How to find out if Ubuntu is using DHCP (Ubuntu 12.04 LTS GUI)

Question

How can I find out if my IP address is DHCP, Fixed or Dynamic?

I need to tell my network administrator what the IP address is, that my Virtual Machine is using. I know the numbers, but I don't know if it is fixed or not.

I have tried: ifconfig and that returned my IP address.

Answer 1

Right click on the Network Manager icon on Ubuntu top panel and select edit. Go to Wired Network or Wireless Network tab and select the network name. Click on the edit button and go to IPv4 settings tab on the new window. If the method is Automatic (DHCP) you are using dhcp.

Other method is cat /var/log/syslog and check for some thing like below

DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 6
DHCPOFFER from 10.100.1.254
DHCPREQUEST on eth0 to 255.255.255.255 port 67
DHCPACK from 10.100.1.254

If you have some thing similar to above. You are using DHCP (IP addresses could be different)

Answer 2

Use command:

cat /etc/network/interfaces

It will show you all interfaces and which one of them are using dhcp.

Whether dhcp is fixed or dynamic you must check on your Virtual Machine configuration. If you are using VM Ware then under edit you got Virtual Networking properties. There you will be able to check if for particular virtual interface you got dhcp reservations for specific machines. If you did not checked it yet then we can assume you did not changed default configuration which is dynamic. You can setup reservation if you wish.

Details of NAT configuration in VM Ware environment: click.

If bridged connection:

f you use bridged networking, your virtual machine needs to have its own identity on the network. For example, on a TCP/IP network, the virtual machine needs its own IP address. Your network administrator can tell you whether IP addresses are available for your virtual machine and what networking settings you should use in the guest operating system. Generally, your guest operating system may acquire an IP address and other network details automatically from a DHCP server, or you may need to set the IP address and other details manually in the guest operating system.

If you use bridged networking, the virtual machine is a full participant in the network. It has access to other machines on the network and can be contacted by other machines on the network as if it were a physical computer on the network.

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Be aware that if the host computer is set up to boot multiple operating systems and you run one or more of them in virtual machines, you need to configure each operating system with a unique network address. People who boot multiple operating systems often assign all systems the same address, since they assume only one operating system will be running at a time. If you use one or more of the operating systems in a virtual machine, this assumption is no longer true.

Answer 3

I'm using debian but directories should be the same or similar. Check if you have the directory /var/lib/dhcp. Then:

ls -lrt /var/lib/dhcp/

You should see files named /var/lib/dhcp/dhclient-random-numbers-eth1.lease. Look for the most recent file associated with the interface you're interested in and open it up:

cat /var/lib/dhcp/dhclient-...-eth1.lease

The output should be something like this:

lease {
  interface "eth1";
  fixed-address 192.168.10.12;
  rebind 4 2012/08/02 03:56:17;
  expire 4 2012/08/02 04:41:17;
}

If /var/lib/dhcp directory doesn't exist or if it is empty, you're most likely not getting your IPs from DHCP.