Standards/Ethernet/arp

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Address Resolution Protocol (ARP) is a protocol used to find a hardware address associated to a given logical address. As such, it is used to find the MAC address associated to a given IP address.

Manually add entry to ARP Table in Windows

Display the following command to have a list of the ARP Tabels of all Interfaces

arp -a
Interface: 192.168.56.1 --- 0x13
  Internet Address      Physical Address      Type
  192.168.56.255        ff-ff-ff-ff-ff-ff     static
  224.0.0.2             01-00-5e-00-00-02     static
  224.0.0.22            01-00-5e-00-00-16     static
  224.0.0.251           01-00-5e-00-00-fb     static
  224.0.0.252           01-00-5e-00-00-fc     static
  239.255.255.250       01-00-5e-7f-ff-fa     static
  239.255.255.253       01-00-5e-7f-ff-fd     static
  255.255.255.255       ff-ff-ff-ff-ff-ff     static

Add an ARP entry statically

#       IP-Address      MAC-Address
arp -s 192.168.111.1 e4-af-a1-39-02-01 

ARP frames

The ARP broadcast frames are sent to the local network and the device with the corresponding IP address answers to it. They are sent to MAC broadcast address FF:FF:FF:FF:FF:FF with Ethertype 0806.

ARP frames can be filtered in Wireshark with the help of the expression:

arp

An ARP frame for IP/MAC pairing is as follows:

offset (bytes) 0 1 2 3
0 HTPYE = 0x0001 PTPYE = 0x0800
4 HLEN = 0x06 PLEN = 0x04 OPER
8 SHA = source MAC address
12 SHA (end) SPA = source IP address
16 SPA (end) THA = destination MAC address
20 THA (end)
24 TPA = destination IP address

For a query:

  • OPER is 0x0001
  • THA is 00:00:00:00:00:00
  • TPA is the IP address which is to be matched to a MAC address

For a response

  • OPER is 0x0002

Host discovery

Query

Typing in a terminal:

ping 153.109.5.142

causes the computer to send the following ARP query (who has 153.109.5.142?):

ff ff ff ff ff ff 40 6c  8f 54 b8 79 08 06 00 01
08 00 06 04 00 01 40 6c  8f 54 b8 79 99 6d 05 b7
00 00 00 00 00 00 99 6d  05 8e  

This frame contains the Ethernet header, with multicast broadcast MAC address FF:FF:FF:FF:FF:FF, source MAC address and ARP Ethertype (0806):

ff ff ff ff ff ff 40 6c  8f 54 b8 79 08 06

ARP header with element sizes:

                                            00 01
08 00 06 04

request type:

            00 01

sender MAC and IP address:

                  40 6c  8f 54 b8 79 99 6d 05 b7

empty MAC address and target IP address:

00 00 00 00 00 00 99 6d  05 8e

Answer

To this, the selected device should respond something like "153.109.5.142 is at 00:0D:B9:0E:33:F4". The corresponding frame is:

40 6c 8f 54 b8 79 00 0d  b9 0e 33 f4 08 06 00 01
08 00 06 04 00 02 00 0d  b9 0e 33 f4 99 6d 05 8e
40 6c 8f 54 b8 79 99 6d  05 b7 

This frame contains Ethernet header (MAC addresses and Ethertype):

40 6c 8f 54 b8 79 00 0d  b9 0e 33 f4 08 06 

ARP header (response):

                                           00 01
08 00 06 04 00 02

response MAC and IP addresses:

                  00 0d  b9 0e 33 f4 99 6d 05 8e

destination MAC and IP addresses:

40 6c 8f 54 b8 79 99 6d  05 b7
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