What is arp cache in linux

How to clear the ARP cache on Linux?

There are several reasons when you might need to clear your ARP cache. There are two common ways on Linux systems, typically using the arp or ip utility. Depending on your Linux distribution and the availability, we suggest using the ip tool.

Clearing cache with ip

Newer Linux distributions have the ip utility. The ip tool has a more advanced way to clear out the full ARP cache.

The first -s will provide a more verbose output. By adding one more, we can select the neighbor table. The neighbor table with the ip command equals both the ARP and NDISC cache. Note that the -s options are not available on all versions of the ip command. If it not supported for your version of ip, then simply remove them from the command.

The output of the flush all command will produce the following output.

The ARP cache is cleared, with verbose output

Clearing cache with arp command

The arp utility does not accept an option to clear the full cache. Instead, it allows to flush out entries found with the -d option.

After deleting, have a look with the arp utility again to see the new list:

The output of this command will typically show the active ARP entries.

The 192.168.1.1 entry now shows as incomplete, which means the ARP entry will be refreshed when it is needed again.

Conclusion

Depending on your distribution, the ip utility is quicker if you want to flush out the full ARP cache. For individual entries, the arp tool will do the job as quickly. Both tools are available for most distributions, including Arch Linux, CentOS, Debian, Fedora, RHEL, and Ubuntu.

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5 comments

I need to apply flush-clean the arp table, and to have one option as Winsock for the case of win7 and if exist clean the register as Ccleaner, because I have the problem ;
ubuntu@ubuntu:

$ netstat -nat
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN
tcp6 0 0 ::1:631 . * LISTEN
tcp6 1 0 ::1:53003 ::1:631 CLOSE_WAIT
How can to resolve this case , in other case appear following: I think that need to clean , by the “listen”
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 127.0.1.1:53 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN
tcp 0 0 172.252.37.7:46799 94.31.29.192:80 ESTABLISHED
tcp 0 0 172.252.37.7:51130 91.189.94.232:443 ESTABLISHED
tcp 0 0 172.252.37.7:45273 91.189.95.69:80 ESTABLISHED
tcp 0 0 172.252.37.7:44964 149.210.134.182:443 ESTABLISHED
tcp 0 0 172.252.37.7:45270 91.189.95.69:80 ESTABLISHED
tcp 0 0 172.252.37.7:60107 64.233.176.95:80 TIME_WAIT
tcp 0 0 172.252.37.7:35899 216.58.219.110:80 TIME_WAIT
tcp 0 78 172.252.37.7:40483 216.58.219.72:443 LAST_ACK
tcp 0 0 172.252.37.7:59036 216.58.219.67:80 TIME_WAIT
tcp 0 78 172.252.37.7:38157 64.233.185.94:443 LAST_ACK
tcp 0 0 172.252.37.7:45275 91.189.95.69:80 ESTABLISHED
tcp 0 0 172.252.37.7:51127 91.189.94.232:443 ESTABLISHED
tcp 0 0 172.252.37.7:45271 91.189.95.69:80 ESTABLISHED
tcp 0 0 172.252.37.7:41876 216.58.219.68:80 ESTABLISHED
tcp 0 0 172.252.37.7:43539 64.233.185.154:80 ESTABLISHED
tcp 0 0 172.252.37.7:45272 91.189.95.69:80 ESTABLISHED
tcp 0 0 172.252.37.7:39388 24.139.135.147:80 ESTABLISHED
tcp 0 0 172.252.37.7:60106 64.233.176.95:80 TIME_WAIT
tcp 0 0 172.252.37.7:51131 91.189.94.232:443 ESTABLISHED
tcp 0 0 172.252.37.7:37724 216.58.219.78:80 TIME_WAIT
tcp 0 0 172.252.37.7:59708 216.58.219.66:80 ESTABLISHED
tcp 0 0 172.252.37.7:51586 173.194.219.95:80 TIME_WAIT
tcp 0 0 172.252.37.7:51128 91.189.94.232:443 ESTABLISHED
tcp 0 0 172.252.37.7:37227 216.58.219.98:80 TIME_WAIT
tcp 0 0 172.252.37.7:45274 91.189.95.69:80 ESTABLISHED
tcp 0 0 172.252.37.7:51587 173.194.219.95:80 TIME_WAIT
tcp 0 0 172.252.37.7:35809 64.233.176.94:80 ESTABLISHED
tcp 0 0 172.252.37.7:59035 216.58.219.67:80 TIME_WAIT
tcp 0 0 172.252.37.7:51132 91.189.94.232:443 ESTABLISHED
tcp 0 0 172.252.37.7:51129 91.189.94.232:443 ESTABLISHED
tcp 0 0 172.252.37.7:35666 216.58.219.110:80 TIME_WAIT
tcp 0 0 172.252.37.7:34426 173.194.219.94:80 ESTABLISHED
tcp6 0 0 ::1:631 . * LISTEN
tcp6 1 0 ::1:53003 ::1:631 CLOSE_WAIT

Thanks for your attention ,

These are your active connections (to your web server software). ARP is a protocol one level below these network connections. For details for flushing the ARP table, see the article on how to do that. For easily resetting the active connections, reload your web server daemon (Apache, nginx etc). The ports which state “LISTEN”, have a daemon running (53 = DNS, 631 = SAMBA or CUPS).

