in the
devices' logic and it probably isn't being disabled by 'port security'.
Switches have some features that are marketed as security benefits, and some
even are, but switches remain devices designed to provide efficient
connectivity, i.e. mainly speed. They are not designed as security
enforcers, while other devices, e.g. firewalls, are (or should be).
One can even argue that it is a Bad Idea (TM) to use managed switches in
sensitive environments, because the switch constitutes a further point of
attack and often a single point of failure. The point is strengthened by the
fact that the OS on switches is generally not designed with security a top
priority, it usually supports a number of services (Cisco switches, e.g.
support NTP, SNMP, TFTP, Telnet, CDP) that can be exploited. And more often
than not, I wager, the personnel is pretty clueless with regards to the
switch configuration, since these boxes are typically plug'n'play.
Now don't get me started on VLANs. Just this much: don't use VLANs to
'separate' networks of (more or less substantially) different trust.
Instead, implement physical separation.
Security Basics: Re: ARP Poisoning
From: ATD ()
Date: Nov 08 2002
Next message:
Previous message:
In reply to:
Messages sorted by:
One easy way to ID this is to monitor for the ARP broadcast, or check for hosts doing this broadcast.
For example... when using ettercap (one of those nice arp tools) ot does:
Building host list for netmask 255.255.255.0, please wait...
Sending 7 ARP request... &--- You can detect this.
Another thing that you can do is to run checks for other systems doing
arp poisoning, ettercap offers this feature as well:
[cC] - check for other poisoner...
So, one way to defend against this sniffing is to check for these
poisoners every X minutes and notify the admin IF such a thing happens.
Cerebrum Gateway]
&gawd# ettercap -c -N
ettercap 0.6.7 (c) 2002 ALoR & NaGA
Your IP: xxx.xxx.xxx.xxx with MAC: 00:10:4B:C8:2A:4E on Iface: de0
Building host list for netmask 255.255.255.0, please wait...
Sending 7 ARP request...
* |==================================================| 100.00 %
Resolving 5 hostnames...
* |==================================================| 100.00 %
Checking for poisoners...
MAC of xxx.xxx.xxx.xxx and xxx.xxx.xxx.xxx are identical !
you got a poisoner!!! =o)
From: Trevor Cushen (Trevor.Cushen_at_sysnet.ie)
Subject: ARP Poisoning
Date: Nov 08 2002
Next message: Matt Hemingway: &Re: ARP Poisoning&
Previous message: Mailing Lists: &Re: Biometric question&
Maybe in reply to: Michael Ungar: &ARP Poisoning&
Next in thread: Jeff Dickison: &Re: ARP Poisoning&
Reply: Jeff Dickison: &Re: ARP Poisoning&
Reply: Matt Hemingway: &Re: ARP Poisoning&
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] [ attachment ]
Hello Michael,
I am looking at that at the moment. Encryption is the best way to go to protect against sniffing
and there are millions of ways to enable it around a network in one form or another. On the other side
I am putting together a series of perl scripts and web front ends to monitor devices on the network
because I want to detect new and unauthorised MAC addresses on my network.
Ettercap has a flag that will detect arp poisoning on the network as well as a flag for running
arp requests across the network. What I have done is set this up to test my network at MAC level only.
I gather the results and match it off against a list of my valid mac addresses etc etc. A nice colour
coded web front end will show red for unrecognised and online mac addresses. Green online and recognised
etc. A history option to tell me when machines went online and offline.
This way if any new device is added to my network then I know about it even if it does spoof the
mac address later to sniff only. This came about after it was suspected that people could come in with
laptops and copy of files which of course will not trigger any IDS system as it is valid traffic.
But if a wireless AP was added to the network then I will detect that too because it will be an
unknown MAC address.
I am nearly finished developing this but if anyone knows of a utility that already does this well
then please let me know.
