Address Resolution Protocol (ARP) hovers in the shadows of most networks. Because of its simplicity, by comparison to higher layer protocols, ARP rarely intrudes upon the network administrator's routine. All modern IP-capable operating systems provide support for ARP. The uncommon alternative to ARP is static link-layer-to-IP mappings.
ARP defines the exchanges between network interfaces connected to an Ethernet media segment in order to map an IP address to a link layer address on demand. Link layer addresses are hardware addresses (although they are not immutable) on Ethernet cards and IP addresses are logical addresses assigned to machines attached to the Ethernet. Subsequently in this chapter, link layer addresses may be known by many different names: Ethernet addresses, Media Access Control (MAC) addresses, and even hardware addresses. Disputably, the correct term from the kernel's perspective is "link layer address" because this address can be changed (on many Ethernet cards) via command line tools. Nevertheless, these terms are not realistically distinct and can be used interchangeably.
Address Resolution Protocol (ARP) exists solely to glue together the IP and Ethernet networking layers. Since networking hardware such as switches, hubs, and bridges operate on Ethernet frames, they are unaware of the higher layer data carried by these frames [9]. Similarly, IP layer devices, operating on IP packets need to be able to transmit their IP data on Ethernets. ARP defines the conversation by which IP capable hosts can exchange mappings of their Ethernet and IP addressing.
ARP is used to locate the Ethernet address associated with a desired IP
address. When a machine has a packet bound for another IP on a locally
connected Ethernet network, it will send a broadcast Ethernet frame
containing an ARP request onto the Ethernet. All machines with the same
Ethernet broadcast address will receive this packet
[10].
If a machine receives the ARP request and it hosts the IP requested,
it will respond with the link layer address on which it will receive
packets for that IP address.
N.B., the
arp_filter
sysctl will alter this behaviour
somewhat.
Once the requestor receives the response packet, it associates the MAC address and the IP address. This information is stored in the arp cache. The arp cache can be manipulated with the ip neighbor and arp commands. To learn how and when to manipulate the arp cache, see Section B.1, “arp”.
In Example 1.2, “Testing reachability of a locally connected host with
ping”, we used ping to
test reachability of masq-gw. Using a packet sniffer to capture
the sequence of packets on the Ethernet as a result of tristan's
attempt to ping, provides an example of ARP in flagrante
delicto. Consult the
example network map for a
visual representation of the network layout in which this traffic
occurs.
This is an archetypal conversation between two computers exchanging relevant hardware addressing in order that they can pass IP packets, and is comprised of two Ethernet frames.
Example 2.1. ARP conversation captured with tcpdump [11]
|
This broadcast Ethernet frame, identifiable by the
destination Ethernet address with all bits set
(ff:ff:ff:ff:ff:ff) contains an ARP request from | |
The ARP reply from
The machine which initiated the ARP request ( | |
| The final two packets in Example 2.1, “ARP conversation captured with tcpdump ” display the link layer header and the encapsulated ICMP packets exchanged between these two hosts. Examining the ARP cache on each of these hosts would reveal entries on each host for the other host's link layer address. |
This example is the commonest example of ARP traffic on an Ethernet. In summary, an ARP request is transmitted in a broadcast Ethernet frame. The ARP reply is a unicast response, containing the desired information, sent to the requestor's link layer address.
An even rarer usage of ARP is gratuitous ARP, where a machine announces its ownership of an IP address on a media segment. The arping utility can generate these gratuitous ARP frames. Linux kernels will respect gratuitous ARP frames [12].
Example 2.2. Gratuitous ARP reply frames
|
The frames generated in Example 2.2, “Gratuitous ARP reply frames” are ARP replies to a question never asked. This sort of ARP is common in failover solutions and also for nefarious sorts of purposes, such as ettercap.
Unsolicited ARP request frames, on the other hand, are broadcast ARP requests initiated by a host owning an IP address.
Example 2.3. Unsolicited ARP request frames
|
These two uses of arping can help diagnose Ethernet and ARP problems--particularly hosts replying for addresses which do not belong to them.
