DHCP: how a device gets its address

The idea

A laptop and a phone join a home network and ask for their settings; dhcpServer lends them, while the printer's were typed by hand. homeRouter, the router, is their way out to a web server. See how DHCP works, message by message.

Steps

1. An address nobody typed

   Typing settings on every device that joins does not scale, so nobody set up this laptop. Read its routing table and DNS server, then the printer's, set by hand.

   laptop:~# ip route
   laptop:~# cat /etc/resolv.conf
   printer:~# ip route

2. Discover, Offer, Request, ACK

   A device with no address cannot be sent anything, so asking for one takes a special exchange. Make the laptop ask again while the server records, and find the four DHCP-Message lines.

   laptop:~# dhcp-renew eth0
   laptop:~# ip -4 addr show eth0

3. The server's side

   Why .11 again, not any free address? The server decides, so its settings explain it. Print the DHCP server's settings and its lease list, then play a visitor from the printer.

   dhcpServer:~# cat /etc/dnsmasq.conf
   dhcpServer:~# cat /var/lib/misc/dnsmasq.leases
   printer:~# nmap --script broadcast-dhcp-discover -e eth0

4. Renewing a lease

   An address is only lent, for 120 seconds here, so the laptop must keep renewing it. Renew the lease now, while the server records. Then the server stops: check the address and fetch the page.

   laptop:~# dhcp-renew eth0
   laptop:~# ip -4 addr show eth0
   laptop:~# curl -s --max-time 5 http://webServer.lab/

5. Joining while the server is down

   A device that joins during an outage meets the server's silence. The server is still off, and the lab made the laptop leave: now it joins again. Record its card while it asks, then try the page.

   laptop:~# dhcp-renew eth0
   laptop:~# ip -4 addr show eth0
   laptop:~# curl -sS --max-time 5 http://webServer.lab/

6. An address it gives itself

   With no answer, a device can still reach its neighbours by giving itself an address from 169.254.0.0/16. Check with ARP that 169.254.77.77 is free, put it on the card, then read the routing table.

   laptop:~# arping -D -c 2 -I eth0 169.254.77.77
   laptop:~# ip addr add 169.254.77.77/16 dev eth0
   laptop:~# ip route

7. The server comes back

   Outages end, and a network should heal without anyone touching each device. The DHCP server starts again; nobody touches the laptop: wait for its address, then fetch the page.

   laptop:~# ip -4 addr show eth0
   laptop:~# curl -s --max-time 5 http://webServer.lab/

Run these steps on real machines