How to find IP address conflicts in Linux

Time:2020-6-28

You all know what an IP address is, don’t you? They are assigned to devices on the network to represent them. They are assigned through the DHCP server and change frequently. There are now two IP addresses. The dynamic one often changes (once a few days), while the static one is static just like its name, which means they won’t change.

Sometimes this leads to some conflict. When a dynamic IP is assigned and another network device already has the same IP. Or there are multiple DHCP servers with IP assigned on the same network subnet. If you have connectivity problems and assume it’s caused by IP conflicts, you can scan them with a tool called ARP scan.

The tool will send ARP (address resolution protocol) packets on the local network to collect addresses. If more than one MAC address claims to have the same IP address, then there is a conflict.

To install ARP scan on Ubuntu or Debian, type:

bash/shell CodeCopy content to clipboard
  1. sudo apt-get install arp-scan     

For Fedora, CentOS or RedHat:

bash/shell  CodeCopy content to clipboard
  1. $ sudo yum install arp-scan   

To detect IP conflicts, run the following command:

bash/shell CodeCopy content to clipboard
  1. sudo arp-scan –I eth0 -l  

Output example:

bash/shell CodeCopy content to clipboard
  1. 192.168.1.10   00:1b:a9:63:a2:4c       BROTHER INDUSTRIES, LTD.   
  2. 192.168.1.30   00:1e:8f:58:ec:49       CANON INC.   
  3. 192.168.1.33   00:25:4b:1b:10:20       Apple, Inc   
  4. 192.168.1.37   10:9a:dd:55:d7:95       Apple Inc   
  5. 192.168.1.38   20:c9:d0:27:8d:56       (Unknown)   
  6. 192.168.1.39   d4:85:64:4d:35:be       Hewlett Packard   
  7. 192.168.1.39   00:0b:46:e4:8e:6d       Cisco (DUP: 2)   
  8. 192.168.1.40   90:2b:34:18:59:c0       (Unknown)   

In this case, the IP of 192.168.1.39 conflicts because it appears twice.

The above is the whole content of this article, I hope to help you learn.