Posts tagged ‘dns’

Recently, a site I frequent went down for an extended period. I waited patiently for it to return (it said it was upgrading). A week later, I hopped on Twitter and asked the author WTF? I learn that the site has been back up for days. Only I, and probably half of the internet, had no idea. Why? DNS. Done wrong, it’s a bitch.

I think there’s an object lesson here.

So, let’s say that you’ve decided that your webhost sucks rocks, and you want to change hosts for your domain from your existing server called junkhost to a much better and new fangled one called goodrack.

How To Do DNS The Wrong Way

(or why the site is currently totally missing from the internet as I see it)

Step 1: Your domain’s nameservers are currently set to dns1.junkhost.com and dns2.junkhost.com.

Step 2: Your new hosting company tells you to set them to dns1.goodrack.com and dns2.goodrack.com. You do so.

Step 3: You wait a day or two or possibly flush your home router’s DNS and voila, your host is now visible to you.

Why this is the wrong way: DNS is a caching system. When I try to lookup the IP address for yoursite, then I don’t ask your nameserver for it. I ask my nameserver for it. My nameserver then asks you, but only if it doesn’t already know.

See, you communicate with your site regularly. So you are doing lots of queries and flushing the DNS on a regular basis. Home router DNS systems tend to flush a lot, as do cable modems and cable companies, etc. So you might see that nameserver switch in just a day or so. Maybe less.

Me, on the other hand, am sitting here at BigCompany, which has weird fiber and satellite links to all sorts of places. If my DNS has your nameservers cached, it could be cached for weeks. And you don’t have a whole hell of a lot of control over what is called “TTL”, or “Time-To-Live”.

Time-To-Live is what tells DNS caching servers when to drop the info and get fresh data. If the TTL for, say, a normal DNS lookup is 2 hours, then my nameserver isn’t constantly querying yours whenever I look at your site. It only queries it once every two hours, tops. More or less.

Sitting here looking at the thing in debug mode, I see that there’s another 46 hours left to go before the old data I now know exists gets flushed out of the DNS server. By then, I won’t have been able to access his site for a bit under 2 weeks. And I’m not the only one.

How To Do DNS The Right Way

(or why I can change hosts in under a couple hours)

Step 1: My domain’s nameservers are currently set to dns1.myregistrar.com and dns2.myregistrar.com. My registrar offers DNS hosting for free, as part of my having bought my domain through them. If they didn’t, I could use a free service like ZoneEdit, or similar.

Step 2: My new hosting company tells me to set the nameservers to dns1.goodrack.com and dns2.goodrack.com. I look them right in the eye, tell them to “get bent”, and ask what the IP address of my new server will be. If they put up a fight, I tell them I can always find a better web host. That usually shuts them up.

Step 2 (alternate): I say “sure, whatever you say”, then do the following at a command line:

C:\otto\>nslookup
Default Server:  dns.mycompany.com
Address:  1.2.3.4

> server dns1.goodrack.com
Default Server:  dns1.goodrack.com
Address:  5.6.7.8

> set type=any
> mydomain.com
Server:  dns1.goodrack.com
Address:  5.6.7.8

mydomain.com      internet address = 111.222.111.222
(root)  nameserver = dns1.goodrack.com
(root)  nameserver = dns2.goodrack.com
dns1.goodrack.com    internet address = 5.6.7.8
dns1.goodrack.com    internet address = 5.6.7.9

Now I know my new server’s IP address from there (it’s 111.222.111.222).

Step 3: I go to myregistrar.com’s site, edit my DNS settings (specifically, I change the A record for mydomain.com to point to that new IP address), and then I wait for a couple hours or so, max (since my registrar lets me set my own TTL, I can do what I like with it. Two hours is a decent tradeoff of time vs. functionality). Within 2 hours, every site on the whole blamed internet sees my new host.

Why is this better? Well, note that my nameservers never actually changed. Nameservers are set in the root domain lookup systems. Changes there not only take a while to propagate, but those servers are overloaded so caching values for nameservers are usually quite high. Individual host lookups though, not so much.

How a DNS request works:

  1. I ask my nameserver for example.com.
  2. It doesn’t know, so it goes and ask the root nameservers “Hey, who is the nameserver for example.com?”
  3. One of the 13 root nameservers says “Yes, example.com is serviced by dns1.somebody.com and dns2.somebody.com and here’s their IP address’s. Now leave me alone, I’m busy.”
  4. So my name server now goes and asks dns1.somebody.com, “Hey, do you know the IP for example.com?”
  5. And then dns1.somebody.com says “Sure, it’s 1.2.3.4″.

Lot of work. But note that my nameserver talked to TWO nameservers here. The root, and the actual nameserver for example.com. The TTL on the root lookup is not something I can adjust, usually. Or if I can, then it’s very difficult to do for most people. The TTL on the second is not nearly as hard, and doesn’t require all the root nameservers to be updated.

If you own a domain, then you need to own your DNS. Use a nameserver that you have control over, and which you will almost never change. You can change the addresses, you can change the MX records (for email), you can add TEXT records for SPF, and other DNS tricks all you want. But keep the nameserver location itself unchanged for as long as possible. Because when you do change it, the internet takes a long, long time to adjust to that fact. And you are then “offline” for quite a while.

To think of it another way: He who controls the DNS, controls the domain, as least for a while. Well, you’re going to change hosts a lot more often than you change domain names, right? So don’t give them your DNS control. No webhost should ever have your nameserver pointed to them unless you bought the domain name from them in the first place.

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