To avoid IP address collisions on dynamic networks (where hosts are turning on and off, connecting and disconnecting and otherwise changing IP addresses) duplicate address detection becomes important. Fortunately, arping provides this functionality as well. A startup script could include the arping utility in duplicate address detection mode to select between IP addresses or methods of acquiring an IP address.
Example 2.4. Duplicate Address Detection with ARP
|
Address Resolution Protocol, which provides a method to connect physical network addresses with logical network addresses is a key element to the deployment of IP on Ethernet networks.
In simplest terms, an ARP cache is a stored mapping of IP addresses with link layer addresses. An ARP cache obviates the need for an ARP request/reply conversation for each IP packet exchanged. Naturally, this efficiency comes with a price. Each host maintains its own ARP cache, which can become outdated when a host is replaced, or an IP address moves from one host to another. The ARP cache is also known as the neighbor table.
To display the ARP cache, the venerable and cross-platform arp admirably dispatches its duty. As with many of the iproute2 tools, more information is available via ip neighbor than with arp. Example 2.5, “ARP cache listings with arp and ip neighbor” below illustrates the differences in the output between the output of these two different tools.
Example 2.5. ARP cache listings with arp and ip neighbor
|
A major difference between the information reported by ip neighbor and arp is the state of the proxy ARP table. The only way to list permanently advertised entries in the neighbor table (proxy ARP entries) is with the arp.
Entries in the ARP cache are periodically and automatically
verified unless continually used. Along with
net/ipv4/neigh/$DEV/gc_stale_time,
there are a number of other parameters in
net/ipv4/neigh/$DEV which control the
expiration of entries in the ARP cache.
When a host is down or disconnected from the Ethernet, there is a
period of time during which other hosts may have an ARP cache entry
for the disconnected host. Any other machine may display a neighbor
table with the link layer address of the recently disconnected host.
Because there is a recently known-good link layer address on which
the IP was reachable, the entry will abide. At
gc_stale_time the state of the entry will change,
reflecting the need to verify the reachability of the link layer
address. When the disconnected host fails to respond ARP requests,
the neighbor table entry will be marked as
incomplete
Here are a the possible states for entries in the neighbor table.
Table 2.1. Active ARP cache entry states
| ARP cache entry state | meaning | action if used |
|---|---|---|
| permanent | never expires; never verified | reset use counter |
| noarp | normal expiration; never verified | reset use counter |
| reachable | normal expiration | reset use counter |
| stale | still usable; needs verification | reset use counter; change state to delay |
| delay | schedule ARP request; needs verification | reset use counter |
| probe | sending ARP request | reset use counter |
| incomplete | first ARP request sent | send ARP request |
| failed | no response received | send ARP request |
To resume, a host (192.168.99.7) in tristan's ARP cache on the
example network has just
been disconnected. There are a series of events which
will occur as tristan's ARP cache entry for 192.168.99.7 expires and
gets scheduled for verification. Imagine that the following commands
are run to capture each of these states immediately before state
change.
Example 2.6. ARP cache timeout
|
Before the entry has expired for 192.168.99.7, but after the
host has been disconnected from the network. During this
time, tristan will continue to send out Ethernet frames with
the destination frame address set to the link layer address
according to this entry.
| |
It has been gc_stale_time seconds since
the entry has been verified, so the state has changed to
stale.
| |
| This entry in the neighbor table has been requested. Because the entry was in a stale state, the link layer address was used, but now the kernel needs to verify the accuracy of the address. The kernel will soon send an ARP request for the destination IP address. | |
The kernel is actively performing address resolution for the
entry. It will send a total of
ucast_solicit frames to the last known
link layer address to attempt to verify reachability of the
address. Failing this, it will send
mcast_solicit broadcast frames before
altering the ARP cache state and returning an error to any
higher layer services.
| |
| After all attempts to reach the destination address have failed, the entry will appear in the neighbor table in this state. |
The remaining neighbor table flags are visible when initial ARP
requests are made. If no ARP cache entry exists for a requested
destination IP, the kernel will generate
mcast_solicit ARP requests until receiving an
answer.
