DNS troubleshooting

DNS problems can cause all sorts of strange network behavior. Not being able to resolve Internet host names is an obvious symptom, but because of reverse lookups, general server slowness can be caused by the same thing. Your server tries to look up a host name for a local machine trying to connect to it; if it takes too long, the other machine can give up.

Often machines are configured to use the ISP's DNS server, which of course knows nothing about local machines. As long as it returns failure quickly, that's no problem: the server will continue on to look in its local files or its local DNS server. Sometimes the ISP's server isn't actually broken, but simply takes too long to respond, causing timeouts or extreme delays locally.

On a Unix or Linux box, DNS servers can be easily tested at the command line. Let's pretend your ISP DNS is 61.2.3.4:

dig @61.2.3.4 192.168.2.3

If that doesn't return quickly, local machines could experience problems. However, that test may not be enough to prove the issue. The absolute test is to change your configuration to take the ISP server right out of your configuration. If you comment out the nameserver in /etc/resolv.conf and local server issues improve, there obviously is a problem and it probably isn't at your end.

Jump to Comments

Many of the products and books I review are things I purchased for my own use. Some were given to me specifically for the purpose of reviewing them. I resell or can earn commissions from the sale of some of these items. Links within these pages may be affiliate links that pay me for referring you to them. That's mostly insignificant amounts of money; whenever it is not I have made my relationship plain. I also may own stock in companies mentioned here. If you have any question, please do feel free to contact me.

Specific links that take you to pages that allow you to purchase the item I reviewed are very likely to pay me a commission. Many of the books I review were given to me by the publishers specifically for the purpose of writing a review. These gifts and referral fees do not affect my opinions; I often give bad reviews anyway.

We use Google third-party advertising companies to serve ads when you visit our website. These companies may use information (not including your name, address, email address, or telephone number) about your visits to this and other websites in order to provide advertisements about goods and services of interest to you. If you would like more information about this practice and to know your choices about not having this information used by these companies, click here.