Trevor Cushen Sysnet Ltd
www.sysnet.ie
Tel: +353 1 2983000
Fax: +353 1 2960499
-----Original Message-----
From: Michael Ungar [mailto:m_ungar_]
Sent: 07 November
To: security-basics_
Subject: ARP Poisoning
From security books I've read it's not hard to eavesdrop on network communication using tools like
dsniff, even in a switched environment. My understanding is that it is accomplished quite easily by
ARP poisoning your victim in thinking your machine's MAC as the router MAC & after interception, re-forwarding
the traffic back to the true router MAC. Assuming the network environment is large (e.g., configuring
port switches for specific MAC addresses not practical) & desktop security cannot be guaranteed (and
thereby cannot prevent people from allowing machines to IP forward), how can one defend against other
than encrypting data. Thanks....Mike
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Softpanorama Recommended
Intro to ARP spoofing
&AntiCode Website&
van Hauser. &Parasite 0.5&
Song, Dug. &Dsniff&
by Gereon Ruetten und Oliver Stutzke (2005)
Nicht nur aus dem Internet werden PCs angegriffen, um Trojaner und Spyware zu installieren. Auch im
LAN, beispielsweise in der Firma, versuchen b?sartige Zeitgenossen Informationen auszusp?hen. Selbst
geswitchte Netzwerke bieten keinen Schutz, wenn Angreifer die Verbindung mittels ARP-Spoofing über sich
by US Secret Service (2005)
This study [...] is the first of its kind to provide a comprehensive analysis of insider actions
by analyzing both the behavioral and technical aspects of the threats. The findings underscore the
importance of organizations' technology, policies and procedures in securing their networks against
insider threats, as most of the cases showcased in the report were perpetrated by insiders with minimal
technical skills. Various proactive practices are among the suggestions offered by the report.
ARP Cache Poisoning Prevention and Detection. Silky Manwani. CS 298 Project. 1. San Jose State University.
ARP Cache Poisoning Detection and Prevention ...
The Cain Sniffer
A sniffer is a program that can capture data as it travels on a network. As previously discussed,
the amount of traffic the sniffer has access to depend on whether or not the network card is in promiscuous
mode. In addition, with the advent of the switch, a computer no longer has access to all the data flowing
on the network. Thanks to smart switching devices, a computers network card will only be passed information
to which it is addressed, or if the data is meant for all connected computers (broadcast). As a result,
even if a network card is placed in promiscuous mode, it is not necessarily going to be able to sniff
network traffic.
When a switch is initialized, it determines the MAC address of the card connected to each port.
As the network cards communicated, the switch monitors the data and creates a table linking IP address
with MAC address. Then when a data packet enters into the switch, it compares the destination IP address
with the list of devices in its MAC address table to see which port it is to send the data on to. This
table is called the ARP table.
The Address Resolution Protocol (ARP) is the standard that defines how MAC addresses are linked
to IP addressed. However, there is a sight weakness in the protocol that allows a user to lie to a
computer about the IP to MAC address translation. As a result, it is possible to trick two network
devices into passing all their network traffic to a deceitful computer that can simply capture and
then pass the information on to the intended target. This process is known as ARP Spoofing, and it
allows a computer to sniff network traffic even if there is a switch.
The ARP table, cached in memory, stores requested Ethernet addresses for up to 20 minutes. If an
Ethernet address does not appear in the ARP table, an ARP request is sent.
Solicited ARP entries cache for 20 minutes, while unsolicited ARP entries cache for 5 minutes.
Most important commands
arp -a -- view cashe
arp hostname To examine a specific ARP table entry, perform the command.
arp -s hostname ethernet_address To add a static (until reboot) ARP table entry, perform
the command.
# arp -s hostname ethernet_address pub To add a published ARP table entry, perform the command:
To add ARP entries from a file, perform the command:
# arp -f filename
Entries in the file should be in the form:
hostname ethernet_address [pub]
To delete an ARP table entry, perform the command:
# arp -d hostname
Anatomy of an ARP Poisoning Attack. by Corey Nachreiner, WatchGuard Network Security Analyst
ARP poisoning enables local hackers to cause general networking mayhem. Because it's mostly &incurable,&
every administrator should be aware of how this attack works.
ARP Refresher
In , we explained that
is how network devices associate
devices on the local network can find each other. ARP is basically a form of networking roll call.