During this discovery period, the ARP cache
entry will be listed in an incomplete state. If
the lookup does not succeed after the specified number of ARP
requests, the ARP cache entry will be listed in a
failed state. If the lookup does succeed, the
kernel enters the response into the ARP cache and resets the
confirmation and update timers.
After receipt of a corresponding ARP reply, the kernel enters the response into the ARP cache and resets the confirmation and update timers.
For machines not using a static mapping for link layer and IP addresses, ARP provides on demand mappings. The remainder of this section will cover the methods available under linux to control the address resolution protocol.
Complete ARP suppression is not difficult at all. ARP suppression can be accomplished under linux on a per-interface basis by setting the noarp flag on any Ethernet interface. Disabling ARP will require static neighbor table mappings for all hosts wishing to exchange packets across the Ethernet.
To suppress ARP on an interface simply use ip link set dev $DEV arp off as in Example B.7, “Using ip link set to change device flags” or ifconfig $DEV -arp as in Example C.5, “Setting interface flags with ifconfig”. Complete ARP suppression will prevent the host from sending any ARP requests or responding with any ARP replies.
When a linux box is connected to a network segment with multiple network cards, a potential problem with the link layer address to IP address mapping can occur. The machine may respond to ARP requests from both Ethernet interfaces. On the machine creating the ARP request, these multiple answers can cause confusion, or worse yet, non-deterministic population of the ARP cache. Known as ARP flux [13], this can lead to the possibly puzzling effect that an IP migrates non-deterministically through multiple link layer addresses. It's important to understand that ARP flux typically only affects hosts which have multiple physical connections to the same medium or broadcast domain.
This is a simple illustration of the problem in a network where a server has two Ethernet adapters connected to the same media segment. They need not have IP addresses in the same IP network for the ARP reply to be generated by each interface. Note the first two replies received in response to the ARP broadcast request. These replies arrive from conflicting link layer addresses in response to this request. Also notice the greater time required for the sending and receiving hosts to process the broadcast ARP request frames than the unicast frames which follow (probes two and three).
Example 2.7. ARP flux
|
There are four solutions to this problem. The common solution for
kernel 2.4 harnesses the
arp_filter
sysctl, while the common solution for kernel 2.2 takes
advantage of the
hidden
sysctl. These two solutions alter the behaviour of ARP on a
per interface basis and only if the functionality has been enabled.
Alternate solutions which provide much greater control of ARP (possibly documented here at a later date) include Julian Anastasov's ip arp tool and his noarp route flag. While these tools were conceived in the course of the Linux Virtual Server project, they have practical application outside this realm.
One method for preventing ARP flux involves the use of
net/ipv4/conf/$DEV/arp_filter. In
short, the use of arp_filter causes the recipient
(in the
case below,
real-server) to perform a route lookup to
determine the interface through which to send the
reply, instead of the default behaviour
(shown above), replying
from all Ethernet interfaces which receive the request.
The arp_filter solution can have unintended
effects if the only route to the destination
is through one of the network cards. In
Example 2.8, “Correction of ARP flux with
conf/$DEV/arp_filter”, real-client will
demonstrate this. This instructive example should highlight
the shortcomings of the arp_filter solution in
very complex networks where finer-grained control is required.
In general, the arp_filter solution
sufficiently solves the ARP flux problem. First, hosts do not
generate ARP requests for networks to which they do not have a
direct route (see
Section 4.2, “Routing to Locally Connected Networks”) and second, when such a route
exists, the host normally
chooses a source
address in the same network as the destination. So, the
arp_filter solution is a good general solution,
but does not adequately address the occasional need for more control
over ARP requests and replies.