The statement “The second one defines the neighbor table” is not accurate. Both -s are for verbosity (providing 2 increases it). The basic command is simply “ip neigh flush all”

Depending on your version of the ip utility, you may not have the -s option available. The text have been extended to reflect that. Thanks for the feedback!

ip neigh flush all does not flush the cache completely, i.e. it does not delete the entries from the neighbor table. Instead it only clears the cached MAC addresses in the neighbor table, i.e. it sets all entries to state FAILED. But the entries with IP and IPv6 address are still kept in the cache. In former times the kernel expired entries after some time without usage, but unfortunately, this is not done anymore and there seems to be no way to remove entries manually.

Even a normal user could fill the neighbor cache with lots of entries which will stay until the next reboot. E.g. with “for n in <1..255>; do ping -c1 10.0.0.$n; done”

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Linux arp command

On Linux operating systems, the arp command manipulates or displays the kernel’s IPv4 network neighbour cache. It can add entries to the table, delete one, or display the current content.

ARP stands for Address Resolution Protocol, which is used to find the address of a network neighbor for a given IPv4 address.

Installing arp

Arp is part of the net-tools package. For example, on systems that use APT for package management, it can be installed with apt-get:

Syntax

Modes

arp with no mode specifier prints the current content of the table. It is possible to limit the number of entries printed, by specifying a hardware address type, interface name or host address.

arp -d address deletes an ARP table entry. Root privilege is required to do this. The entry is found by IP address. If a hostname is given, it will be resolved before looking up the entry in the ARP table.

is used to set up a new table entry. The format of the hw_addr parameter is dependent on the hardware class, but for most classes one can assume that the usual presentation can be used. For the Ethernet class, this is 6 bytes in hexadecimal, separated by colons. When adding proxy arp entries (that is those with the publish («pub«) flag set a netmask may be specified to proxy arp for entire subnets. This is not good practice, but is supported by older kernels because it can be useful. If the temp flag is not supplied entries will be permanent stored into the ARP cache. To simplify setting up entries for one of your network interfaces, you can use the «arp -Ds address ifname» form. In that case the hardware address is taken from the interface with the specified name.

Options

-v, —verbose	Display information verbosely.
-n, —numeric	shows numerical addresses instead of trying to determine symbolic host, port or user names.
-H type, —hw-Type type	When setting or reading the ARP cache, this optional parameter tells arp which class of entries it should check. The default value of this parameter is ether (i.e., hardware code 0x01 for IEEE 802.3 10 Mbps Ethernet). Other values might include network technologies such as ARCnet (arcnet), PROnet (pronet), AX.25 (ax25) and NET/ROM (netrom).
-a [hostname], —all [hostname]	Displays the entries of the specified hosts. If the hostname parameter is not used, all entries display. Hostnames are displayed using alternate BSD-style output format (with no fixed columns).
-d hostname, —delete hostname	Remove any entry for the specified host. This can be used if the indicated host is brought down, for example.
-D, —use-device	Instead of a hw_addr, the given argument is the name of an interface. arp uses the MAC address of that interface for the table entry. This is usually the best option to set up a proxy ARP entry to yourself.
-e	Shows entries in default (Linux) style.
-i If, —device If	Select an interface. When dumping the ARP cache, only entries matching the specified interface will be printed. When setting a permanent or temp ARP entry this interface will be associated with the entry; if this option is not used, the kernel will guess based on the routing table. For pub entries, the specified interface is the interface on which ARP requests will be answered. NOTE: This has to be different from the interface that the IP datagrams will be routed. NOTE: As of kernel 2.2.0 it is no longer possible to set an ARP entry for an entire subnet. Linux instead does automagic proxy arp when a route exists and it is forwarding. Also, the dontpub option that is available for delete and set operations cannot be used with 2.4 and newer kernels.
-s hostname hw_addr, —set hostname	Manually create an ARP address mapping entry for host hostname with hardware address set to hw_addr class, but for most classes one can assume that the usual presentation can be used. For the Ethernet class, this is 6 bytes in hexadecimal, separated by colons. When adding proxy arp entries (that is those with the publish flag set) a netmask may be specified to proxy arp for entire subnets. This is not good practice, but is supported by older kernels because it can be useful. If the temp flag is not supplied entries will be permanent stored into the ARP cache. NOTE: As of kernel 2.2.0 it is no longer possible to set an ARP entry for an entire subnet. Linux instead does automagic proxy arp when a route exists and it is forwarding.
-f file name, —file file name	Similar to the -s option, only this time the address info is taken from file file name set up. The name of the data file is very often /etc/ethers, but this is not official. If no file name is specified /etc/ethers is used as default. The format of the file is simple; it only contains ASCII text lines with a hostname, and a hardware address separated by whitespace. Additionally, the pub, temp and netmask flags can be used.

Notes

In all places where a hostname is expected, one can also enter an IP address in dotted-decimal notation.

As a special case for compatibility, the order of the hostname and the hardware address can be exchanged.

Each complete entry in the ARP cache will be marked with the C flag. Permanent entries are marked with M and published entries have the P flag.

Examples

Display network card and Ethernet connectivity.

Related commands

ifconfig — View or modify the configuration of network interfaces.
ip — Display and manipulate information about routing, devices, policy routing and tunnels.
netstat — Print information about network connections, routing tables, and more.
route — Display and manipulate the IP routing table.

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