The DNS and DHCP protocols are indirectly used for network communication, but they are no less important. In this video, you’ll learn how to troubleshoot DNS and DHCP configuration issues.
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If you’re having problems with your DNS process or a DNS server, then you may get a notification that your users, that the internet is down. Which really means that they’re in a browser, they’re trying to communicate to a website, and they’re not able to communicate to any of those websites out on the internet.
You might also see a DNS problem if you can ping an IP address but you’re not able to communicate to that address through a web browser using its fully qualified domain name. Even when you know the web service is running, for some reason you’re not able to communicate to that device when you’re inside of a browser.
Applications can also have problems if your name resolution process is not working properly. A good developer is going to use a fully qualified domain name in the application rather than to use an IP address, but of course, that means that a name resolution has to occur, and if it’s not able to occur the application is not going to work.
When troubleshooting DNS issues, the first thing you should check is to see what DNS your device is using. So look at the IP address configuration and see what IP is associated with the primary and secondary DNS for your device. You can then try to perform a name service lookup using nslookup or dig. Simply use a name that you know should be resolving and see if you get a response from this name server.
You might also want to try a different name server. If it’s something that’s external, you could try Google’s name server at 22.214.171.124 or 126.96.36.199. By changing an IP address for your DNS server and then trying a name resolution, you can see if the problem is something associated with one DNS server or if it’s something that’s outside the scope of DNS.
If your dynamic host configuration protocol server is giving your problems, then you may see a number of different symptoms. DHCP is completely automatic, of course, and we don’t even think about it. We plug into the network. We’re automatically provided an IP address. There was no human intervention, and we’re simply using the resources on our network.
If you’re having a misconfiguration or a problem with your DHCP server, then you may be able to access some local devices but not able to access any devices that might be outside of your local network. If you’re looking at your IP address and it doesn’t seem like you received a dynamic IP address, you may have received an APIPA address. That’s the automatic IP addressing address, and you can see that those addresses start with 169.254.x.x. So if you’re noticing that your IP address begins with 169.254, that probably means that there is an issue with DHCP.
The first thing we should check then is our network connection. If we’re sending DHCP requests and we’re getting no responses from any DHCP server, then we’re going to have a locally automatically assigned address, the APIPA address, and that’s very common if you send a request and you’re not getting any response from any DHCP server.
The DHCP server itself might be having problems. It might be down or the service itself may be having issues. Or you might have run out of IP addresses. Even though the DHCP server is running, there are no IP addresses in the pool to assign to any new devices. Or the DHCP server may be completely down or disabled. You may only be able to use static IP addresses until that particular problem is resolved.
We often engineer our networks with multiple DHCP servers so that if you lose one server you have a backup. But if your network was not engineered this way and you’ve lost the primary DHCP server, then you won’t be able to get any IP addressing and you’re going to have problems communicating outside of your local subnet.