Odd DNS behaviour with VIASAT connection.

sgbotsford · 10-11-2012, 09:44 AM

We connect to the world with viasat, one of the latest two way satellite services, purchased through xplornet.com.

We get periodic DNS outages with the following characateristics:

host xplornet.com {ip of xplornet's dns server} will fail.

Established internet connections still function. names that are still in cache will continue to work, at least until they expire.

Pinging by IP address works with normal latency.

If I run hping -c 5 --udp {dns server IP} it fails.
If instead I run hping -c 5 -S {dns server IP} I get a slow response.

The support centre at Xplornet says I'm the only one with this problem.

Ideas?

Power cycling the satellite modem will cure the problem.

agentx · 10-11-2012, 10:28 AM

What version of OS you running ?

Firstly - You can try and turn off IPv6 on your network interface (Ethernet/WIFI aka Airport) and see if it helps.

Secondly - manually enter the ISP DNS servers (or even try a third party but not so good on SAT links) into your Network preferences, it could be a mixture of modem/OS X network stack not passing DNS requests or servers correctly.

In short many of these issues on OS X can be related to the crappy mDNSresponder process which handles all DNS functionality. Often many DNS related issues on OS X can be sorted by restarting the process.
Next time it happens just as a test...try this....

Code:

sudo launchctl unload /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
sudo launchctl load /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist

or

Code:

sudo killall -HUP mDNSResponder

Also
sudo killall -INFO mDNSResponder will give you detailed information about DNS into your Console logs

In Mac OS X v10.6+, the default DNS server searching behavior is that when a server does not return a result (returning SERV_FAIL for a query), and other servers are available to query, the server is temporarily disabled in the search order for about thirty seconds. If there is more than one server for the query and all of them have returned SERV_FAIL, the servers will be queried in the order that they were disabled (that is, the server that has been disabled the longest will be used first).

I have in the past on SAT links run a local caching DNS server on a server or local machine to help with "time out" issues when SAT DNS slow to respond or general connectivity issues. But this was a long time ago and many of the issues have gone nowadays ;-)

sgbotsford · 10-12-2012, 11:24 AM

The previous statement about hping -c 5 --udp {dns server IP} is inaccurate.

That command will always fail. Since udp has no setup and tear down, you only get a response from the server, not from the network stack. So to test a udp server, you need to send it a well formed packet so that the server will parse it and send you something back.

Does anyone know of a public echo server?

sgbotsford · 10-14-2012, 02:56 PM

I'll check out the mDNS issue. The comman 'host' apparently doesn't use this subsystem so it's a good way to check that it is not just a mac issue.

We have 3 macs in the house, as well as 2 iphones and an ipad. When it fails, it fails for all of them at once, allthough with cached results it's hard to tell.

I've also bypassed the router and plugged in a single mac while this problem was occuring, shutting the router off to ensure it could not interfere wirelessly. Doesn't help.

But power cycling the modem always works. For a while.

sgbotsford · 10-14-2012, 02:59 PM

Oh: As to OS versions. All macs are running snow leopard. IOS devices are all at 5.

agentx · 10-14-2012, 04:59 PM

It could well to be modem/router issue then. Thy can go bad.
Try what I said anyway and see if it helps.

10-11-2012, 09:44 AM	#1
sgbotsford Prospect Join Date: Jan 2011 Posts: 13	Odd DNS behaviour with VIASAT connection. We connect to the world with viasat, one of the latest two way satellite services, purchased through xplornet.com. We get periodic DNS outages with the following characateristics: host xplornet.com {ip of xplornet's dns server} will fail. Established internet connections still function. names that are still in cache will continue to work, at least until they expire. Pinging by IP address works with normal latency. If I run hping -c 5 --udp {dns server IP} it fails. If instead I run hping -c 5 -S {dns server IP} I get a slow response. The support centre at Xplornet says I'm the only one with this problem. Ideas? Power cycling the satellite modem will cure the problem.

10-11-2012, 10:28 AM	#2
agentx Hall of Famer Join Date: Feb 2003 Location: Brighton, UK Posts: 3,832	What version of OS you running ? Firstly - You can try and turn off IPv6 on your network interface (Ethernet/WIFI aka Airport) and see if it helps. Secondly - manually enter the ISP DNS servers (or even try a third party but not so good on SAT links) into your Network preferences, it could be a mixture of modem/OS X network stack not passing DNS requests or servers correctly. In short many of these issues on OS X can be related to the crappy mDNSresponder process which handles all DNS functionality. Often many DNS related issues on OS X can be sorted by restarting the process. Next time it happens just as a test...try this.... Code: sudo launchctl unload /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist sudo launchctl load /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist or Code: sudo killall -HUP mDNSResponder Also sudo killall -INFO mDNSResponder will give you detailed information about DNS into your Console logs In Mac OS X v10.6+, the default DNS server searching behavior is that when a server does not return a result (returning SERV_FAIL for a query), and other servers are available to query, the server is temporarily disabled in the search order for about thirty seconds. If there is more than one server for the query and all of them have returned SERV_FAIL, the servers will be queried in the order that they were disabled (that is, the server that has been disabled the longest will be used first). I have in the past on SAT links run a local caching DNS server on a server or local machine to help with "time out" issues when SAT DNS slow to respond or general connectivity issues. But this was a long time ago and many of the issues have gone nowadays ;-)

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10-12-2012, 11:24 AM	#3
sgbotsford Prospect Join Date: Jan 2011 Posts: 13	The previous statement about hping -c 5 --udp {dns server IP} is inaccurate. That command will always fail. Since udp has no setup and tear down, you only get a response from the server, not from the network stack. So to test a udp server, you need to send it a well formed packet so that the server will parse it and send you something back. Does anyone know of a public echo server?

10-14-2012, 02:56 PM	#4
sgbotsford Prospect Join Date: Jan 2011 Posts: 13	I'll check out the mDNS issue. The comman 'host' apparently doesn't use this subsystem so it's a good way to check that it is not just a mac issue. We have 3 macs in the house, as well as 2 iphones and an ipad. When it fails, it fails for all of them at once, allthough with cached results it's hard to tell. I've also bypassed the router and plugged in a single mac while this problem was occuring, shutting the router off to ensure it could not interfere wirelessly. Doesn't help. But power cycling the modem always works. For a while.

10-14-2012, 02:59 PM	#5
sgbotsford Prospect Join Date: Jan 2011 Posts: 13	Oh: As to OS versions. All macs are running snow leopard. IOS devices are all at 5.

10-14-2012, 04:59 PM	#6
agentx Hall of Famer Join Date: Feb 2003 Location: Brighton, UK Posts: 3,832	It could well to be modem/router issue then. Thy can go bad. Try what I said anyway and see if it helps.