Re: Netsurf timing out
- From: Erik G <erikgspm@xxxxxxxxxxxxxx>
- Date: Mon, 20 Jul 2009 18:36:48 +0200
As Brian Carroll wrote on 18 Jul 2009:
In article <4833d87c50.root@xxxxxxxxxxxxxxxx>, Erik G
<erikgspm@xxxxxxxxxxxxxx> wrote:
[your permanently-on ADSL modem will pick up changes in IP adresses
of ISP hosts (like pop.your-isp.co.uk) automatically through the
wonders of the Domain Name System.]
Thank you. That's a clear and helpful description of the process
of serving names of required sites, but I (and I think Chris
Evans) was writing about getting the address of the ISP's server
itself, not the addresses it serves. I'm a bit confused; does the
process you describe apply to that as well?
I assume you mean with "the ISP's server itself" the DNS server of the
ISP. Suppose the ISP decides, for one reason or another, to change the
IP address of their DNS server. Will your permanently-on modem/router
pick this up?
In short, there are two cases. For the first case, the answer is "No".
For the second, the answer is once again "Yes, but it may take a while".
If your router uses fixed values for the IP addresses of the DNS
machines of the ISP (there are always at least two, the primary and
the secondary server), then it will never automatically change to the
new values. In this case it will not matter if you switch your router
off regularly or leave it on all the time. Resetting the router will not
help either. The only way to follow the change at the ISP is to change
the settings in your router.
If your router uses DHCP to collect information about the connection
with the ISP [1] then the answer is once again: "Yes, but it may take a
while." The Dynamic Host Configuration Protocol (DHCP) has a "lease
time" associated with the information. When half this time has run out,
the router will fetch new information from the ISP, and so discover any
change in the IP address of the DNS server of the ISP.
Resetting the modem (or switching off and switching on again) will make
it fetch new information. This will presumably contain the new IP
adresses of the DNS servers, even when the lease time has not yet
elapsed.
The length of the lease time is set by the DHCP server, which in this
case is at the ISP. You can probably find the lease time (and whether
DHCP or fixed values are used to find the DNS at the ISP [1]) by
browsing the modem/router settings.
[1] Note: Not to be confused with your computers using DHCP to connect
to the modem/router.
On a practical note, an ISP will rarely change the IP addresses of their
DNS servers. If they bluntly change them, all their customers will
immediately experience problems and bombard the ISP with complaints.
Probably by phone, because pretty soon they won't be able to send or
receive email either.
If an ISP is forced to change, and they are sensible, they will execute
the change in stages over several months. First they will inform the
customers of the coming change. Then they will set up the new DNS
servers on the new IP addresses while keeping the old ones active. Next
they will inform the customers of the need to change over to the new IP
addresses.
For customers who use DHCP to get the ISP's DNS server IPs, the change
can be as simple as waiting for the lease to expire. For those who
don't, manual intervention will be required. Because most customers know
sod-all about the setup of a modem/router, the ISP will probably provide
software that will upload the new settings into the router.
By measuring the traffic on both sets of DNS servers, they can determine
what percentage of the customers has changed over. New information
campaigns may be needed if the changeover is too slow.
When the use of the old servers is down to, say, 10%, the ISP can start
sending targeted information to those users who have not made the change
yet (their IP addresses will be in the DNS requests sent to the old
servers).
When down to, say, 1% or 0.1% it is time to shut down the old servers.
Customers who have not made the change by then will find they have
trouble with their connection and call customer service, who will help
them with the required changes. In extreme case, an engineer may have to
be sent round.
All in all a major and expensive operation, and one that an ISP will not
undertake lightly. In fact, an ISP will fight very hard to keep the IP
addresses that were assigned to them.
Changing the IP address of a service like smtp.your-isp.com, for example
to switch over to a new machine with fancier software, is rather easy
and will not cause a great deal of trouble for the customers.
Changing the IP address of a DNS server is a big thing and will cause no
end of troubles. ISPs will try hard to avoid it.
--
Erik G http://www.xs4all.nl/~erikgrnh
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