Linux Arpwatch

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Installing Arpwatch in Linux

By default, Arpwatch tool is not installed on any Linux distributions. We must install it manually using ‘yum‘ command on RHELCentOSFedora and ‘apt-get‘ on UbuntuLinux Mint and Debian.

# yum install arpwatch
$ sudo apt-get install arpwatch

Let’s focus on the some most important arpwatch files, the location of the files are slightly differ based on your operating system.

  1. /etc/rc.d/init.d/arpwatch : The arpwatch service for start or stop daemon.
  2. /etc/sysconfig/arpwatch : This is main configuration file…
  3. /usr/sbin/arpwatch : Binary command to starting and stopping tool via the terminal.
  4. /var/arpwatch/arp.dat : This is main database file where IP/MAC addresses are recorded.
  5. /var/log/messages : The log file, where arpwatch writes any changes or unusual activity to IP/MAC.

Type the following command to start the arpwatch service.

# chkconfig --level 35 arpwatch on
# /etc/init.d/arpwatch start
$ sudo chkconfig --level 35 arpwatch on
$ sudo /etc/init.d/arpwatch start
Arpwatch Commands and Usage

To watch a specific interface, type the following command with ‘-i‘ and device name.

# arpwatch -i eth0

So, whenever a new MAC is plugged or a particular IP is changing his MAC address on the network, you will notice syslog entries at ‘/var/log/syslog‘ or ‘/var/log/message‘ file.

# tail -f /var/log/messages
Sample Output
Apr 15 12:45:17 tecmint arpwatch: new station 172.16.16.64 d0:67:e5:c:9:67
Apr 15 12:45:19 tecmint arpwatch: new station 172.16.25.86 0:d0:b7:23:72:45
Apr 15 12:45:19 tecmint arpwatch: new station 172.16.25.86 0:d0:b7:23:72:45
Apr 15 12:45:19 tecmint arpwatch: new station 172.16.25.86 0:d0:b7:23:72:45
Apr 15 12:45:19 tecmint arpwatch: new station 172.16.25.86 0:d0:b7:23:72:45

The above output displays new workstation. If any changes are made, you will get following output.

Apr 15 12:45:17 tecmint arpwatch: changed station 172.16.16.64 0:f0:b8:26:82:56 (d0:67:e5:c:9:67)
Apr 15 12:45:19 tecmint arpwatch: changed station 172.16.25.86 0:f0:b8:26:82:56 (0:d0:b7:23:72:45)
Apr 15 12:45:19 tecmint arpwatch: changed station 172.16.25.86 0:f0:b8:26:82:56 (0:d0:b7:23:72:45)
Apr 15 12:45:19 tecmint arpwatch: changed station 172.16.25.86 0:f0:b8:26:82:56 (0:d0:b7:23:72:45)
Apr 15 12:45:19 tecmint arpwatch: changed station 172.16.25.86 0:f0:b8:26:82:56 (0:d0:b7:23:72:45)

You can also check current ARP table, by using following command.

# arp -a
Sample Ouput
tecmint.com (172.16.16.94) at 00:14:5e:67:26:1d [ether] on eth0
? (172.16.25.125) at b8:ac:6f:2e:57:b3 [ether] on eth0

If you want to send alerts to your custom email id, then open the main configuration file ‘/etc/sysconfig/arpwatch‘ and add the email as shown below.

# -u <username> : defines with what user id arpwatch should run
# -e <email>    : the <email> where to send the reports
# -s <from>     : the <from>-address
OPTIONS="-u arpwatch -e [email protected] -s 'root (Arpwatch)'"

Here is an example of an email report, when a new MAC is connected on.

        hostname: centos
      ip address: 172.16.16.25
       interface: eth0
ethernet address: 00:24:1d:76:e4:1d
 ethernet vendor: GIGA-BYTE TECHNOLOGY CO.,LTD.
       timestamp: Monday, April 15, 2012 15:32:29

Here is an example of an email report, when a IP changing his MAC address.

            hostname: centos
          ip address: 172.16.16.25
           interface: eth0
    ethernet address: 00:56:1d:36:e6:fd
     ethernet vendor: GIGA-BYTE TECHNOLOGY CO.,LTD.
old ethernet address: 00:24:1d:76:e4:1d
           timestamp: Monday, April 15, 2012 15:43:45
  previous timestamp: Monday, April 15, 2012 15:32:29 
               delta: 9 minutes

As you can see above, it records, HostnameIP addressMAC addressVendor name and timestamps. For more information, see the arpwatch man page by hitting ‘man arpwatch’ on the terminal.

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