Posts Tagged ‘broadcast’

How to scan for DHCP available servers in a Network range on Linux and FreeBSD

Thursday, December 8th, 2011

GNU / Linux and FreeBSD had a nifty little program (tool) called dhcping . dhcpingsend a DHCP request to DHCP server to see if it’s up and running. dhcping is also able to send a request to DHCP servers on a whole network range and therefore it can e asily be used as a scanner to find any available DHCP servers in a network.
This makes dhcping a nmap like scanner capable to determine if dhcp servers are in a network 😉
To scan an an entire network range with dhclient and find any existing DHCP servers:

noah:~# dhcping -s 255.255.255.255 -r -v
Got answer from: 192.168.2.1
received from 192.168.2.1, expected from 255.255.255.255
no answer

In above’s output actually my Dlink wireless router returns answer to the broadcast DHCP LEASE UDP network requests of dhcping .
On a networks where there is no DHCP server available, the requests dhcping -s 255.255.255.255 -r -v returns:

noah:~# dhcping -s 255.255.255.255 -r -v
no answer

This article was inspired by a post, I’ve red by a friend (Amridikon), so thx goes to him.

 

How to configure manually static IP address on Debian GNU/Linux / How to fix eth0 interface not brought up with error (networking restart is deprecated)

Friday, July 29th, 2011

I’ve recently had to manually assign a static IP address on one of the servers I manage, here is how I did it:             

debian:~# vim /etc/network/interfaces

Inside the file I placed:

# The primary network interface
allow-hotplug eth0
auto eth0
iface eth0 inet static address 192.168.0.2 netmask 255.255.255.0 broadcast 192.168.0.0 gateway 192.168.0.1 dns-nameservers 8.8.8.8 8.8.4.4 208.67.222.222 208.67.220.220

The broadcast and gateway configuration lines are not obligitory.
dns-nameservers would re-create /etc/resolv.conf file with the nameserver values specified which in these case are Google Public DNS servers and OpenDNS servers.

Very important variable is allow-hotplug eth0
If these variable with eth0 lan interface is omitted or missing (due to some some weird reason), the result would be the output you see from the command below:

debian:~# /etc/init.d/networking restart
Running /etc/init.d/networking restart is deprecated because it may not enable again some interfaces ... (warning).
Reconfiguring network interfaces...

Besides the /etc/init.d/networking restart is deprecated because it may not enable again some interfaces … (warning). , if the allow-hotplug eth0 variable is omitted the eth0 interface would not be brough up on next server boot or via the networking start/stop/restart init script.

My first reaction when I saw the message was that probably I’ll have to use invoke-rc.d, e.g.:
debian:~# invoke-rc.d networking restart
Running invoke-rc.d networking restart is deprecated because it may not enable again some interfaces ... (warning).

However as you see from above’s command output, running invoke-rc.d helped neither.

I was quite surprised with the inability to bring my network up for a while with the networking init script.
Interestingly using the command:

debian:~# ifup eth0

was able to succesfully bring up the network interface, whether still invoke-rc.d networking start failed.

After some wondering I finally figured out that the eth0 was not brought up by networking init script, because auto eth0 or allow-hotplug eth0 (which by the way are completely interchangable variables) were missing.

I added allow-hotplug eth0 and afterwards the networking script worked like a charm 😉