IP


A simple way to find which DHCP server gave you an IP

Recently, we were trying to find which DHCP server was responding to the messages on the network. Using a DHCP-enabled client on a Fedora 26 GNU/Linux we grepped the contents of journalctl to find the DHCP acknowledgment messages (DHCPACK) and figure out the IP of the DHCP server.

The command we used was the following:

sudo journalctl | grep DHCPACK;

And it gave us results such as the ones below:

[[email protected] ~]$ sudo journalctl | grep DHCPACK
Nov 12 13:08:28 sys-net dhclient[578]: DHCPACK from 10.1.101.252 (xid=0x80ec760c)
Nov 12 13:08:34 sys-net dhclient[720]: DHCPACK from 10.1.101.252 (xid=0x2ed6486f)
Nov 12 11:51:19 sys-net dhclient[1248]: DHCPACK from 10.1.101.252 (xid=0xe3dd491c)
Nov 12 12:02:09 sys-net dhclient[1407]: DHCPACK from 10.1.101.252 (xid=0x1fa42c2d)
Nov 12 12:11:03 sys-net dhclient[1508]: DHCPACK from 10.1.101.252 (xid=0x91c3990a)
Nov 12 12:14:06 sys-net dhclient[1607]: DHCPACK from 10.1.101.252 (xid=0x57ebb515)
Nov 12 12:19:27 sys-net dhclient[1710]: DHCPACK from 10.1.101.252 (xid=0x5450c250)
Nov 12 12:19:39 sys-net dhclient[1776]: DHCPACK from 10.1.101.252 (xid=0x2c38d517)
Nov 12 12:39:53 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:40:51 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:41:51 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:42:44 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:43:33 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:44:31 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:46:20 sys-net dhclient[2053]: DHCPACK from 192.168.1.1 (xid=0xbb006001)

It is important to use sudo or else you will not be seeing messages from other users and the system. As, only users in groups ‘adm’, ‘systemd-journal’, ‘wheel’ can see all messages.

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How to set a static IP Address from the Command Line in GNU/Linux using ip addr and ip route

Assuming you want to make the following changes to the network device eth0

  1. Change the IP to the static value 192.168.1.2
  2. Set the Subnet Mask to 255.255.255.0
  3. Set the Default Gateway for the device to be 192.168.1.1

and you want to avoid using ifconfig and route that are obsolete you can perform these changes using the following two commands

sudo ip addr add 192.168.1.2/24 dev eth0;
sudo ip route add default via 192.168.1.1 dev eth0;

Please note that the netmask is given in CIDR notation (it is the /24 right after the IP of the device in the ip addr command).

A subnet mask (netmask) is a bitmask that encodes the prefix length in quad-dotted notation: 32 bits, starting with a number of 1 bits equal to the prefix length, ending with 0 bits, and encoded in four-part dotted-decimal format: 255.255.255.0. A subnet mask encodes the same information as a prefix length, but predates the advent of CIDR. In CIDR notation, the prefix bits are always contiguous, whereas subnet masks may specify non-contiguous bits.

From Wikipedia: https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing


Fedora/Bash: Get the IP of enp0s3

Following is a small snippet that will print on screen the IP of enp0s3 (or any other device if you change the name) while in Fedora.
As you will see, it is not a very sound solution as it depends on the structure of the output of ifconfig enp0s3.

Nevertheless is works (for Fedora at least)! 🙂

ifconfig enp0s3 | grep "inet " | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//' | cut -d ' ' -f 2

What this line does is: first it prints out the configuration information for enp0s3, then finds the line that contains the inet, then using sed it will trim the result (in other words, it will remove all leading and all trailing white-space from the pipe), finally cut gets the second column of the data after separating the line using the space symbol.

The Fedora version that was used for this tutorial is

$cat /etc/fedora-release 
Fedora release 23 (Twenty Three)

The version of ifconfig for this tutorial is

$ifconfig --version
net-tools 2.10-alpha

In case you want to assign the IP of enp0s3 to a variable, you can easily do as follows

IP=`ifconfig enp0s3 | grep "inet " | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//' | cut -d ' ' -f 2`;

Ubuntu/Bash: Get the IP of eth0

Following is a small snippet that will print on screen the IP of eth0 while in Ubuntu (Both server and desktop versions).
As you will see, it is not a very sound solution as it depends on the structure of the output of ifconfig eth0.

Nevertheless is works (for Ubuntu at least)! 🙂

ifconfig eth0 | grep "inet addr" | cut -d ':' -f 2 | cut -d ' ' -f 1

What this line does is: first it prints out the configuration information for eth0, then finds the line that contains the inet addr, using cut it gets the second column of the data after separating the line using the : symbol. Right now in the pipe we will have something similar to this 192.168.1.37 Bcast, so we need to filter out the last part as well. We do the last filtering by using cut again, this time by getting the first column while using the space character as the delimiter.

The Ubuntu version that was used for this tutorial is

$lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 14.04.4 LTS
Release:	14.04
Codename:	trusty

The version of ifconfig for this tutorial is

$ifconfig --version
net-tools 1.60
ifconfig 1.42 (2001-04-13)

In case you want to assign the IP of eth0 to a variable, you can easily do as follows

ETH0=`ifconfig eth0 | grep "inet addr" | cut -d ':' -f 2 | cut -d ' ' -f 1`;