ARP, a very simple protocol, consists of merely four basic message types:
An ARP Request. Computer A asks the network, &Who has this IP address?&
An ARP Reply. Computer B tells Computer A, &I have that IP. My MAC address is [whatever it is].&
A Reverse ARP Request (RARP). Same concept as ARP Request, but Computer A asks, &Who has this
MAC address?&
A RARP Reply. Computer B tells Computer A, &I have that MAC. My IP address is [whatever it is]&
All network devices have an ARP table, a short-term memory of all the IP addresses and MAC
addresses the device has already matched together. The ARP table ensures that the device doesn't have
to repeat ARP Requests for devices it has already communicated with.
Here's an example of a normal ARP communication. Jessica, the receptionist, tells Word to print
the latest company contact list. This is her first print job today. Her computer (IP address 192.168.0.16)
wants to send the print job to the office's HP LaserJet printer (IP address 192.168.0.45). So Jessica's
computer broadcasts an ARP Request to the entire local network asking, &Who has the IP address, 192.168.0.45?&
as seen in .
All the devices on the network ignore this ARP Request, except for the HP LaserJet printer. The
printer recognizes its own IP in the request and sends an ARP Reply: &Hey, my IP address is 192.168.0.45.
Here is my MAC address: 00:90:7F:12:DE:7F,& as in
Now Jessica's computer knows the printer's MAC address. It sends the print job to the correct device,
and it also associates the printer's MAC address of 00:90:7F:12:DE:7F with the printer's IP address
of 192.168.0.45 in its ARP table.
Hey ARP, Did You Know Gullible Is Not in the Dictionary?
The founders of networking probably simplified the communication process for ARP so that it would
function efficiently. Unfortunately, this simplicity also leads to major insecurity. Know why my short
description of ARP doesn't mention any sort of
method? Because in ARP, there is none.
ARP is very trusting, as in, gullible. When a networked device sends an ARP request, it simply trusts
that when the ARP reply comes in, it really does come from the correct device. ARP provides no way
to verify that the responding device is really who it says it is. In fact, many operating systems implement
ARP so trustingly that devices that have not made an ARP request still accept ARP replies from other
OK, so think like a malicious hacker. You just learned that the ARP protocol has no way of verifying
ARP replies. You've learned many devices accept ARP replies before even requesting them. Hmmm. Well,
why don't I craft a perfectly valid, yet malicious, ARP reply containing any arbitrary IP and MAC address
I choose? Since my victim's computer will blindly accept the ARP entry into its ARP table, I can force
my victim's gullible computer into thinking any IP is related to any MAC address I want. Better yet,
my faked ARP reply to my victim's entire network and fool all his computers. Muahahahahaa!
Back to reality. Now you probably understand why this common technique is called ARP Cache Poisoning
(or just ARP Poisoning): the attacker lies to a device on your network, corrupting or &poisoning& its
understanding of where other devices are. This frighteningly simple procedure enables the hacker to
cause a variety of networking woes, described next.
All Your ARP Are Belong To Us!
The ability to associate any IP address with any MAC address provides hackers with many attack vectors,
including Denial of Service, Man in the Middle, and MAC Flooding.
Denial of Service
A hacker can easily associate an operationally significant IP address to a false MAC address. For
instance, a hacker can send an ARP reply associating your network router's IP address with a MAC address
that doesn't exist. Your computers believe they know where your default gateway is, but in reality
they're sending any packet whose destination is not on the local segment, into the Great Bit Bucket
in the Sky. In one move, the hacker has cut off your network from the Internet.
Man in the Middle
A hacker can exploit ARP Cache Poisoning to intercept network traffic between two devices in your
network. For instance, let's say the hacker wants to see all the traffic between your computer, 192.168.0.12,
and your Internet router, 192.168.0.1. The hacker begins by sending a malicious ARP &reply& (for which
there was no previous request) to your router, associating his computer's MAC address with 192.168.0.12
Now your router thinks the hacker's computer is your computer.
Next, the hacker sends a malicious ARP reply to your computer, associating his MAC Address
with 192.168.0.1 (see
Now your machine thinks the hacker's computer is your router.
