[ntp:questions] Re: sendto( Invalid argument

Brad Knowles brad at stop.mail-abuse.org
Sat Aug 13 11:14:30 UTC 2005

At 12:18 PM +0200 2005-08-13, Edrusb wrote:

>  Yes probably, it is too bad the same socket has not been used to send
>  data too, just leting the system put the appropriate Source IP and
>  select the adequate output interface according to system's routing
>  decision. This way there would not have any need to scan available
>  network interfaces for changes.

	Nice theory, but it doesn't work that way in practice.  Dig deep 
into the code of programs like ntpd, BIND, and sendmail if you want 
to understand why, but be prepared to spend a significant amount of 
time learning about cross-platform issues, problems with IPv4 versus 
IPv6, guaranteeing that reply packets always come from the same 
interface and IP address that the query was sent to, and a whole host 
of other problems.

	It's just not that simple.

>  I have a "tap" interfaces that handles the traffic when the link is
>  down for dial on demand (see "diald" interesting and powerful
>  program), unfortunately, ntpd does not bind a socket to this
>  interface! Too bad.

	Well, ntpd should bind to all interfaces that are up at the time 
it starts, and remain bound to them until it is restarted.  So, if 
that tap interface works at all times, and remains consistent at all 
times, you shouldn't have any problems.

>  I guess, the best solution waiting for next ntpd release, as described
>  by Steve Kostecke, is to have my local ntpd server for my local network
>  be "inside" the network without any dynamic IP address.

	Either that, or stopping and restarting ntpd every time the 
dynamic IP address changes.

Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.

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