Debug DHCP troubles, see exactly what the DHCP server is sending me

Scenario: Trouble with DHCP on a client machine. User claims that the DHCP server is not sending the right information. Need to see exactly what DHCP server is sending.

System: openSUSE 13.1


The first thing to realize is that there are two DHCP clients shipped with openSUSE: dhcpcd (the "DHCP Client Daemon") and the ISC DHCP client (package name dhcp-client and binary name /sbin/dhclient).

dhcpcd

The most basic test with dhcpcd is to run the dhcpcd-test bash script as root:

# dhcpcd-test enp0s25
info, enp0s25: hardware address = ...
... etc. lots of output ...

For details take a peek into the script.

dhclient

1. On a client machine somewhere in the network segment, open the file /etc/sysconfig/network/dhcp and change the DHCLIENT_DEBUG value to "yes"
2. Read the comment:
   

   ## Type:        yesno
   ## Default:     no
   #
   # Start in debug mode? (yes|no)
   # (debug info will be logged to /var/log/messages for dhcpcd, or to
   # /var/log/dhclient-script for ISC dhclient)
   #
   DHCLIENT_DEBUG="yes"
   

3. run dhclient as root
   
   • if I want to debug a particular interface, add -d $IFACE
   • if I want to dump the entire lease to STDOUT, do dhclient -4 -d -v -sf /bin/true -lf /dev/stdout $ifname
     
   • if I want to debug a particular DHCP server, add -s $SERVER_IP_ADDR
   
   
   
4. Study the dumped data in /var/log/dhclient-script
5. HINT: try deleting the DHCP lease file(s) on the client