Clearing arp cache in linux

Original oneliner

Be sure to do it all at once, so you don’t break network connectivity before you’re able to turn ARP back on.

Interface discovering copy-paste command

interfaces=$( arp -n | awk ' NR == 1  END > ' ); for interface in $interfaces; do echo "Clearing ARP cache for $interface"; sudo ip link set arp off dev $interface; sudo ip link set arp on dev $interface; done 

Note: The semicolons allow you to condense this command into a oneliner, but it looks terrible in a code block on SO.

Example output on Raspbian

[email protected]:~ $ arp -n Address HWtype HWaddress Flags Mask Iface 10.0.0.1 ether 58:19:f8:0d:57:aa C wlan0 10.0.0.159 ether 88:e9:fe:84:82:c8 C wlan0 [email protected]:~ $ interfaces=$( arp -n | awk ' NR == 1  END > '); for interface in $interfaces; do echo "Clearing ARP cache for $interface"; sudo ip link set arp off dev $interface; sudo ip link set arp on dev $interface; done Clearing ARP cache for wlan0 [email protected]:~ $ arp -n Address HWtype HWaddress Flags Mask Iface 10.0.0.159 ether 88:e9:fe:84:82:c8 C wlan0 

Solution 2

Your first solution works, it just takes a little time (5-10 seconds in my test on Kali) to go from «(incomplete)» to no entries.

Presumably it is in some sort of transitional state on its way to being deleted.

Solution 3

Your mentioned solution is the correct and safe approach to flush the ARP table:

If certain entries are changed into invalid, that’s temporary and part of the ARP protocol. The important aspect is that the mapping is gone, an ARP entry flagged as incomplete is not an IP-MAC entry on the table.

I’ve just tried this and in my case it immediately cleared the table and left no incomplete entries.

Solution 4

In certain system, ip command is not available.

[email protected]:~$ ip -bash: ip: command not found [email protected]:~$ 

So this the alternative of the ip link set arp off dev eth0; ip link set arp on dev eth0 command.

If you want to delete all entries from the table in a single command, use for loop like the following example.

[email protected]:~$ arp ? (10.0.0.1) at 00:00:00:aa:aa:12 [ether] on eth1 ? (172.168.0.3) at 00:00:00:aa:aa:11 [ether] on eth2 [email protected]:~$ 

REMOVING ARP WITH ARP -D COMMAND

[email protected]:~$ for i in 10.0.0.1 172.168.0.3; do sudo arp -d $i; done [email protected]:~$ 

AFTER: MAC ADDRESS REMOVED FROM THE ENTRIES WITH INCOMPLETE MESSAGE

[email protected]:~$ arp ? (10.0.0.1) at on eth1 ? (172.168.0.3) at on eth2 [email protected]:~$ 

That’s true, by deleting arp with arp -d command, the arp entries are still there with incomplete message.

To solve this problem, use ifconfig ethx up/down like this

[email protected]:~$ arp ? (10.0.0.1) at on eth1 ? (172.168.0.3) at on eth2 [email protected]:~$ 

DISABLE & ENABLE THE INTERFACES ETH1 & ETH2 IN A SINGLE COMMAND

[email protected]:~$ for i in 1 2; do sudo ifconfig eth$i down; sudo ifconfig eth$i up; done [email protected]:~$ 

TADAAA . PROBLEM SOLVED 🙂

Related videos on Youtube

blueFast

Updated on September 18, 2022

Comments

sudo ip -s -s neigh flush all 

Instead of clearing the arp cache they seem to just invalidate entries (they will appear as incomplete ). Even after some minutes, the ARP cache looks like:

$ arp -n Address HWtype HWaddress Flags Mask Iface 192.168.0.103 (incomplete) eth0 192.168.0.1 ether DE:AD:BE:EF:DE:AD C eth0 

(The MAC of the gateway has been refreshed — that is ok) How can I really clear the ARP cache, like in «delete all entries from the table»? I do not want to keep incomplete entries, I want them removed. Is this possible?

EDIT

» arp --version net-tools 1.60 arp 1.88 (2001-04-04) +I18N AF: (inet) +UNIX +INET +INET6 +IPX +AX25 +NETROM +X25 +ATALK +ECONET +ROSE HW: (ether) +ETHER +ARC +SLIP +PPP +TUNNEL -TR +AX25 +NETROM +X25 +FR +ROSE +ASH +SIT +FDDI +HIPPI +HDLC/LAPB +EUI64 » lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 14.04.2 LTS Release: 14.04 Codename: trusty » uname -a Linux polyphemus.xxx-net 3.13.0-46-generic #77-Ubuntu SMP Mon Mar 2 18:23:39 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux 

@MichaelMartinez because xy problem. It seems likely that question isn’t the root problem and there might be some underlying problem that the OP should be working on instead.

@EEAA: It is a bit long to explain. I am having troubles with my sip client in one of my routers, so I need to restore factory settings and reconfigure. For this I need to reconfigure my network topology, because restoring factory settings stops this router from playing nicely in my LAN. This happens often, and every time I have trouble locating the IP of the device. Since I am unable of tricking my DHCP server on giving fixed IPs, I have decided to create a personal mapping of MAC addresses to hostnames, so that I can automatically identify hosts by MAC, processing NMAP output.

For this I need first to manualy identify some important hosts by MAC address, but I am seeing too much rubbish in the ARP cache, which annoys me. So I want to clean it, but the clear operation does not do what it is supposed to do, which annoys me way more

@jeckyll2hide Please edit that information into your original answer, so more people can see it without digging into the comments.

The reason why this information is not in the question is because it is not relevant. I want a clean ARP cache. Full stop. You do not trust me that I want that, but that is your problem, not mine. I assure you: I want a clean ARP cache.

In case ARP (or the kernel?) does not support this (!), a simple «not possible» would suffice. Some rationale on «why» would be welcome.

Источник

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.

Address HWtype HWaddress Flags Mask Iface 192.168.1.1 (incomplete) eth0 192.168.1.2 ether 00:02:9b:a2:d3:f3 C eth0 192.168.1.3 ether 00:02:9b:d9:d1:a2 C eth0

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.

Did this article to clear the ARP cache help you as well? Wonderful! Become part of the community and share this on social media to let others know. Got questions or suggestions? Join us in the comments.

One more thing.

Keep learning

So you are interested in Linux security? Join the Linux Security Expert training program, a practical and lab-based training ground. For those who want to become (or stay) a Linux security expert.

Security scanning with Lynis and Lynis Enterprise

Run automated security scans and increase your defenses. Lynis is an open source security tool to perform in-depth audits. It helps with system hardening, vulnerability discovery, and compliance.

Continue reading

The purpose of the /etc/networks file

Linux Security Guide for Hardening IPv6

List network interfaces on Linux

Which Linux process is using a particular network port?

7 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 , Reply

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). Reply

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” Reply

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! Reply

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” Reply

I expected arp entries of things in use to be more or less immediately re-populated, as the next packet to them would initiate an arp query and response in a few milliseconds. This was not the case. It took about 2 minutes for everything to return to normal. So be advised. Reply

Источник