Example 2.8. Correction of ARP flux with
conf/$DEV/arp_filter
|
Set the sysctl variables to enable the
arp_filter functionality. After this,
you might expect that ARP replies for 10.10.20.67 would only
advertise the link layer address on eth0 (00:80:c8:e8:1e:fc).
| |
Here is the expected behaviour. Only one reply comes in for
the IP 10.10.20.67 after the arp_filter
sysctl has been enabled. The reply originates from the
interface on real-server which actually hosts the IP
address. Note that the source address on the ARP queries is
10.10.20.33, and that the ARP query causes real-server to
perform a route lookup on 10.10.20.33 to choose an interface
from which to send the reply.
| |
Here, real-client requests the link layer address of the
host 192.168.100.1, but the source IP on the request packet
(chosen according to the
rules for source
address selection) is 10.10.20.33. When
real-server looks up a route to this destination, it
chooses its eth0, and replies with the link layer address of
its eth0. Conventional networking needs should not run
afoul of this oddity of the arp_filter
ARP flux prevention technique.
| |
| Remove the entry in the neighbor table before testing again. | |
| By adding an IP address in the same network as the intended destination (which would be rather common where multiple IP networks share the same medium or broadcast domain), the kernel can now select a different source address for the ARP request packets. | |
Note the source address of the ARP queries is now
192.168.100.2. When real-server performs a route lookup
for the 192.168.100.0/24 destination, the chosen path is
through eth1. The ARP reply packets now have the correct
link layer address.
|
In general, the arp_filter solution should
suffice, but this knowledge can be key in determining whether or not
an alternate solution, such as an
ARP filtering solution
are necessary.
The ARP flux problem can also be combatted with a kernel patch by Julian Anastasov, which was incorporated into the 2.2.14+ kernel series, but never into the 2.4+ kernel series. Therefore, the functionality may not be available in all kernels.
The sysctl net/ipv4/conf/$DEV/hidden toggles
the generation of ARP replies for requested IPs. It marks an
interface and all of its IP addresses invisible to other
interfaces for the purpose of ARP
requests. When an ARP request arrives on any interface, the kernel
tests to see if the IP address is locally hosted anywhere on the
machine. If the IP is found on any interface, the kernel will
generate a reply.
Since this is not always desirable, the hidden
sysctl can be employed. This prevents the kernel from finding the
IP address when testing to see what IP addresses are locally hosted.
The kernel can always find IPs hosted on the interface on which the
packet arrived, but it cannot find addresses which are
hidden.
As shown in
Example 2.9, “Correction of ARP flux with
net/$DEV/hidden”, not only can ARP flux be
corrected, but sensitive information about the IP addresses
available on a linux box can be safeguarded
[14].
This makes the hidden sysctl useful for
preventing unwanted IP disclosure via ARP on multi-homed hosts,
in addition to preventing ARP flux on hosts connected to the
same network medium.
Example 2.9. Correction of ARP flux with
net/$DEV/hidden
|
These are two examples of methods to prevent ARP flux. Other alternatives for correcting this problem are documented in Section 2.3, “ARP filtering”, where much more sophisticated tools are available for manipulation and control over the ARP functions of linux.
[9] Some networking equipment vendors have built devices which are sold as high performance switches and are capable of performing operations on higher layer contents of Ethernet frames. Typically, however, a switching device is not capable of operating on IP packets.
[10] The kernel uses the Ethernet broadcast address configured on the link layer device. This is rarely anything but ff:ff:ff:ff:ff:ff. In the extraordinary event that this is not the Ethernet broadcast address in your network, see Section B.3.7, “Changing hardware or Ethernet broadcast address with ip link set”.
[11] tcpdump is one of a number of utilities for watching packets visible to an interface. For further introduction to tcpdump, see Section G.5, “tcpdump”.
[12] I have repeatedly tested using arping in gratuitous ARP mode, and have found that linux kernels appear to respect gratuitous ARP. This is a surprise. Does anybody have ideas about this? Must research!
[13] I have seen it called names other than ARP flux--anybody out there heard of this called anything besides ARP flux?
[14] Consider a masquerading firewall which answers ARP requests on a public segment for IPs hosted on an internal interface. This amounts to inadvertent exposure of internal addressing, and can be used by an attacker as part of a data-gathering or reconaissance operation on a network.