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 the “old” setting. In this case, the new setting has not yet propagated. Hosting Admins come across this case very often.
• Privacy: 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, such as “dogma.swiftspirit.co.za” or “google.com” will 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 Windows’ DNS 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-click “Command Prompt” -> Run As Administrator

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

ipconfig /flushdns

It is also possible to clear the cache in Windows by restarting the “DNS Client” or “Dnscache” service.

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 and NetBSD. Mac OS X uses lookupd or 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 part 1, the vertical bar indicates that either “lookupd” OR “dscacheutil” are acceptable. The parenthesis indicate that the vertical bar only applies to the “lookupd|dscacheutil” portion of the expression. Thus, the “ -flushcache” is 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

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

Flushing Linux/Unix’ DNS 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 the “path”. If your distribution has nscd in a strange place, locate it first:

locate -r bin/nscd$

Notice that the above “bin/nscd$” is itself a regular expression.

Using nscd, invalidate the “hosts” cache, logged in as a user:

sudo nscd -i hosts

Using nscd, invalidate the “hosts” cache, logged in as root:

nscd -i hosts

Using nscd, invalidate the “hosts” cache, 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 )(service |/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 “service “. 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 (for example: Arch Linux, FreeBSD, or OpenBSD). The best way to know for sure which command to use is to ‘locate’ the 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 between “bind”, “nscd”, and “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.

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

/etc/rc.d/dnsmasq restart

Arch, 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

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 Addon – provides a button to flush the cache.
2. Install the DNS Cache Addon – provides 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; Click [ Clear Private Data Now ].
4. Manually do what DNS Cache does: set the following 2 about:config options “network.dnsCacheExpiration” and “network.dnsCacheEntries” to 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 the “authoritive” host, 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 which lists their resolving DNS servers.

Regex Quick Start

1. A vertical bar |, sometimes known as a pipe, is an “OR” operator in regular expressions. It indicates that the pattern to the left OR to the right is acceptable. On the keyboard, this key is typically the “capital”, or “shifted version”, of the backslash key.
2. Parentheses are round brackets () that select a section of a regular expression. They can be used similarly to in mathematics when the order of operations is adjusted. The “selection” can be referenced elsewhere though we’ll cover that later.
3. A dot . is a special character which matches any character other than “newline” characters.
4. A backslash \ is sometimes called an escape character. There are many characters, such as the parentheses and the vertical bar above, that have special meanings. The backslash is used to take away (escape) those special meanings. For example, to specify an actual full stop, you need to escape the special dot character: \.
5. A star * is an operator which indicates that the preceding character can be repeated zero or more times. This is often combined with the dot to produce a “wildcard” pattern, which matches a string of any length: .*

Using the vertical bar and the escape character

Here we have a number of example web addresses:

http://dogma.swiftspirit.co.za/

http://swiftspirit.co.za/

http://google.com/

If I want a pattern that matches them all, I could use the vertical bar (rule 1 above) to separate them. I’d then also need to use the escape character (rule 4 above) for the full stops:

http://dogma\.swiftspirit\.co\.za/|http://swiftspirit\.co\.za|http://google\.com/

To above will match every URI in my example however it isn’t specifically efficient or elegant.

Using parenthesis

The same can be achieved with the following, shorter, regex, utilising the parenthesis from rule 2 above:

http://((|dogma\.)swiftspirit\.co\.za|google\.com)/

Notice that I’ve simply placed common items, for example “http://” and “/”, outside the selections.

What if you want to match text that might be anything?

Maybe you have more that you still want to match? For example, if I put http://swiftspirit.co.za/downloads/ and I still want the pattern to match that or any other url under my web site. Or maybe its okay for http://anything.google.com/anything to also match. We can use the special dot and star characters from rules 3 and 5 from above:

http://((|dogma\.)swiftspirit\.co\.za|(|.*\.)google\.com)/.*

Simply adding that any character can appear zero or more times lets a lot more match without adding too much to the regex’s complexity:

(|.*\.)

The above is from in front of the google.com pattern. The vertical bar indicates two options here: either blank (so we end up with just google.com), or .*\. (so we have anything followed by a literal dot and that is then followed by google.com).

Practise makes perfect

We can already see from the last example that a regular expression can very quickly become complex. Much like any language, being easily able to read a regular expression takes practice. If you have a relevant need to work with regular expressions, you’re in for a treat.

Part 2 should be due in a few days. I’ll be doing some very indirect “work”, demonstrating how to flush DNS caches in a variety of systems.