DHCP: "no ServerID" messages + no lease

brainwash

Hi,

recently, my wlan Adapter has trouble acquiring a lease. This happens only occassionally:

My Adapter is an Intel 3945 ABG using the iwl3945 driver. My AP/Router is a Speedport W701V modded with Freetz 29.04.80freetz-1.1.3, which is roughly equivalent to a Fritz!Box 7170. Rebooting the router and reloading the iwl3945 allows me to get a lease again. Only reloading the kernel module or rebooting the router does not help. This problem has started appearing only recently (approximately a week ago), and happens only once a day. I did not make any changes to my Gentoo or Router config and have not installed any Software updates.

Does anyone have an idea what could be causing this Problem?

Thanks in advance,

brainwash

Jaglover · Posted: Wed Nov 10, 2010 12:53 pm Post subject:

May it be there is another access point in proximity which is connected to another router?

brainwash · Posted: Wed Nov 10, 2010 1:08 pm Post subject:

This is a single-router home setup. There are other APs near, but they use different ESSIDs and I have set up wpa_supplicant to only connect to my ESSID.

Jaglover · Posted: Wed Nov 10, 2010 3:23 pm Post subject:

Well, Google tells this problem occurs with poorly written DHCP servers. You can edit dhcpcd sources and remove server ID check if you want to.

brainwash · Posted: Wed Nov 10, 2010 3:29 pm Post subject:

Yes, but this still does not explain why wlan0 won't accept the offered lease. It should just ignore the fact that no ServerId is present. Or does the absence of the ServerID also imply that any DHCP lease from that Server will be ignored? And why do I sometimes get a lease if this is a case of misconfigured DHCP Server?

Jaglover · Posted: Wed Nov 10, 2010 4:16 pm Post subject:

This may explain it.
It appears new versions of dhcpcd consider NAK messages without ServerID invalid.
I'm no fan of wireless and DHCP myself, can't tell from experience.

UberLord · Posted: Mon Nov 15, 2010 9:27 am Post subject:

The code has changed a little bit since then.
The ServerID requirement is now enforced in /etc/dhcpcd.conf by default - the user can remove this themselves now so they can work with faulty DHCP servers.

And before anyone goes "bla bla bla dhcpcd is at fault" it is a DHCP requirement that DHCP servers transmit a correct ServerID so that clients know which DHCP server to renew/release their leases from. We can't assume that that computer that sent the message is the DHCP server as that would break DHCP bridges.

The Fritz! box is also a PITA as it enforces a ClientID being present in the DISCOVER/REQUEST which is not a requirement.
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brainwash · Posted: Mon Nov 15, 2010 11:02 am Post subject:

Hi,

thanks for the explanation. The really weird part was this: The problem only occurred when I booted my machine. Then, after restarting the router and reloading the wlan module (causing a restart of dhcpcd) everything would work fine. Maybe I would have tried changing the setting in dhcpcd.conf, but the problem already got on my nerves enough that I decided to replace the DHCP Server of Fritz!' multid with the dnsmasq provided by freetz. Works fine so far.