Manual:IP/ARP

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Applies to RouterOS: 2.9, v3, v4 +

Summary

Sub-menu: /ip arp
Standards: ARP RFC 826


Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. Address Resolution Protocol is used to map OSI level 3 IP addresses to OSI level 2 MAC addreses. Router has a table of currently used ARP entries. Normally the table is built dynamically, but to increase network security, it can be partially or completely built statically by means of adding static entries.


Properties

Property Description
address (IP; Default: ) IP address to be mapped
interface (string; Default: ) Interface name the IP address is assigned to
mac-address (MAC; Default: 00:00:00:00:00:00) MAC address to be mapped to


Read only properties:

Property Description
dhcp (yes | no) Whether ARP entry is added by DHCP server
dynamic (yes | no) Whether entry is dynamically created
invalid (yes | no) Whether entry is not valid


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Note: Maximal number of ARP entries is 8192.


ARP Modes

It is possible to set several ARP modes in interface configuration .....

Disabled

If ARP feature is turned off on the interface, i.e., arp=disabled is used, ARP requests from clients are not answered by the router. Therefore, static arp entry should be added to the clients as well. For example, the router's IP and MAC addresses should be added to the Windows workstations using the arp command:

C:\> arp -s 10.5.8.254  00-aa-00-62-c6-09

Enabled

This mode is enabled by default on all interfaces. ARPs will be discovered automatically and new dynamic entries will be added to ARP table.

Proxy ARP

A router with properly configured proxy ARP feature acts like a transparent ARP proxy between directly connected networks.

This behaviour can be usefull, for example, if you want to assign dial-in (ppp, pppoe, pptp) clients IP addresses from the same address space as used on the connected LAN.

Image10002.gif

Lets look at example setup from image above. Host A (172.16.1.2) on Subnet A wants to send packets to Host D (172.16.2.3) on Subnet B. Host A has a /16 subnet mask which means that Host A believes that it is directly connected to all 172.16.0.0/16 network (the same LAN). Since the Host A believes that is directly connected it sends an ARP request to the destination to clarify MAC address of Host D. (in case when Host A finds that destination IP address is not from the same subnet it send packet to default gateway.)

Host A broadcasts an ARP request on Subnet A:

Info from packet analyzer software:

 No.     Time   Source             Destination       Protocol  Info

 12   5.133205  00:1b:38:24:fc:13  ff:ff:ff:ff:ff:ff  ARP      Who has 173.16.2.3?  Tell 173.16.1.2


Packet details:

Ethernet II, Src: (00:1b:38:24:fc:13), Dst: (ff:ff:ff:ff:ff:ff)
    Destination: Broadcast (ff:ff:ff:ff:ff:ff)
    Source: (00:1b:38:24:fc:13)
    Type: ARP (0x0806)
Address Resolution Protocol (request)
    Hardware type: Ethernet (0x0001)
    Protocol type: IP (0x0800)
    Hardware size: 6
    Protocol size: 4
    Opcode: request (0x0001)
    [Is gratuitous: False]
    Sender MAC address: 00:1b:38:24:fc:13
    Sender IP address: 173.16.1.2
    Target MAC address: 00:00:00:00:00:00
    Target IP address: 173.16.2.3

With this ARP request, Host A (172.16.1.2) is asking Host D (172.16.2.3) to send its MAC address. The ARP request packet is then encapsulated in an Ethernet frame with the MAC address of Host A as the source address and a broadcast (FF:FF:FF:FF:FF:FF) as the destination address. Layer 2 broadcast means that frame will be sent to all hosts in the same layer 2 broadcast domain which includes the ether0 interface of the router, but does not reach Host D, because router by default does not forward layer 2 broadcast.

Since the router knows that the target address (172.16.2.3) is on another subnet but it can reach Host D, it replies with its own MAC address to Host A.