Finally, the hacker turns on an operating system feature called IP forwarding. This feature
enables the hacker's machine to forward any network traffic it receives from your computer to the router
(shown in ).
Now, whenever you try to go to the Internet, your computer sends the network traffic to the hacker's
machine, which it then forwards to the real router. Since the hacker is still forwarding your traffic
to the Internet router, you remain unaware that he is intercepting all your network traffic and perhaps
also sniffing your clear text passwords or
your secured Internet sessions.
MAC Flooding
MAC Flooding is an ARP Cache Poisoning technique aimed at network switches. (If you need
a reminder about the difference between a hub and a switch, see this
When certain switches are overloaded they often drop into a &hub& mode. In &hub& mode, the switch is
too busy to enforce its port security features and just broadcasts all network traffic to every computer
in your network. By flooding a switch's ARP table with a ton of spoofed ARP replies, a hacker can overload
many vendor's switches and then
your network while the switch is in &hub& mode.
Scared? Good, Now Calm Down!
This is scary stuff. ARP Cache Poisoning is trivial to exploit yet it can result in very significant
network compromise. However, before you jump to Defcon-7, notice the major mitigating factor: only
local attackers can exploit ARP's insecurities. A hacker would need either physical access to your
network, or control of a machine on your local network, in order to deliver an ARP Cache Poisoning
attack. ARP's insecurities can't be exploited remotely.
That said, hackers have been known to gain local access to networks. Good network administrators
should be aware of ARP Cache Poisoning techniques.
Since ARP Cache Poisoning results from a lack of security in a protocol that is required for TCP/IP
networking to function, you can't fix it. But you can help prevent ARP attacks using the following
techniques.
For Small Networks
If you manage a small network, you might try using static IP addresses and static ARP tables. Using
CLI commands, such as &ipconfig /all& in Windows or &ifconfig& in 'NIX, you can learn the IP address
and MAC address of every device in your network. Then using the &arp -s& command, you can add static
ARP entries for all your known devices. &Static& this prevents hackers from adding
spoofed ARP entries for devices in your network. You can even create a login script that would add
these static entries to your PCs as they boot.
However, static ARP entries
impossible in large networks. That's because every
device you add to your network has to be manually added to your ARP script or entered into each machine's
ARP table. But if you manage fewer than two dozen devices, this technique might work for you.
For Large Networks
If you manage a large network, research your network switch's &Port Security& features. One &Port
Security& feature lets you force your switch to allow only one MAC address for each physical port on
the switch. This feature prevents hackers from changing the MAC address of their machine or from trying
to map more than one MAC address to their machine. It can often help prevent ARP-based Man-in-the-Middle
For All Networks
Your best defense is understanding ARP Poisoning and monitoring for it. I'd highly recommend deploying
an ARP monitoring tool, such as
, to alert
you when unusual ARP communication occurs. This kind of vigilance is still the greatest weapon against
all kinds of attack -- for, as Robert Louis Stevenson wrote, &The cruelest lies are often told in
Resources:
[Sept 27, 2001]
by Robert Wagner
The classic
relies upon convincing two hosts that the computer in the middle is the other host. This can be accomplished
with a domain name spoof if the system is using DNS to identify the other host or address resolution
protocol (ARP) spoofing on the LAN. This paper is designed to introduce and explain ARP spoofing. The
term Man-in-the-Middle is used from a historical usage, this does not imply that only
men can use these attacks. Perhaps Teenager-in-the-Middle or Monkey-in-the-Middle
may be more accurate terms.
Published on 2001- by DataWizard, (C)PacketStorm Security.
After month's of doing everything except writing a new paper or updating an old one, I'm back with
a new subject. Because I only want to write about subjects that are not very common, I will not publish
much tutorials/papers in the future. This paper is dedicated to ARP tables and how to alter them remotely.
The paper also describes a couple of implemantations of ARP poisoning in a bridge based segment and
a couple of ways to protect yourself. As usual: I'm not responsible for any of your stupid actions
while practicing the following info at places you shouldn't be.
Published on July 12, 2001- by Mike Beekey, (C)Black Hat Briefing.
