DHCP

Dynamic Host Configuration Protocol

lets a client to “lease” a variety of network and administrative parameters (IP address and netmask, gateway, DNS server, syslog host, WINS server, X font server, proxy server, NTP server, TFTP server, …) => autoconfiguration at boot time
clients must report back to the server periodically to renew their leases
backward-compatible replacement of bootp
server listens on udp 67, answers through udp 68
often paired with DNS role
SOHO broadband routers can serve as DHCP servers (and also as NAT routers, switches)
break-even point for using it – 6 - 12 computers
ISC DHCP server – most common; dhcp, dhcp-server, dhcp3-server, isc-dhcp-server
Dnsmasq – DHCP + DNS, good for small networks

How DHCP works

Client broadcasts “Help! Who am I?” (on generic all-ones broadcast address as it doesn’t know its netmask yet)
DHCP server negotiates with the client the IP address and other networking parameters
When the client’s lease time is half over, it tries to renew its lease

Setting up a DHCP

Configure /etc/dhcp/dhcpd.conf file
Start the DHCP server in debug mode to verify that the server is working: /usr/sbin/dhcpd -d -f
To start the server for actual use, enter /etc/init.d/isc-dhcp-server start

/etc/dhcp/dhcpd.conf:

## Parameters -- describe general configuration
ddns-update-style none;

option domain-name "ianet";
option domain-name-servers 195.146.128.60, 192.168.128.1;

default-lease-time 600;
max-lease-time 7200;

log-facility local7;

## Declarations

## Dynamic addresses
subnet 192.168.128.0 netmask 255.255.255.0 {
  range 192.168.128.10 192.168.128.100;
  range 192.168.128.150 192.168.128.200;
  # Default GW
  option routers 192.168.128.1;
}

# Dummy entry for external interface -- every subnet must be declared, even
#  if no DHCP service is provided on it    
subnet 209.180.251.0 netmask 255.255.255.0 {
}


## Fixed addresses (not the same as static addresses)
host neptune {
  hardware ethernet 0a:1b:2c:3d:4e:5f;
  fixed-address 192.168.128.11;
}

## bootp support
host bootpneptune {
  hardware ethernet 0a:1b:2c:3d:4e:5f;
  fixed-addresses 192.168.128.105;
  filename "/tftpboot/bootpneptune.boot";
}

To find out MAC - IP address mapping: ping -c 1 <ip-addr>; /sbin/arp <ip-addr>.

dhcpd.leases

all leases the DHCP server has given out

/var/lib/dhcp/dhcpd.leases:

lease 192.168.128.17 {
    starts 5 2004/01/02 10:53:18;
    ends 5 2004/01/02 20:13:57;
    hardware ethernet 00:02:2d:5e:74:8c;
    client-hostname "lookfar.example.com";
}

DHCP relay

If you have multiple network segments (with routers in between the segments):

run multiple DHCP servers
run the DHCP server on the router
configure the router to route DHCP broadcast (Cisco: ip-helper address)
run DHCP relay agent (must be installed on one computer on each subnet): dhcrelay 172.27.15.2
- it’s the address of DHCP server on remote network

Source

Roderick W. Smith: LPIC-2 (2011)
ULSAH