How can I tell if my Linux server has a static or dynamic IP address assigned to it?

RBMike · 12-14-2009, 05:38 PM

Hi there,

I work for a small company and have been tasked with managing a server that runs linux. My boss has asked me to assign a fixed IP to the server. The server is not new, it has been up and running for months.

I have a hunch that it already has a fixed IP address, but how can I tell for sure? I can run the "ifconfig" command, and it gives me the IP address and some other information. I'd post the results here if I could, but it's on a different machine.

So, is there a command I can run to see if my server has a static or dynamic IP? I'm a total beginner so it would be helpful if you don't get too technical.

Thanks in advance.

Mike

chrism01 · 12-14-2009, 05:48 PM

Start by telling us which distribution (aka name) and version of Linux you have. That'll help enormously.
If you don't already know, open a cmd line terminal and try try the following cmds

uname -a

cat /etc/*release*

and post the results.
Also, post the output from ifconfig.

worm5252 · 12-14-2009, 06:47 PM

You can use cat to view a file. In a command window run this command as root

Code:

cat /etc/network/interfaces

I for example have a static IP so this is what mine looks like

Code:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet static
  address 10.1.1.20
  netmask 255.255.255.0
  network 10.1.1.0
  broadcast 10.1.1.255
  gateway 10.1.1.1

If you have a dynamic IP (DHCP) it will look like this

Code:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

evo2 · 12-15-2009, 12:29 AM

Quote:

Originally Posted by worm5252

You can use cat to view a file. In a command window run this command as root

Code:

cat /etc/network/interfaces

that is only for a Debian style network config.

Chrism asked the OP what distro he is using: best to wait for the reply.

Cheers,

Evo2.

evo2 · 12-15-2009, 12:31 AM

Quote:

Originally Posted by chrism01

cat /etc/*release*

Depending on the distro, may need:

Code:

cat /etc/*version*

Cheers,

Evo2.

resetreset · 12-15-2009, 02:18 AM

Well I don't know about this being distro-dependent - how about this: use ifconfig to note down your IP address, then turn off and turn on your net connection, and (preferably the next day or something) run ifconfig again and see whether you have the same IP - this should solve your problem no?
But this obviously presupposes that you won't get the same IP twice which isn't guaranteed, and may very well be the case.

evo2 · 12-15-2009, 02:24 AM

Quote:

Originally Posted by resetreset

Well I don't know about this being distro-dependent - how about this: use ifconfig to note down your IP address, then turn off and turn on your net connection, and (preferably the next day or something) run ifconfig again and see whether you have the same IP - this should solve your problem no?
But this obviously presupposes that you won't get the same IP twice which isn't guaranteed, and may very well be the case.

I think stopping and restarting the networking is probably something the OP would rather avoid. Also not only is it possible that the IP will be the same even in the DHCP case, but very likely since DHCP clients usually request the same IP that they had previously.

A distro indep. way could just be looking in /var/log/messages.

Cheers,

Evo2.

RBMike · 12-15-2009, 10:29 AM

Quote:

Originally Posted by chrism01

Start by telling us which distribution (aka name) and version of Linux you have. That'll help enormously.
If you don't already know, open a cmd line terminal and try try the following cmds

uname -a

cat /etc/*release*

and post the results.
Also, post the output from ifconfig.

Thanks for everyone's help. Since I don't know how to email the results from that machine, the only thing I could think of was to take a photo of the screen. Here are the results of the three commands.

This machine has been shut down and restarted a few times, and the IP address has always been the same: 192.168.1.109. That's why I suspect that it is configured for static IP.

Thanks,

Mike

Zmyrgel · 12-15-2009, 11:22 AM

Check if you have /etc/sysconfig/network-scripts/ifcfg-eth0 file and whats its contents are.
It seems to be where CentOS keeps its network configuration options.

RBMike · 12-15-2009, 11:42 AM

Quote:

Originally Posted by Zmyrgel

Check if you have /etc/sysconfig/network-scripts/ifcfg-eth0 file and whats its contents are.
It seems to be where CentOS keeps its network configuration options.

I ran this command and it sure looks like it has been set as static. Is that what the "BOOTPROTO=static" means?

If so, then my issue will be resolved. Thank you!

Mike

worm5252 · 12-15-2009, 11:52 AM

Well there you go. It says in your /etc/sysconfig/network-scripts/ifcfg-eth0 file BOOTPROTO=STATIC. That tells you right there it is a static (Fixed) IP.

chrism01 · 12-15-2009, 05:05 PM

And you don't need to keep taking photos, just copy & paste eg from an ssh session into your browser, if necessary.

BTW, Centos (free version of RHEL) 4.4 is very old.
They are currently on 4.8 and the latest is 5.4

Here's the RHEL 4.x Admin guide (which will work for Centos) http://www.linuxtopia.org/online_boo...ide/index.html