Re: what is the RIGHT(TM) way to configure background DHCP?

On Wed, Jul 6, 2011 at 12:47 PM, deeptech71_at_gmail.com
<deeptech71_at_gmail.com> wrote:
> (i intend the discussion to take place primarily on the
> freebsd-hackers list, i'm CCing the freebsd-current list for a reason
> stated below.)
>
> (original first post:
> http://lists.freebsd.org/pipermail/freebsd-questions/2011-June/231301.html)
>
> On Mon, Jul 4, 2011 at 4:20 PM, Lowell Gilbert <Lowell_at_be-well.ilk.org> wrote:
>> You might want to try rewording your question, because it didn't make
>> any sense the first time.
>
> very well. though the question should have made perfect sense the first time.
>
>> [All of your examples were commented out, so
>> it was no surprise that they didn't work.]
>
> i chose to use # characters to separate rc.conf snippets from the
> other sentences i wrote. though perhaps #s were an unfortunate choice,
> because a # has a comment-begins-here meaning in rc.conf. in any case,
> "attempt 1" and "attempt 2" didn't work, but "attempt 3" did work, and
> that rules out the use of # for comments in rc.conf. now i will use
> "===== rc.conf snippet begins =====" and "===== rc.conf snippet ends
> =====" instead of # characters.
>
>> I considered trying to use my chrystal ball to guess that you needed
>> "SYNCDHCP" in your interface config, but you had obviously read the
>> manual for rc.conf(5), so that seemed unlikely...
>
> and "SYNCDHCP" makes things even worse. (i have no idea what
> "SYNCDHCP" is supposed to do, but in my experience:) "SYNCDHCP" makes
> the boot process wait until DHCP server replies before proceeding.
> "SYNCDHCP" is actually almost the same as "DHCP", except that
> "SYNCDHCP" waits indefinitely, while "DHCP" waits for at most
> defaultroute_delay, and also, "DHCP" gets "Starting network: lo0 sk0.
> <insert_bogus_interface_info_here>" printed *before* the IP address
> assignment process starts, while "SYNCDHCP" gets "Starting network:
> lo0 sk0. <insert_current_interface_info_here>" printed *after* the IP
> address assignment process finishes.
>
> so here is the question rephrased:
>
> the boot process of my FreeBSD machines takes a relatively long time.
> it spends 30 seconds idling at some point, because my network
> interface (sk0) is supposed to have an IP address assigned via DHCP,
> and the DHCP server on my LAN takes an extremely long time (~40
> seconds) to reply to IP address requests. this is unacceptable for me;
> i want the FreeBSD boot process to finish 30 seconds earlier, even if
> i won't get the chance to use the network for ~40 seconds after the
> booting has finished.
>
> this was the actual case when my rc.conf had the following options
> related to network interfaces:
> ===== rc.conf snippet begins =====
> ifconfig_sk0="DHCP"
> ===== rc.conf snippet ends =====
>
> it took me 3 rc.conf configuration attempts to find a configuration in
> which the boot process does not idle for 30 seconds.
>
> the following was the first attempt:
> ===== rc.conf snippet begins =====
> background_dhclient="YES"
> background_dhclient_sk0="YES"
> ===== rc.conf snippet ends =====
> with this configuration, the DHCP client isn't even started. ie., the
> boot process does not idle at all (that's good!), but the network
> interface will never receive an IP address automatically (that's very
> bad!).
>
> here is the second attempt:
> ===== rc.conf snippet begins =====
> ifconfig_sk0="DHCP"
> background_dhclient="YES"
> background_dhclient_sk0="YES"
> ===== rc.conf snippet ends =====
> with this configuration, the DHCP client is started, but the boot
> process still idles for 30 seconds at some point (as if the
> background_dhclient and background_dhclient_<if> variables had no
> effect).
>
> the final attempt is:
> ===== rc.conf snippet begins =====
> ifconfig_sk0="DHCP"
> defaultroute_delay="0"
> ===== rc.conf snippet ends =====
> this configuration works, ie., during the boot process, there is no
> idling related to waiting for a DHCP server to reply. but this
> configuration looks hacky.
>
> so what is the RIGHT(TM) way to configure the boot process not to idle
> much in case of a slow DHCP server, and why?
>
> i ask this question partially because there is an rc.conf option
> background_dhclient (also, background_dhclient_<if>), which doesn't
> seem to do anything, though it should, since it exists (either that,
> or the option should be removed). (there has been a recent discussion
> on the freebsd-current list about rc.d parallelization; [reason for
> CCing:] could recent rc.d changes have made the background_dhclient
> option useless?)
>
> i'd like to have an in-depth explanation on what effect should any
> combination of the following options should have on the boot process:
> defaultroute_delay="0", background_dhclient="YES",
> synchronous_dhclient="YES". (for example, using both
> background_dhclient="YES" and synchronous_dhclient="YES" seems stupid;
> i need a clarification if that's not the case.) what's the explanation
> if there is more than 1 network card (all of which are to have IP
> addresses assigned via DHCP)?

Hi,
    You'll need ifconfig_sk0="DHCP" in order to get dhclient to work.
Nothing more, nothing less. The rest of the variables just seem
redundant to me (sans background_dhclient vs synchronous_dhclient, but
it seems like that should be streamlined to one variable instead of
being two anyhow as they're mutually exclusive options that should be
fed into dhclient). I recommend bringing this up with rc_at_ because this
is just inefficient and misleading.
    Look through /etc/network.subr // /etc/rc.d/dhclient if you're
curious and capable of reading sh -- it's straightforward what's going
on -- it's just that the method for doing it isn't clearcut and
straightforward, and it looks like things need to be weedwacked a bit
in that section of code.
Thanks,
-Garrett

PS Please don't cross-post ;).