No.     Time   Source            Destination         Protocol   Info

13   5.133378  00:0c:42:52:2e:cf  00:1b:38:24:fc:13   ARP        172.16.2.3 is at 00:0c:42:52:2e:cf

Packet details:

Ethernet II, Src: 00:0c:42:52:2e:cf, Dst: 00:1b:38:24:fc:13
   Destination: 00:1b:38:24:fc:13
   Source: 00:0c:42:52:2e:cf
   Type: ARP (0x0806)
Address Resolution Protocol (reply)
   Hardware type: Ethernet (0x0001)
   Protocol type: IP (0x0800)
   Hardware size: 6
   Protocol size: 4
   Opcode: reply (0x0002)
   [Is gratuitous: False]
   Sender MAC address: 00:0c:42:52:2e:cf
   Sender IP address: 172.16.1.254 
   Target MAC address: 00:1b:38:24:fc:13
   Target IP address: 172.16.1.2 

This is the Proxy ARP reply that the router sends to Host A. Router sends back unicast proxy ARP reply with its own MAC address as the source address and the MAC address of Host A as the destination address, by saying "send these packets to me, and I'll get it to where it needs to go."


When Host A receives ARP response it updates its ARP table, as shown:

C:\Users\And>arp -a

Interface: 173.16.2.1 --- 0x8
  Internet Address      Physical Address      Type
  173.16.1.254          00-0c-42-52-2e-cf    dynamic
  173.16.2.3            00-0c-42-52-2e-cf    dynamic
  173.16.2.2            00-0c-42-52-2e-cf    dynamic

After MAC table update, Host A forwards all the packets intended for Host D (172.16.2.3) directly to router interface ether0 (00:0c:42:52:2e:cf) and the router forwards packets to Host D. The ARP cache on the hosts in Subnet A is populated with the MAC address of the router for all the hosts on Subnet B. Hence, all packets destined to Subnet B are sent to the router. The router forwards those packets to the hosts in Subnet B.

Multiple IP addresses by host are mapped to a single MAC address (the MAC address of this router) when proxy ARP is used.


Proxy ARP can be enabled on each interface individually with command arp=proxy-arp:

Setup proxy ARP:

 [admin@MikroTik] /interface ethernet> set 1 arp=proxy-arp

 [admin@MikroTik] /interface ethernet> print

 Flags: X - disabled, R - running
   #    NAME                 MTU   MAC-ADDRESS         ARP
   0  R ether1              1500  00:30:4F:0B:7B:C1 enabled
   1  R ether2              1500  00:30:4F:06:62:12 proxy-arp
 [admin@MikroTik] interface ethernet>

Reply Only

If arp property is set to reply-only on the interface, then router only replies to ARP requests. Neighbour MAC addresses will be resolved using /ip arp statically, but there will be no need to add the router's MAC address to other hosts' ARP tables like in case if arp is disabled.

Local Proxy Arp

Since v6.38 - if the arp property is set to local-proxy-arp on an interface, then the router performs proxy ARP to/from this interface only. I.e. for traffic that comes in and goes out of the same interface. In a normal LAN, the default behaviour is for two network hosts to communicate directly with each other, without involving the router.

With local-proxy-arp enabled, the router will respond to all client hosts with the router's own interface MAC address instead of the other host's MAC address.

E.g. If Host A (192.168.88.2/24) queries for the MAC address of Host B (192.168.88.3/24), the router would respond with its own MAC address. In other words, if local-proxy-arp is enabled, the router would assume responsibility for forwarding traffic between Host A (192.168.88.2 and Host B 192.168.88.3). All the arp cache entries on Hosts A and B will reference the router's MAC address. In this case the router is performing local-proxy-arp for the entire subnet 192.168.88.0/24.

Gratuitous ARP

It is possible to create Gratuitous ARP requests in RouterOS. To do so you must use the Traffic-Generator tool, below is an example how to generate a Gratuitous ARP request to update the ARP table on a remote device:

/tool traffic-generator inject interface=ether2 \
data="ffffffffffff4c5e0c14ef78080600010800060400014c5e0c14ef780a057a01ffffffffffff0a057a01000000000000000000000000000000000000"

You must change the MAC address (4c5e0c14ef78) and the IP address (0a057a01) to your router's addresses. The IP address and the MAC address must be from the device that requests an ARP table update. You also need to specify through which interface (ether2) you want to send the Gratuitous ARP request. Make sure that receiving device supports Gratuitous ARP requests.