On Sun, May 11, 2014 at 06:31:23AM -0400, Kenneth Westerback wrote: > On 11 May 2014 06:03, Kenneth Westerback <kwesterb...@gmail.com> wrote: > > > > On 11 May 2014 05:26, "Creamy" <cre...@dishplanning.com> wrote: > >> > >> Hello again! > >> > >> OK, this time it's a bug, (or is it a feature?), in dhclient. > >> > >> Imagine that you have two separate wireless networks, which operate > >> independently using the same private address spaces and offer leases > >> based on the same algorithm computed from the MAC address of the > >> client. > >> > >> For example: > >> > >> Network 1 - SSID foo, AP IP 192.168.64.1 netmask 255.255.255.0 > >> Network 2 - SSID bar, AP IP 192.168.64.1 netmask 255.255.255.0 > >> > >> But these are NOT connected in any way. They are two completely > >> different networks. E.G. 192.168.64.20, (by way of example), on > >> foo, is a different machine to 192.168.64.20 on bar. Maybe one > >> is your office network and one is a home network. > >> > >> Now, imagine that you have another machine, (probably a laptop), > >> which connects to either one, but never both at the same time. > >> > >> This machine is assigned an IP by DHCP when it joins either > >> network. The DHCP server assigns IPs based pseudo randomly based > >> on MAC address. > >> > >> If both networks use identical algorithms to calculate this IP, > >> then the laptop will be assigned the same IP regardless of the > >> network it joins. Imagine that it's assigned 192.168.64.10. > >> > >> This causes problems with dhclient, when you switch between > >> networks whilst a lease is still valid on the first one. It works, > > > > Whew. At least it works. > > > >> but if you pass the -L option to dhclient, it doesn't produce the > > > > At last! Somebody using -L! > > > >> normal output, presumably because it believes that it's continuing > >> with an active lease. > > > > Possibly, since I believe you are saying the dhcp servers have the same IP > > or provide the same server id. dhclient currently has no way to detect that > > the nwid and only the nwid has changed. > > Possibly, but it turns out not. Even if the lease is determined to be > identical, the bind_lease() function will re-write the -L file by > calling rewrite_option_db(). But even this is inapplicable in the > situation you describe since your script starts a new dhclient, and it > will not have a lease bound to the interface and will never conclude > that it is responsible for the address in place. Thus it will do the > full monty of deleting existing addresses and binding the lease it > gets.
Agreed. > > > > >> > >> This causes problems with scripts. > >> > >> A script such as this: > >> > >> #!/bin/sh > >> ifconfig if0 nwid foo wpakey bar > >> sleep 3 > >> dhclient -L /output > >> cat /output > >> > >> fails with cat: /test: No such file or directory. > > > > This seems implausible. Why would cat complain about /test here? > > > > Assuming that's a typo, can you provide some more details on what version of > > OpenBSD you have installed? Some -L changes went in recently. There is also > > a race in your script since dhclient will not be guaranteed to have finished > > with /output before your cat runs. > > There is also a conceivable race between the old and the new dhclient. > One trying to truncate the file to indicate it has not configured an > address and one trying to repopulate the file with new lease info. I > haven't seen that in my testing. > > Are you saying the behaviour you report is consistent? Or occasional? Initially I only saw it in the situation I described above, where it occurs almost all the time. But after testing, it happens occasionally if I just start a new dhclient on the same interface connected to the same wlan. So, most likely it is a race, possibly a timing difference in the response from the servers making one situation worse than the other with my setup. I didn't realise at first that a new dhclient instance always polls the DHCP server afresh, I thought it would use existing leases that had been allocated and not expired. What happens if the dhcp server doesn't respond? Do we re-use an old but not expired lease? -- Creamy! <3