ARP may be one of the most used, but least respected protocol allowing two devices to establish
communications with each other across a network. Unfortunately, even with its critical role of mapping
the logical address to physical address, ARP is inherently susceptible to a variety of spoofing attacks
within local subnets. While there have been discussions surrounding this issue and tools written to
take advantage of these features, its potential to cause nearly indefensible denial of service attacks
with minimal effort, appears to still be understood by only a few.
Published on April 2001- by Sean Whalen, (C)Chocobospore.
This paper deals with the subject of ARP spoofing.
is a method of
exploiting the interaction of IP and
protocols. It is only
applicable to Ethernet networks running IP. The subject will be addressed such that anyone with basic
networking experience can understand key points of the subject. Knowledge of the TCP/IP reference model
is vital to full understanding, as is a familiarity with the operation of switched and non-switched
networks. Some background will be presented in the &Introduction& section, but experienced readers
may wish to skip to &Operation&.
Published on October 2002- by Laurent Licour, Vincent Royer, (C)Althes.
This paper describe a new method for spoofing an IP address with any networking application.
is not new and various
hacking tools have been developed to exploit it. In the following, we will discuss on the way to use
it with any standard application. As a result, we will explain why IP based access control is not reliable
in many cases, and should not be used in many corporate.
Published on - by L33tdawg, (C)HackinTheBox.
(MAC) address, also known as an Ethernet address, is the physical/hardware address for devices that
are connected to a network, usually a LAN. Each node connected to the network has it's own unique
hard coded (burned)
(NIC) by the manufacturer and uses this address to find and communicate with other devices
on the same network domain or wire.
Published on - by Mahesh V. Tripunitara, Partha Dutta, (C)Annual Computer Security Applications
Conference.
We discuss the
(ARP) and the problem of ARP cache poisoning. ARP cache poisoning is the malicious act,
of a host in a LAN, of introducing a spurious IP address to Medium Access Control (MAC) address mapping
in another host's ARP cache. We discuss design constraints for a solution: the solutions needs to be
implemented in middleware, without access or change to any operating system source code, be backward-compatible
to the existing protocol, and be asynchronous.
Published on - by Frédéric Raynal, (C)ARP-SK.
If ARP is a well known protocol, the attacks it allows are often restricted to sniffing, while so
many are possible.
Published on August 11, 2001- by Jonathan Wilkins, (C)Phrack Magazine.
Taranis redirects traffic on switch hardware by sending spoofed ethernet traffic. This is not the
same as an ARP poisoning attack as it affects only the switch, and doesn't rely on ARP packets. Plus,
it is virtually invisible because the packets it sends aren't seen on any other port on the switch.
Evading detection by an IDS that may be listening on a monitoring port is as simple as changing the
type of packet that is sent by the packet spoofing thread.
Published on October 22, 2001- by Bob Fleck, Jordan Dimov, (C)Cigital, Inc..
Wireless networks, specifically 802.11b, have received a tremendous amount of interest and
scrutiny from the security community over the past few months. The security community agrees that wireless
networks introduce a new point of entry into previously closed wired networks and must thus be treated
as an untrusted source, just like the Internet. Standard technologies enable wireless client machines
to connect to a local area network made up of other wireless hosts. For wireless networking to be most
useful, the wireless networks must pass data on to standard wired networks connected to the Internet.
This paper describes the application of a well understood class of attacks on wired networks to the
emerging mix of wired and wireless networking equipment.
Arp Spoofing (cache poisoning)
ARP cache poisoning / ARP
Many people think that once they use a switch for connecting their local network they're safe from
network sniffing. Basically this is right because the traditional way of sniffing where a host can
read all network packets just by accepting them (the so called &promiscuous mode&) is not possible.
However there are other means to achieve the same and because maybe some SysAdmins think they're safe
from sniffing thus designing their network a bit more open it's even more dangerous.
The tool used here is called arpspoof and is distributed in the
What we do is the following: We constantly send the victim computer ARP answers telling him that
the MAC address belonging to the IP of the gateway machine (router) is our MAC address. After some
time the victim computer will believe us and makes a wrong entry in his ARP cache. Next time the victim
wants to send an IP packet to the gateway he sends the ethernet frame to our MAC address so actually
we get the IP packet. We do the same thing with the gateway machine just the other way round.
describes the ARP protocol.
