Network setup - How to - Routers - Cisco - Linux

DHCP (Dynamic Host Configuration Protocol) is a network protocol that allows customers to get their network settings automatically. In this case, I will explain how to configure a small DHCP server for a small network, assigned by a range.

The first thing to do is make a copy of the original file of DHCP, to ensure that we always have a copy of the original file as a backup.

linux# cp /etc/dhcpd.conf /etc/dhcpd.conf.original

For the DHCP service is running, you must specify the network interfaces in which the service is running. For this, you must edit the /etc/sysconfig/dhcpd file.

linux# vi /etc/sysconfig/dhcpd

## Path:        Network/DHCP/DHCP server
## Description: DHCP server settings
## Type:        string
## Default:     ""
## ServiceRestart: dhcpd
#
# Interface(s) for the DHCP server to listen on.
#
# Instead of the interface name, the name of its configuration can be given.
# If the configuration file is named
#    /etc/sysconfig/network/ifcfg-eth-id-00:50:fc:e4:f2:65
# then id-00:50:fc:e4:f2:65 would be suitable to identify the configuration.
#
# Examples: DHCPD_INTERFACE="eth0"
#           DHCPD_INTERFACE="eth0 eth1 eth2 tr0 wlan0"
#           DHCPD_INTERFACE="internal0 internal1"
#           DHCPD_INTERFACE="id-00:50:fc:e4:f2:65 id-00:a0:24:cb:cc:5c wlan0"
#
DHCP_INTERFACE="eth0"

In the "DHCP_INTERFACE" label should specify the interfaces that will be used to provide the service.

The configuration file "dhcpd.conf"

The DHCP is basically divided into two sections. The first is the general options and are global. The second and last define the network segment where the DHCP will reside. There may be more than one section of this type. The parameters here writings are more global preference.


authoritative;

This statement allows you to define the DHCP server is authoritative for the defined network segment and can send warning messages to misconfigured clients.


default-lease-time 21600;

This standard defines how many seconds will "rent" an IP address to a computer that request before it has to request an extension


max-lease-time 43200;

Defines the maximum time that a device can retain an IP number assigned by the DHCP server without applying for it an extension (max-lease-time).


ddns-update-style none;

This parameter controls whether the server will attempt, or not, make a DNS update when a loan is confirmed.


subnet ……… netmask ………

Defines a network with subnet mask


range

Select the address range used by the DHCP daemon to assign IP addresses to clients who consult. For this example, are all the addresses between 172.16.3.2 and 172.16.3.10


option domain-name-servers dns1.intranet.labtest;

Enter up to three DNS servers. These are responsible for resolving IP addresses to hostnames (and vice versa).


option domain-name "intranet.labtest";

Defines the default domain of your network


option routers 172.16.3.1;

Defines where to be sent data packets that can not be delivered to the local network (due to the direction of the source host and the destination host and the subnet mask). This router usually acts as the gateway to the Internet for small networks.


option subset-mask 255.255.255.0;

Provides customer network mask to deliver.


Below is an example configuration file: /etc/dhcpd.conf

linux# vi /etc/dhcpd.conf

#
# Section Global parameters
#

authoritative;
default-lease-time 21600;
max-lease-time 43200;
ddns-update-style none;

#
# Section Network Configuration
#

subnet 172.16.3.0 netmask 255.255.255.0 {
 range 172.16.3.2 172.16.3.16;
 option domain-name-servers dns1.intranet.labtest;
 option domain-name "intranet.labtest";
 option routers 172.16.3.1;
 option subset-mask 255.255.255.0;
}

After this brief setup, you should be able to activate the DHCP daemon with the command rcdhcpd start or /etc/init.d/dhcpd start

It is also possible to control the syntax of the configuration using the "rcdhcpd check-syntax" command. If there is a problem and the server gives an error check with "tail-f /var/log/messages".

The Dynamic Host Configuration Protocol server, DHCP is a protocol that allows individual devices on a network to get their own network configuration information such as IP address, subnet mask, gateway, DNS servers, etc. . The main purpose is to make it easier to manage large networks.

Without the help of a DHCP server would have to manually set each IP address of each computer on the network. A DHCP server monitors and distributes IP addresses in a local area network by assigning an IP address to each computer that is attached to the Local Area Network.

There are three methods of assignment in the DHCP protocol: Manual assignment, automatic assignment and dynamic assignment. In the following example, we analyze two of these cases.

This document explains in simple terms how to configure a Linux server to assign addresses via DHCP to multiple vlans.


For the example of the graph, it is considered that a server is configured Linux1 routing packets, that is configured as a router, as well as function as a DHCP server.

In small or medium networks can use the same equipment as DHCP server and as a router, however when the network grows, it is advisable to separate these functions, on different devices. Always remember that the switch port where the DHCP server is connected must be configured as Trunk (IEEE 802.1Q).

In the example also sees a DNS server, it may be a server configured in Linux or Windows, this server belongs to vlan 100 and have configured the IP address 172.16.100.2


VLAN2 Network Data

IP Number VLAN2 network: 172.16.2.0
Subnet Mask: 255.255.255.0
Gateway: 172.16.2.254 (IP Address Interface VLAN2 on the server)
Name Server: 172.16.100.2

VLAN3 Network Data

IP Number VLAN3 network: 172.16.3.0
Subnet Mask: 255.255.255.0
Gateway: 172.16.3.254 (Interface IP Address VLAN3 on the server)
Name Server: 172.16.100.2


Configuration File /etc/dhcpd.conf

# Configuration for the network 172.16.3.0/24
# The 172.16.3.0 network will be configured statically,
# ie always assigned the same IP address to computers.
# ------------------------------------------------------

subnet 172.16.3.0 netmask 255.255.255.0
{
option subnet-mask 255.255.255.0;
option broadcast-address 172.16.3.255;
option domain-name "test.com";
option domain-name-servers 172.16.100.2
option routers 172.16.3.254;

# IP allocation to each PC

host pc01
{
option host-name "pc01.test.com";
hardware ethernet 00:06:AB:AB:01:01;
fixed-address 172.16.3.1;
}

host pc02
{
hardware ethernet 00:06:AB:AB:02:02;
fixed-address 172.16.3.2;
}
}

# Settings for network 172.16.2.0/24

# Network data vlan2 dynamically allocated
subnet 172.16.2.0 netmask 255.255.255.0
{
range 172.16.2.10 172.16.2.20;
option subnet-mask 255.2255.255.0;
option broadcast-address 172.16.2.255;
option domain-name "test.com";
option domain-name-servers 172.16.100.2;
option routers 172.16.2.254;
}

Configuring network interfaces where DHCP service work

A good safety measure is to make the dhcpd service only works through the network interface used by the LAN, that in the case of multiple network devices.

Edit the file /etc/sysconfig/dhcpd and add as parameter argument value DHCPDARGS eth0, eth1, eth2, or in our case eth0.2, eth0.3.

# Command line options here

DHCPDARGS=eth0.2, eth0.3

Note: For example, do not add the interface eth0.100 because normally address assignment on the servers is not done via DHCP.

Then just start the service is dhcp.

/sbin/service dhcpd start or in the case of opensuse /etc/init.d/dhcpd start