How to fix MAC Address with IP Address on DHCP server.

Today, I would like to monitor my internet network bandwidth. When my internet network is slow I often have questions from my boss “Why my internet network is slow, who are using a lot of internet bandwidth?”. I cannot answer these questions because I don’t have informations of current network status.

First step, I think I should know which IP Address is using a lot of internet bandwidth and second step I should know who is owner that IP Address.

The answer of the first step , I will use ntop software to answer this question. For the second step, I will fix all MAC Address of Laptop’s wireless card with IP Address on a DHCP Server. And this is my solutions :

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1. Get a MAC Address from Client’s laptop such as  A1:B2:C3:A1:B2:C3

2. Add the MAC Address into the file /etc/dhcpd.conf such as

ddns-update-style interim
ignore client-updates

subnet 192.168.1.0 netmask 255.255.255.0 {

   # The range of IP addresses the server
   # will issue to DHCP enabled PC clients
   # booting up on the network

   range 192.168.1.201 192.168.1.220;

   # Set the amount of time in seconds that
   # a client may keep the IP address

   default-lease-time 86400;
   max-lease-time 86400;

   # Set the default gateway to be used by
   # the PC clients

   option routers 192.168.1.1;
   # Don't forward DHCP requests from this
   # NIC interface to any other NIC
   # interfaces

   option ip-forwarding off;

   # Set the broadcast address and subnet mask
   # to be used by the DHCP clients

   option broadcast-address 192.168.1.255;
   option subnet-mask 255.255.255.0;

   # Set the NTP server to be used by the
   # DHCP clients

   option ntp-servers 192.168.1.100;

   # Set the DNS server to be used by the
   # DHCP clients

   option domain-name-servers 192.168.1.100;

   # If you specify a WINS server for your Windows clients,
   # you need to include the following option in the dhcpd.conf file:

   option netbios-name-servers 192.168.1.100;

   # You can also assign specific IP addresses based on the clients'
   # ethernet MAC address as follows (Host's name is "laser-printer":

   host user-one {
      hardware ethernet A1:B2:C3:A1:B2:C3;
      fixed-address 192.168.1.222;
   }
}
#
# List an unused interface here
#
subnet 192.168.2.0 netmask 255.255.255.0 {
}

3. Check your configuration file is correct :

# /usr/sbin/dhcpd -t -cf /etc/dhcpd.conf

4. Restart your DHCP Service :

# service dhcpd restart

5. Set DHCP relay in your CISCO switch points to your DHCP Server (Assume your IP Address of DHCP Server is 192.168.1.201)

$ conf t
$ conf t > interface vlan 10
$ conf t > ip address 192.168.1.1 255.255.255.0
$ conf t > no ip directed-broadcast
$ conf t > ip helper-address 192.168.1.201
$ conf t > end

6. You can check all IP Addresses which receive from DHCP Server in file /var/lib/dhcp/dhcpd.leases
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References :
1. http://www.redhatmagazine.com/2008/08/05/linux-dhcp-server-static-ips-are-gone-in-60-seconds/
2. http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch08_:_Configuring_the_DHCP_Server
3. http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a00800f0804.shtml

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