I Learned Something About IP Caching Today That You Might Find Interesting

[Steven Wagenheim]

This is something that I had never run into before and I am passing it along
since some of you may have sites (such as membership sites) that your
customers access on a regular basis.

Now, I am not a tech person so I can't really explain this in really good
technical terms (which may be just as well for you non techies) so I am
going to explain it in simple English.

Let's say John Doe accesses your site and has it bookmarked and their ISP
also has the IP address cached. This way, when you go to the site, you'll
notice that it comes up right away.

Well, if for some reason, your hosting company makes some updates and
in the process, changes the IP address of the site, your customer's PC, when
they go to access the site again, will NOT find it, even though it is still up.

Now, according to my tech guy, eventually this gets flushed out and
when the customer goes back to the site, maybe a day later or whenever,
the site WILL come up. But until the IP or DNS gets flushed, it won't.

This just happened to me. I could get to my sites from one PC (hadn't been
on it before the update) but couldn't get to the sites from the PC that had
just accessed it before the update.

If you have a very active site, this could cause a customer service
nightmare during that period before the DNS gets flushed.

You can manually flush your DNS by pulling up a DOS prompt and typing in

ipconfig/flushdns

However, your customers and/or members won't know to do this.

It took me forever to figure this out, which was later confirmed by my
tech guy at the hosting company.

I don't know how often something like this will happen. It is the first time
this has happened to me since I've been online, but in case you ever find
that you can get to one of your sites from one computer but not from
another, this is probably why.

[rmholla]

So this only applies to someone using a bookmark to find your site?


Rhonda

[Steven Wagenheim]

Originally Posted by rmholla

So this only applies to someone using a bookmark to find your site?


Rhonda

Rhonda, I'm not sure if it's just that OR if they had also just recently been
to your site and have the IP cached.

But certainly, yes, if the site is bookmarked and they go to it regularly,
they're toast until the dns gets flushed.

[Steven Fullman]

For those who go goggle-eyed at the thought of 'pulling up a DOS box'...rebooting your PC will achieve the same thing.

However, Steve has touched upon some great points...nearly all correct

'DNS For Marketers' is making me LOL at the moment though. I could write it in an evening, but sadly it'd never be 'Mass Control' popular.

Steve 'Network Guy' Fullman (...and I mean network in the geek sense of the word, not as in 'shake 'n fake'...)

[Steven Wagenheim]

Originally Posted by Steven Fullman

For those who go goggle-eyed at the thought of 'pulling up a DOS box'...rebooting your PC will achieve the same thing.

However, Steve has touched upon some great points...nearly all correct

'DNS For Marketers' is making me LOL at the moment though. I could write it in an evening, but sadly it'd never be 'Mass Control' popular.

Steve 'Network Guy' Fullman (...and I mean network in the geek sense of the word, not as in 'shake 'n fake'...)

As I said Steve, I'm not a techie person and this was all news to me. I
figured each time you went to a site, your PC just looked for the site and
if it was up and running, found it.

Turns out, not so.

[Ben Roy]

A while ago they made some changes to the DNS infrastructure on the internet. In the process, the timeouts that are generally associated with caching that information were drastically lowered. Most users, with most ISPs, should only suffer a problem like this for a very short period of time (minutes).

And of course they would only experience this problem if your host had changed your IP address while they were accessing the site (or someone else on the same DNS servers). This should happen...almost never. I'd guess most server accounts don't change ip address even once a year.

[Steven Fullman]

Originally Posted by Steven Wagenheim

As I said Steve, I'm not a techie person and this was all news to me. I
figured each time you went to a site, your PC just looked for the site and
if it was up and running, found it.

Turns out, not so.

I know...it's crazy!

Trivia: There's a silent period '.' at the end of every domain name. It got dropped from popular speech (and browsers) simply because it was always there...

So google.com is really google.com.

The periods are the DNS root servers. They know where all the top-level domain name servers are...(.com, .org, .edu etc).

So, you want to visit www.iveneverbeenherebefore.com ?

Your PC checks whether the address is in cache, and if not, it querys its primary DNS server (most likely, your ISP)...

Your ISP checks whether the address is in its cache, and if not, it queries the root servers.

The root servers say, "I know where the .com's are!", and sends back a message to your ISP.

Your ISP sends a reply to the .com servers, asking where iveneverbeenherebefore.com is. The .com server replies, "I know where iveneverbeenherebefore.com is!"

The .com server sends your ISP the IP address.

Your ISP sends a message to the iveneverbeenherebefore.com server, asking, "Do you know where www.iveneverbeen....etc" is?"

The iveneverbeenherebefore DNS server answers the question, and gives your ISP the answer.

Boom...We have a connection.