In order to tell the victim host that now we (our MAC address) are the one belonging to the
IP of the gateway enter the following command:
# arpspoof -t victim gateway
In a seperate shell we start the matching command to fool gateway to believe we are victim.
# arpspoof -t gateway victim
Don't forget to enable IP forwarding on your host so that the traffic goes through your host. Otherwise
victim will loose connectivity.
# echo 1 & /proc/sys/net/ipv4/ip_forward
Now watch all the traffic between the victim host and the outside network going through your
# tcpdump host victim and not arp
Frightening easy...
SysAdmins beware of that threat! If you have users on your network you can't trust (e.g. in universities)
use tools like
monitor the changes of the MAC / IP address tables.
Nov 15, 2006.
By default, every device on a wireless network has a unique address that's used to distinguish one
wireless network interface card (WNIC) from another. This address is called the Media Access Control
(MAC) address. In theory, since every WNIC has been pre-assigned a unique MAC address by the hardware
vendor, an access point can be set up to only allow a pre-selected list of WNICs to connect. For example,
the Linksys WAP11 includes a MAC filtering option in its software that allows an administrator to define
who can connect to the WLAN by listing all the allowed MAC addresses. Once you have the MAC address
of all the connecting WNICs, you can set up MAC filtering and enable it accordingly. This will stop
many connection attempts made by unauthorized addresses.
However, while in theory this is an excellent way to stop hackers from accessing your WLAN, there's
a serious flaw in MAC filtering: MAC addresses can be spoofed (faked) by changing WNIC settings. For
example, the Dell TrueMobile includes software that allows a hacker to alter his MAC address to any
address he chooses. Thus, this option is about as useful as trying to keep people from accessing a
chat room by restricting chat handle names. To bypass such a restriction, a person only has to change
his or her name.
Why would a software/hardware vendor want to allow a user to change a MAC address? Having the power
to adjust a MAC address can provide a network administrator with more tools to keep control over the
network. However, this increased power gives just as much control to an attacker. This is one example
of how the ancient power struggle between user needs and security often plays right into an attacker's
Regardless, if MAC filtering is an option, you should implement it on your WLAN. Just as with enabling
WEP, MAC filtering does require a modicum of sniffing and network expertise. Thus, it can also serve
as an intellectual barrier to most of the potential intruders of your wireless network. Note that if
your WLAN supports more than 100 users, managing the MAC control list could be a time-consuming chore.
MAC Filtering MAC Filtering is very important if you are on a wireless network. You are legally
responsible for what is done on your network. MAC Filtering allows you to setup your network in two
ways. MAC filtering usually allows you to prevent certain MAC addresses from accessing the internet.
It will also allow certain MAC Addresses to access the internet and deny all others. The second setup
is the one that you need to use. Most people take home their wireless router, plug it in, and never
bother to setup MAC filtering. This is really bad. If MAC filtering is not setup anyone can use their
network to access the internet. Hackers drive arround towns in their cars with their laptops trying
to connect to the internet. These wireless routers are setup to hand out ip addresses via DHCP, so
the second that a hacker gets in range they are on the net. The hacker is on your network what now?
Well he could sniff the traffic on your network to find out what you are doing. He could also watch
your network for credit card information or other vital financal information. That is not really likely.
Most likely he will start hacking other peoples networks from yours. This is a huge concern, because
it looks like you are attacking others. If you are in the states, you could have the FBI knocking on
your door. I know this for a fact because I assisted the FBI in some instances while I worked for an
isp. So here is fair warning! Setup MAC filtering now!
You can use MAC filtering to allow only certain MAC addresses to access the internet and prevent
all others. This is really easy to do. First off you need to find the MAC address of every computer
that will be connecting to the internet. Take a look at our
help with this. I'm planning on writing a how to for every router I can find on this. For instructions
on your specific router please visit our
Q1: What is the meaning of P flag in arp table listing
A: P is published entry flag. A system can be configured to publish 9advertize) an ARP entry
on behave of systems that cannot respond to ARP requests.
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