What can cause "dhclient/dhcpcd" to fail to get an address?

[sundialsvc4]

Can we start a summary thread here of:

• What can cause "dhclient" or "dhcpcd" (or both of them...) to fail to get an IP-address from a particular router? (Basically, "no DHCPOFFERs.")
• What can cause the "/etc/resolv.conf" file to fail to be updated?

[batistuta]

let me start with the obvious reasons:
- No proper drivers for the physical device (WLAN card or Eth adapter)
- No connection (no eth cable, no wlan authentication). These can be tested with iwconfig ifconfig
- No dhcp server on router activated

[Ravak]

Also old/non standard dhcp server. In my case I needed to do some "voodoo" for dhcpcd (since by default it follows RFC2131) to work. I think I had to add -I ''. Even though the server kindly accepts me after two minutes of discover & requests so I also had to set up big timeout (5 minutes).

Many dhcp servers require that a hostname be set. A particular hostname can be set by calling dhcpcd with the -h <hostname> option. Using the -H option, dhcpcd can be configured to set the hostname that is recieved from the dhcpcd server. Moreover, dhcpcd can be configured to set the domainname of the client to the domainname supplied by the dhcp server with the -H option.
Some dhcp servers require that clients send two DHCP_DISCOVER messages before it will reply. dhcpcd will not do this by default and this behavior can be specified with the -S option.
Some dhcp servers use outdated or uncommon protocols and dhcpd can be configured to use these protocols. Detailed instructions can be found in the man page for dhcpcd.

from http://wiki.linuxquestions.org/wiki/Dhc ... mon_Issues