Re: Netsurf timing out

As Chris Evans wrote on 17 Jul 2009:

I wonder what happens when people leave their router powered 24/7 would it
pick up a DNS change at the ISP?

In short: Yes, but it may take a while.

As I understand it this is one of the main features of the Domain Name
System. See <http://en.wikipedia.org/wiki/Domain_Name_System> for
details.

When a name (like bbc.co.uk) is first looked up by your computer, it
asks the DNS in the modem. This in turn asks the DNS of the ISP. The
result is passed back to your computer, and the modem will store a copy
in its cache.

The next time the computer asks for the same name to be resolved
(bbc.co.uk again), the modem will use the entry in the cache to reply,
without asking the ISP again. This system of caching is not specific to
modems or certains operating systems. It is an integral part of the
internet. It reduces the data trafic for name lookups enormously and
results in quicker access to host names you have visited before.

The entries in the cache do not last forever. The trick is that the copy
in the cache is tagged with a Time To Live (TTL) field. This gives the
number of seconds the cache entry is valid. If the time runs out then
the next time the name is looked up the modem will ask the ISP again.

I'm not sure what values are used for the TTL, but I think they are
usually in the order of one day. They may be as short as one minute, or
as long as a week. The value of the TTL is set by the administrator who
defined the host name. For the example of bbc.co.uk, that would be the
BBC system administrator. For your ISP's email machine
(smtp.your.isp.com) that would be the system administrator of your ISP.

This means that any change of an IP address at the service provider may
take a day before it is noticed by your computer. To hasten this (in
case you know or suspect there has been a change), you can reset your
modem, which flushes the cache. Or you can temporarily change the DNS
settings in your computer, putting the ISP DNS IP addresses on top,
rather than the IP address of your modem. This will make your computer
look the names up directly at the ISP.

Note that it is possible that your computer also has a cache. I'm not
sure if the RISC OS resolver does this. Can anyone comment?

Also note that in the whole of internet, DNS entries change all the
time. These changes are noticed as soon as their respective TTL runs
out.

--
Erik G http://www.xs4all.nl/~erikgrnh
== 'From:' address is a spam trap. Do not use
== See web site for email address
.

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