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Sunnuntai, April 26th, 2009 | Kirjailija:

Trust me. We’re still dealing with regexesjust in a roundabout (and vaguely practical) way. This is a pretty comprehensive listing of how to go about flushing DNS caches while using regexes to show where similar methods deviate.

Why do we want to clear DNS caches exactly?

There are a number of reasons to clear DNS caches, though I believe these are the most common:

  • An intranet service has an private (internal) IP address when on the company network but it has a public IP address for outside access. When you try to access that service from outside after accessing it from inside, there’s a chance that you would have cached the private (inaccessible) IP. A good long-term solution is to make the service inaccessible except via VPN. A simpler solution is to leave work at work. 😛
  • An internet service or web site changes their DNS settings and your desktop/laptop is looking at theoldsetting. In this case, the new setting has not yet propagated. Hosting Admins come across this case very often.
  • Yksityisyys: If someone can track your DNS history then it wouldn’t be too hard to figure out which web sites you’ve been viewing. Though the individual pages you’ve viewed can’t be tracked in this way, the hostnames, kuten “dogma.swiftspirit.co.za” tai “google.comwill be in the DNS cache, likely in the order you first accessed each site. There are better ways to do this though. One example is to use a Tor network for all DNS requests.

Flushing WindowsDNS cache, from command prompt:

Evidence suggests that prior to Windows 2000, Windows OS’s didn’t cache DNS results. The ipconfig command, run from the command prompt, was given some control over the DNS cache and has remained roughly the same since.

To get to the prompt if using Vista as non-Admin: Start -> Programs -> Accessories -> Right-clickCommand Prompt” -> Run As Administrator

Otherwise: Start -> Run -> [cmd ] -> [ Kunnossa ]

ipconfig /flushdns

Flush the DNS Resolver Cache in Windows

It is also possible to clear the cache in Windows by restarting theDNS Client” tai “Dnscache” palvelu.

Flushing Mac OS X DNS cache, from shell prompt:

Since Mac OS X, Apple Macs have been running a Unix-based, POSIX-compliant, operating system based on Nextstep, itself originally containing code from FreeBSD ja NetBSD. Mac OS X uses lookupd tai dscacheutil to manage the DNS cache, depending on the version.

To get to the prompt: Applications -> Utilities -> Terminal

(lookupd|dscacheutil) -flushcache

What have we here? As per osa 1, the vertical bar indicates that eitherlookupdORdscacheutilare acceptable. The parenthesis indicate that the vertical bar only applies to thelookupd|dscacheutilportion of the expression. Thus, the ” -flushcacheis not optional and must be included in the command in order for it to work. Note that these commands produce no output unless there is an error.

Use dscacheutil if you’re using Mac OS X 10.5 (Leopard) or later.

Mac OS X:

lookupd -flushcache

Mac OS X Leopard:

dscacheutil -flushcache

Use dscacheutil to flush the cache in Mac OS X Leopard

There is also a GUI tool, DNS Flusher, which automatically uses the correct command available.

Flushing Linux/UnixDNS cache, from shell prompt:

N.B. If you don’t already have either bind (with caching lookup enabled), nscd, or dnsmasq installed and running on your *nix-based desktop/server, you are probably not caching DNS at all and there is nothing to flush. In that case you will be utilising your DNS server for every web request, probably slowing your web experience.* If so, I recommend at least installing nscd as it is the easiest to set up. **

Flushing nscd’s cache

As with the Mac OS command, this produces absolutely no output unless there is an error:

(|sudo )(|/usr/sbin/)nscd -i hosts
  • Use sudo if you’re not already root otherwise the first selection is blank.
  • Specify /usr/sbin/ if nscd is not already within thepath”. If your distribution has nscd in a strange place, locate it first:
locate -r bin/nscd$

Notice that the abovebin/nscd$is itself a regular expression. 🙂

Using nscd, invalidate thehosts” kätkö, logged in as a user:
sudo nscd -i hosts
Using nscd, invalidate thehosts” kätkö, logged in as root:
nscd -i hosts
Using nscd, invalidate thehosts” kätkö, logged in as root, specifying the full path:
/usr/sbin/nscd -i hosts

Flushing bind’s cache

To flush bind’s cache, we issue a command via rndc. Use sudo if you are not already root:

(|sudo )rndc flush

Restarting the cacheing services also works!

