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   /  General Technology (No Console Threads)
      /  Why DNS lookups are so slow?
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elwood 
Why DNS lookups are so slow?
Posted on 1-Aug-2016 13:30:25
#1 ]
Elite Member
Joined: 17-Sep-2003
Posts: 3428
From: Lyon, France

... on Windows !

I know this site is full of skilled IT engineers so I have to ... test your skills

I had this same problem so many times in the last decade (yes, 10 years) that I would love to know a solution.

At work, the program from my company access a database using ODBC.
I have a Windows system (whatever version of Windows). If I do an ODBC access to 127.0.0.1 or the IP address of the server, the access is instantaneous.
If I use the computername "myserver", it needs 2 seconds to complete.

Often if I add an entry to the host file with "127.0.0.1 myserver", the access is instantaneous again.
So it shows the DNS lookup is too slow.
But sometimes even adding the entry in the host file, the access is still slow. I suspect this behaviour happens because IPV6 is enabled. When I disable IPV6 (poking the registry) lookups are fast again.

So the questions are:
- why DNS lookups are so slow?
- what is the correct way to have them go fast?
- should I disable IPV6 despite the "you shouldn't" warnings?

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Philippe 'Elwood' Ferrucci
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BinoX 
Re: Why DNS lookups are so slow?
Posted on 1-Aug-2016 15:18:26
#2 ]
Cult Member
Joined: 2-Nov-2005
Posts: 711
From: Cornwall, UK

@elwood

It's probably the IPv6/IPv4 compatibility layers...

The DNS lookups themselves are probably quite fast, but most likely returning an IPv6 address...
The database software is probably only bound on an IPv4 address, meaning that the IPv6 connection attempt has to timeout before the compatibility layer takes over and tries an IPv4 connection.

The delay is most likely the IPv6 connection timeout... (hosts file is a little unreliable under windows, I have seen situations where it ignores what is in there)

As for how to speed it up... It depends on the db software that you're using...
If you can make it use an IPv6 connection as well, then doing that may well solve your problems...

Disabling IPv6 on your network adapter (by unticking the box in the adapter settings) might solve your problem as well, rather than completely disabling it (assuming you don't have any need for IPv6 on the network)

Maybe also this:

Open RegEdit
Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\tcpip6\Parameters
Create a DisabledComponents DWORD registry value (or modify if it already exists), set its value to 20 in Hex (32 Decimal)
Reboot machine

This will get your machine to PREFER IPv4 connections.

Last edited by BinoX on 01-Aug-2016 at 03:22 PM.

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elwood 
Re: Why DNS lookups are so slow?
Posted on 1-Aug-2016 15:28:51
#3 ]
Elite Member
Joined: 17-Sep-2003
Posts: 3428
From: Lyon, France

@BinoX

Ah yes, that must the V4->V6 translation that takes time.

You are right, the database (OpenEdge) is bound on V4 only but I see that it can do IPv6 as well. I'll need to check my DBA.

Yes, the registry trick is what I did.
Note that I read unticking V6 on the adapter does not disable V6. I don't remember the details though.

Thanks.

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Philippe 'Elwood' Ferrucci
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BinoX 
Re: Why DNS lookups are so slow?
Posted on 1-Aug-2016 15:35:08
#4 ]
Cult Member
Joined: 2-Nov-2005
Posts: 711
From: Cornwall, UK

@elwood

Quote:
Note that I read unticking V6 on the adapter does not disable V6. I don't remember the details though.


It doesn't disable it for the system completely, but it will stop it from doing IPv6 over that adapter. (The local loopback adapter and any other NICs that you have will still do IPv6)

From personal experience (network of about 600 stations) just unticking the box does the trick...

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elwood 
Re: Why DNS lookups are so slow?
Posted on 1-Aug-2016 15:41:01
#5 ]
Elite Member
Joined: 17-Sep-2003
Posts: 3428
From: Lyon, France

@BinoX

It seems it does disable it on Win7 or Win2008 but on 2012R2 I had to modify the registry.

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