Arp . They are the abbreviations in English of Address Resolution Protocol ( Protocol of resolution of directions). It is a protocol for link layer responsible for finding the hardware address ( Ethernet MAC ) corresponding to a specific IP address.
Summary
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- 1 Specification
- 2 Use
- 3 ARP tables
- 4 Operation I
- 5 Operation II
- 6 Procedures
- 7 Sources
Specification
ARP is documented in RFC (Request For Comments) 826.
Use
ARP is used in 4 cases regarding communication between 2 hosts:
- When 2 hosts are on the same network and one wants to send a packet to another.
- When 2 hosts are on different networks and must use a gateway / router to reach another host.
- When a router needs to send a packet to a host through another router.
- When a router needs to send a packet to a host on the same network.
ARP tables
The philosophy is the same as we would have to locate Mr. “X” among 150 people: ask everyone by name, and Mr. “X” will answer us. Thus, when “A” receives a message with an IP source address and it does not have that address in its ARP table, it will send its ARP frame to the broadcast (physical) address, with the IP of which it wants to know its physical address . Then, the computer whose IP address matches the one asked will respond to “A” by sending it its physical address. At this point “A” can add the entry for that IP to its ARP table. The entries in the table are deleted from time to time, since the physical addresses of the network can change (Ex: if a network card breaks down and needs to be replaced, or simply a user on the network changes its IP address ).
Operation I
If A wants to send a frame to the IP address of B (same network ), it will look at its ARP table to put in the frame the physical destination address corresponding to the IP of B. In this way, when the frame reaches all of them, they will not have to undo it to check if the message is for them, but instead it is done with the physical address.
Operation II
If A wants to send a message to C (a node that is not on the same network ), the message must go off the network . Thus, A sends the frame to the outgoing physical address of the router. This physical address will be obtained from the IP of the router , using the ARP table. If this entry is not in the table, it will send an ARP message to that IP (it will reach everyone), so that it can answer it by indicating its physical address.
Once in the router , it will consult its routing table, obtaining the next node (hop) to reach the destination, and it outputs the message through the corresponding interface. This is repeated for all nodes, until reaching the last router, which is the one that shares the medium with the destination host. Here the process changes: the router interface will have to find out the physical address of the destination IP that has reached it. It does this by looking at its ARP table , and if the corresponding IP entry does not exist, it obtains it by multicasting.
Procedures
View the current ARP cache.
harp
It should return something similar to the following, in the case of a single system:
m254.alcancelibre.org (192.168.1.254) at 00: 14: 95: 97: 27: E9 [ether] on eth0
When it comes to a proxy server, the table can look like this:
m051.redlocal.net (10.1.1.51) at 00: 13: 20: D0: 09: 1E [ether] on eth1m046.redlocal.net (10.1.1.46) at 00: 0F: 1F: B1: 71: 14 [ether] on eth1m073.redlocal.net (10.1.1.73) at 00: 11: 25: F6: 93: F1 [ether] on eth1m070.redlocal.net (10.1.1.70) at 00: 11: 25: F6: A2: 52 [ether] on eth1m040.redlocal.net (10.1.1.40) at 00: 0D: 60: 6E: 27: 34 [ether] on eth1m036.redlocal.net (10.1.1.36) at 00: 0D: 60: 6E: 25: FB [ether] on eth1m011.redlocal.net (10.1.1.11) at 00: 11: 2F: C7: D0: D7 [ether] on eth1
The arp command accepts several more options. If you want to display the information in Linux style, use the -e parameter. example:
arp -e
The above should return output similar to the following:
Address HWtype HWaddress Flags Mask Ifacem051.redlocal.net ether 00: 13: 20: D0: 09: 1E C eth1m046.redlocal.net ether 00: 0F: 1F: B1: 71: 14 C eth1m073.redlocal.net ether 00: 11: 25: F6: A2: 52 C eth1m070.redlocal.net ether 00: 11: 25: F6: 95: 8E C eth1m040.redlocal.net ether 00: 0D: 60: 6E: 26: 6F C eth1m036.redlocal.net ether 00: 11: 25: F6: 5F: 81 C eth1
If you want to see the above in numerical format, use the -n parameter. example:
arp -n
The above should return something similar to the following:
Address HWtype HWaddress Flags Mask Iface10.1.1.46 ether 00: 0F: 1F: B1: 71: 14 C eth110.1.1.70 ether 00: 11: 25: F6: A2: 52 C eth110.1.1.73 ether 00: 11: 25: F6: 93: F1 C eth110.1.1.40 ether 00: 0D: 60: 6E: 27: 34 C eth110.1.1.34 ether 00: 0D: 60: 6E: 26: 6F C eth1
If you want to specify a particular interface, use the -i parameter followed by the name of the interface. Example:
arp -i eth0
The above should return something similar to the following, in the case of a single system:
Address HWtype HWaddress Flags Mask Ifacem254.alcancelibre.org ether 00: 14: 95: 97: 27: E9 C eth0
If you want to add a record manually, it can be done using the -s parameter followed by the name of a host and the corresponding MAC address. Example:
arp -s m200.redlocal.net 00: 08: A1: 84: 18: AD
If you want to delete a record from the table, just use the -d parameter followed by the host name to delete. Example:
arp -d m200.redlocal.net
To clear the entire cache, you can use a loop like the following:
for i in `arp -n | awk ‘{print $ 1}’ | grep -v Address`doarp -d $ idone
The previous script asks to create the variable i from arp with the -n option to return the numeric addresses, showing through awk only the first column of the generated table, and eliminating the Address string. This generates a list of IP addresses that are assigned as values of the variable i in the loop, where each of these IP addresses is removed using arp -d.
The purpose of clearing the ARP cache is to allow correcting the table records in certain scenarios where, for example, a server or workstation was turned on with an IP address that is already in use.