udhcpc

A DHCP client broadcasts a Discover, gets an Offer, Requests it and receives an Ack (DORA). The lease carries more than the address: the default gateway and the DNS servers come from it too. In the labs the client is BusyBox's udhcpc (dhclient on many distributions). A lab device that takes its address from DHCP keeps one running, and dhcp-renew eth0 asks it to renew the lease now (dhcp-release eth0 gives it back), printing what it got, as ipconfig /renew does on Windows. Watch the exchange with tcpdump -ni eth0 port 67 or port 68.

udhcpc command usage

udhcpc -i CARD [-n] [-q] [-f]

Options

-i eth0

the card to configure

-n

exit if no lease is offered

-q

exit once a lease is obtained

-f

stay in the foreground

-t 4 -T 2

send 4 discovers, 2 seconds apart, before giving up

-s /bin/true

run this script instead of the one that applies the lease: only ask, change nothing

udhcpc examples

udhcpc -i eth0 -n -q

Asks for a lease on eth0 once, and exits whether or not one comes.

udhcpc -i eth0 -n -q -f -s /bin/true

Asks for a lease and prints it, leaving the card's address alone.

dhcp-renew eth0

Has the device's running DHCP client renew eth0's lease now, and prints the lease.

tcpdump -ni eth0 port 67 or port 68

Shows the DHCP exchange: discover, offer, request, ack.

Pitfalls

• DHCP is broadcast: it never crosses a router, so the server must be on the same network unless a router relays it (ip helper-address, dhcrelay).

Run it on real machines