[OpenWrt-Devel] ddns-scripts "sleep 10" before updating?

Catalin Patulea cat at vv.carleton.ca
Thu Jan 15 21:07:50 EST 2015

On Thu, Jan 15, 2015 at 3:28 PM, Christian Schoenebeck
<christian.schoenebeck at gmail.com> wrote:
> I did a lot of testing with a user using satellite Internet connection.
> He quickly gets his IP from the modem but he needs to use a global dns server to verify his registered ip via tcp
> because the modem buffers all udp dns requests not looking for any timeouts.
> For him it takes same seconds until the connection was established to the web after getting his ip from the modem.
Let me see if I understand.. so after his interface comes up,
dynamic_dns_updater was getting stuck on "get_registered_ip" because
the connection to the web is not yet up? Perhaps this could be handled
by a timeout on the host/nslookup command?

I also think this is an edge case - in most cases, you can call
get_registered_ip immediately and it will work.

> The "5 minutes" I tested the easy way.
> On my ubuntu box I ran nslookup for my own ddns-address against Google's public dns in an endless loop.
> Then I send out a new ip to my ddns provider via ddns-scripts and wait.
> The longest time it takes to have a stable answer from nslookup was 4 minutes 10 seconds.
> Plus reserve = 5 minutes. Also the reason why check_interval minimum is set to 5 minutes = 300 seconds
> in line 176 of dynamic_dns_updater.sh.

I understand that the latency through Google public dns is 5 minutes,
but not everyone uses Google public dns, and your test is not
representative of queries for cache-cold records.

I tried adding a new record to my domain to see what happens on a
query that is not already in the cache. This is the same case as
updating an existing DDNS entry that hasn't been accessed in a long

The record was immediately (<15 seconds, while I typed the commands)
available in my DNS provider's server, my ISP's server, and
The intermediate servers didn't even know if the record exists, so
they had to contact the authoritative server. Probably if I update the
IP now, it will take a long time to propagate, but this is not a
common case for me.

What I want is that if I get a new IP/router crashes/power outage, for
my IP to be updated in the authoritative DNS server as soon as
possible. It is rare that I access my DDNS just before this
(crash/power outage/etc.) happens, so the record will not be cached,
so if I access it now, I will see the fresh IP in any DNS server. In
this case the "sleep 10" is the limiting factor.

I think probably this is the case for most DDNS users - they probably
access their records very rarely, so they are not cached, so "sleep
10" is the biggest factor to how quickly their new address is visible.
But I can't speak definitively about any but my case.

Perhaps the sleep could be a configuration option which defaults to 0.
Specific users can set it if they need it.
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