Network setup - How to - Routers - Cisco - Linux

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