Here’s how to restart either of the caching daemons:

(|sudo )(palvelu |/etc/(rc\.d|rc\.d/init\.d|init\.d)/)(bind|dnsmasq|nscd) restart

That’s starting to get difficult to read. *** Luckily I’ve explained in detail:

  • As with the previous command, use sudo if you’re not already root.
  • The second selection has the first option “palvelu “. This applies mainly to Red Hat/CentOS and Fedora systems.
  • The “/etc/(rc\.d|rc\.d/init\.d|init\.d)/” needs to be expanded further. This is for most other systems. Generally, the rc.d is for if you’re using a BSD-style init system (esimerkiksi: Arch Linux, FreeBSD, or OpenBSD). The best way to know for sure which command to use is to ‘locatethe correct nscd or dnsmasq path. Most Unix flavours, even Solaris, use nscd:
locate -r \.d/nscd$ ; locate -r \.d/dnsmasq$ ; locate -r \.d/rndc$
  • The last choice is betweenbind”, “nscd”, ja “dnsmasq”. This depends entirely on which is installed and in use.
  • The last of the pattern, ” restart”, is the instruction given to the daemon’s control script.

Kaari, using dnsmasq, restarting the cache daemon, logged in as root:

/etc/rc.d/dnsmasq restart

Kaari, using nscd, restarting the cache daemon, logged in as user:

sudo /etc/rc.d/nscd restart

CentOS / Red Hat, using nscd, restarting the daemon, as root:

service nscd restart

nscdrestart

Flush Mozilla Firefox’s internal DNS cache:

Mozilla Firefox keeps its own DNS cache for performance. Firefox 2 would cache only 20 entries for up to 60 seconds. The default setting as of Firefox 3 appears to be 512 entries for up to 60 minutes which seems much more reasonable for every-day browsing. If your desktop has a built-in cache (which most now do) then the cache here is actually redundant. I’m not aware of any other browsers that implement DNS caching.

I’ve found a few solutions for when you need to clear the cache. It seems there are many ways to do this however these are the easiest, which I’ve put into order of preference.:

  1. Install the Firefox DNS Flusher Addonprovides a button to flush the cache.
  2. Install the DNS Cache Addonprovides a toggle which disables or enables the DNS cache.
  3. Clear Cache (clears browser cache as well as DNS Cache): Select Tools -> Clear Private Data; Deselect all checkboxes except for Cache; Naksahdus [ Clear Private Data Now ].
  4. Manually do what DNS Cache does: set the following 2 about:config optionsnetwork.dnsCacheExpiration” ja “network.dnsCacheEntries” kohteeseen 0 and then back to the default.

I had a bad cached record and I cleared my browser’s cache. But its still giving me the wrong info. What gives?

Because of how DNS propagation works, you preferably need to flush the DNS on all DNS hosts between yourself and theauthoritivehost, starting with the host closest to the authoritive host (furthest away from your browser).

As an example, if you have a router that is caching DNS, reset the router’s cache before restarting the DNS cache of your operating system, and only then should you clear the cache in Firefox. The reason is that even if you only clear your OS and Firefox’s caches, your desktop is still going to ask the router for its bad record anyway.

What if my DNS server is a server on the net outside my control?

You could try temporarily using a different nameserver, possibly even a publicly open server. OpenDNS shows some good information on how to do this. If you’d like, you should also be able to get relevant information from your own ISP regarding their resolving DNS servers. A local example (South Africa) is SAIX joka lists their resolving DNS servers.

* Likely the reason why Firefox has a DNS cache built-in ****
** “((Pacman|jogurtti) -S|emerge|(yum|soveltuvuus|apt-get) install) nscdand then ensure that the service is added to the startup scripts. Refer to your distribution’s installation documentation.
*** I’m looking for a syntax highlighting plugin that can work with regex
**** I’ve read statements that restarting the network(ing|) service also clears the DNS cache however I haven’t seen any evidence that this is true. If anyone has a example where this is true, please provide me with the details.
Jakaa
Maanantai, November 17th, 2008 | Kirjailija:

I’d never really had the need to connect to a VPN until this weekend. After connecting, I found that my Internet access was rather non-functional except to the VPN in question. A colleague happened to be on hand (he’d given me the access details in the first place) and he quickly suggested this workaround.

Today, a client had the same issue. Perhaps this problem is more common than I first thought.

When connecting to the VPN, Windows updates the default gateway on your desktop to reflect the VPN’s settings. Most likely, kuitenkin, you only need to access specific subnets on the VPN and you want all unrelated traffic to use youroldsettings.

It turns out that its a simple checkbox that needs to be unchecked. The jist (sp.??) of finding the setting: Right-click the VPN in Network Connections -> Properties -> Internet Protocol (TCP/IP); [Properties] ; [Kehittynyt], and uncheck the “[ ] Use default gateway on remote network”.

Then click the usual OK/Apply/Yes-of-course-your-dialogue-ness (all the while reading and absorbing any warnings appropriately) until you’re back to your Network Connections window. Right-click the VPN connection and disable / re-connect.

You should be able to confirm that the Default Gateway does not change by running the command-line app ipconfig before and after enabling the VPN connection. Look specifically for the line labelledDefault Gateway”.

[edit reason=moore”]…

It turns out that a possible reason for this setting being the default setting is for security. If your desktop happens to be compromised or inadvertently routing traffic, connecting to the VPN might expose the supposedlyprivatenetwork to the Internet.

Jakaa
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