NSLookup is a command-line tool used to verify DNS resolution by returning the IP addresses associated with a domain name. MX Toolbox extends DNS testing through a web interface, supporting lookups for MX, A, AAAA, SPF records, and blacklist checks.
DNS Testing Tools
We know that we don't type in IP addresses to get to certain websites. In fact, that becomes even more critical when we're dealing with IP version 6 addresses, because they're so long. So instead, what we do is type in some sort of name using the naming convention that we talked about before. An example of this that we're going to be using is www.google.com, where www is the host and google.com is the domain.
What we can use to test that domain lookup is working on this machine is nslookup. To use nslookup, all I have to do is type in nslookup and the domain that I want to access. So I'm going to type in nslookup www.google.com and hit enter.
It's showing me the server that it's using to look this up, which is 192.168.42.1. That is my home router that it's using, and that home router then is in turn going and looking up other DNS servers and getting the information that I need. It's coming back with an IP version 6 address, and it's coming back with an IP version 4 address. So we have now successfully figured out what IP addresses are associated with www.google.com.
Now there are some additional features with nslookup. To use those additional features, I just type in nslookup, and that accesses the program and opens it up. It's showing me which server it's using, and all I have to do now is type in www.google.com, and it gives me the same answer here. So I can see that there's the same answer there.
But perhaps I want to change some of the options — maybe I want to reference a different server. Then I just type in server and then the server that I want to use. In this case I'm going to use the 8.8.8.8 server, which is a DNS server that's publicly available out there. So I'll hit enter, and now I'm going to type in www.google.com and see what I come up with. Here we have the same answer once again, but it's from a different server.
So those are some ways that you can use nslookup to test to make sure DNS is working.
There is one other tool that I wanted to show you, a tool that I like, and that is MX Toolbox. I'm going to bring up a browser and type in MX Toolbox, and we're just going to take a look. I'll click on this MX Toolbox.
What I can do here is lookups. This particular one is for MX records, so let's take a look at the MX record for google.com and hit enter. It's going to actually show me the mail exchange record for it. I forgot an E in there, so — there, that looks a little better. It's showing me some SMTP addresses that it has, and showing me some other records that are mail exchange records.
In fact, I can actually look up a lot of different types of records. The records that we were talking about before were just an A record, or I could look up just the AAAA record, the quad A record, which would be the IP version 6 address for these domains. You can see that I can look up a lot of different information, and also do things like check the domain for blacklists. So there are a lot of cool features with this MX Toolbox. I recommend going and checking it out, because it'll give you some aspects to the different records.
Another one that I'll use this a lot on is the SPF record. The SPF record is a text record that tells machines what servers are allowed to send mail on behalf of it. This is one of the records that I'll quite often look up, and then it gives you this information of what servers are allowed to send mail on behalf of it.
So MX Toolbox is a great resource, one of those things that you can jump on and play around with.
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