Your ISP gives you the connection details for www.iveneverbeenherebefore.com and you visit the site.

Your PC, and your ISP cache the IP address, just in case you need to go through that rigamarole again in the next few minutes.

...even though your initial request may have travelled the globe many times, and in milliseconds.

That's how the internet works.

Steve

[BillyParadise]

Yep, if you want to find out what your computer knows about any particular domain, open up a command window (start / run / cmd ) :

C:>nslookup
Default Server: UnKnown
Address: 192.168.165.1

> set type=soa
> etfautopilot.com
Server: UnKnown
Address: 192.168.165.1

Non-authoritative answer:
etfautopilot.com
primary name server = ns1.dreamhost.com
responsible mail addr = hostmaster.dreamhost.com
serial = 2008061101
refresh = 17700 (4 hours 55 mins)
retry = 1800 (30 mins)
expire = 1814400 (21 days)
default TTL = 14400 (4 hours)

etfautopilot.com nameserver = ns2.dreamhost.com
etfautopilot.com nameserver = ns3.dreamhost.com
etfautopilot.com nameserver = ns1.dreamhost.com
ns1.dreamhost.com internet address = 66.33.206.206
ns2.dreamhost.com internet address = 208.96.10.221
ns3.dreamhost.com internet address = 66.33.216.216
>

Note this is rarely a problem - only when you (or your host) switch the IP address of your website. If they know they are going to do this in advance, ask them to "reduce the TTLs" beforehand. This means - lower the numbers you see above (refresh/retry/expire), so that the outage will be minimal.

If you're changing hosts, your IP will also change. If it's a static site, no big deal, leave the old website up and running for a couple of days during the switchover. Hard to do if it's a database-driven site, but for basic sales pages, no issue at all.

[Bishop81]

There is a similar thing that happens on your internet provider's end too.

If you ever notice that your site's ip address changes and you can no longer get to it, but somebody else can from a different area, then you might just have to wait for a day or two for the cache to clear at your provider. An ipconfig /flushdns doesn't work at that point because it's your provider with the problem.

It doesn't happen often, but I've had it happen a couple times over the years.

[Dave777]

On a related issue...

For those of you that may not know about the following Free tools. Another easy and simple way to keep your computer real clean and fast without having to do a separate disc cleanup , defrag, temporary file cache etc...

Amazing Free tool or by donation. Over 195 million downloads. Options allows you to adjust all settings re: cookies etc...
CCleaner - Home
CCleaner - Features

And...
Defraggler - Defragment your files!

Dave

[seasoned]

Actually, the way it works is THIS! If everyone went to the MAIN servers for .COM, it would cause timeouts everywhere, and the internet would just COLLAPSE! There are only about 13 such servers serving the PLANET!!!!!!! When you go to a site, your system checks itself. If the TTL ( Time to live - Wikipedia, the free encyclopedia ) has elapsed, it goes to the next most authoritative source, etc.... EVENTUALLY, it may hit one of those 13 servers, and go back to the main DNS, but that is UNLIKELY!

By doing that, the 13 servers now become MILLIONS! Of course, the problem is that it could take DAYS for the cache to be updated. BTW this is the DNS, and affects EVERYTHING! Browser, FTP, etc....

Steve

[jacktackett]

Nice thread, thanks to everyone.

There's also something else to be aware of - before a DNS server is ever queried windows will check a file called hosts - its in the windows install folder\system32\drivers\etc

This file contains a mapping of IP addresses to host names - similar to what DNS provides.

I always write protect this file to make sure malicious programs (or people) can't send me to a fake site if I put in www.paypal.com etc. Since entries in this file will override DNS entries.

--Jack

ps - to the networking guys - ever get bit by forgetting to clear the arp cache on your router after changing out a dead ethernet card? - took a while to figure that one out, given the router was in Guam and I was in RTP, NC....

[talewins]

My dns change took 12 days to propagate back to me. That is 14 days.
I had to call my local Internet server to ask what was going on. Turns out this company only flushes its cache out once a month. Apparently the situation is repeated occasionally all over the web. Just yesterday another customer reported my site is down, because she was going to the old server.

[pjs]

Originally Posted by talewins

My dns change took 12 days to propagate back to me. That is 14 days.
I had to call my local Internet server to ask what was going on. Turns out this company only flushes its cache out once a month. Apparently the situation is repeated occasionally all over the web. Just yesterday another customer reported my site is down, because she was going to the old server.

This is highly uncommon. 24 hour cache setting is way to long, I can't imagine 14 days. Normal TTL (time to live) for most hosts is around 1 hour (3600 seconds). Some ISP's will ignore the set TTL and cache for whatever period they want. The idea is sound but it's really not practical in today's "internet"..