.NET Developers’ user guide for troubleshooting networking problems Part 1

Introduction

As a programmer I normally don’t need to deal with hard-core networking issues in my job. The company I work at has a group of well-trained network engineers that fix network related problems for developers. However, I sometimes have the need to check some more basic things within networking to debug my code. Also, it can be beneficial to be able to follow along when network engineers discuss subnets, DNS records, ports and the like.

This is exactly the goal of this series: to help developers get to grips with the most basic concepts within networking. You certainly won’t become a professional networking engineer but you may not need that either.

Note: I did all demos on a Windows 7 machine. Other versions of Windows may output the values in a different format.

A network request

What happens when you enter a URL in your browser and press enter?

Networking diagram

The client wants to view http://www.bbc.co.uk to read the news so she enters that URL in the browser. The URL must then be converted into an IP address by the client computer therefore it needs to find out the IP address of http://www.bbc.co.uk. It performs this task by a service called DNS or Domain Naming System.

So it consults its configured DNS servers for the IP address of bbc.co.uk. The DNS server looks up the IP address and sends it back to the client. The client can now go out to the Internet through its switch and router and reach the data centre where the server is located. It will then pass through a firewall and switches to finally arrive at the web server. In the web server it enters the networking stack of the operating system, usually followed by a host based firewall and at last it reaches the process that’s the actual web server.

The data is then sent back to the client in the form of HTML, JSON, XML or whatever the format of the web application and it is rendered on the client machine.

The IP address

Each node in the network has an IP address, which is analogous to the unique address of your home. The postman needs to find you somehow so he will read the address on the letter and deliver it to your letterbox.

An IPv4 address is made up of 4 octets separated by a period similar to the following: 83.183.46.130.

Then we have the subnet mask which defines which part of the IP address is the subnet and which part is the specific node on that network. A subnet mask may look as follows: 255.255.255.0. We’ll look at subnets in a future post but for the time being it’s enough to know that if you try to reach an IP address which is not part of your subnet then it has to go through the default gateway. The default gateway can have an address such as 192.168.0.254.

Then we have the DNS servers that the client computer will use to turn names into IP addresses. Their IP typically looks like 75.75.75.75 or 75.75.75.76.

It’s easy to check your own IP configuration. Open a command prompt and run the ‘ipconfig’ command. The no-args version of the command will show your basic network configuration:

ip config no args

You will see the IPv4 address, the subnet mask and the default gateway. If you run the command ‘ipconfig -all’ then you’ll get a lot more information. You’ll see your host name at the top of the output. You’ll also find the DNS server somewhere in the middle. Your computer is configured to point to that DNS server to translate http://www.bbc.co.uk into numbers. Also, you’ll see something called the DHCP server. The DHCP server, which stands for Dynamic Host Configuration Protocol, is where your computer obtains the IP configuration.

So when a machine comes online and needs an IP configuration then it sends out a message asking for one. The DHCP server will catch that message and will respond with an IP address, a subnet mask, a default gateway and one or more DNS servers. The client machine will then take that information to configure itself and respond to the DHCP server saying that it will use that address. The DHCP server will then know that this IP is in use and will not hand it out to any other online machine for a specified period of time:

ObtainingAnAddressFromDHCP

Starting with Windows 2000 if the client is unable to get hold of an IP address then eventually it will give itself an address in the 169.254 address range which is a range owned by Microsoft. The client will eventually send out a message saying “I’m using 169.254.x.x”. This scenario occurs extremely rarely but if you see that your computer is struggling to get an IP and gets an IP in this range then it’s telling you that something is wrong and you’re not getting a response from the DHCP server.

What’s IPv6?

The current IP version is use is version 4, or IPv4. With the format mentioned above, i.e. 4 octets we get 2^32 – 2 raised to the power of 32 – different addresses. That’s quite a large number but is definitely finite and we’re soon reaching its upper limit.

IPv6 has been developed to extend the number of possible variations to 2^128 which is so large that we’ll enough left for all visiting extraterrestrials in the year of 10000.

Now IPv4 and IPv6 are running parallel. That’s why the ipconfig command gave you both and IPv4 and an IPv6 address. The ultimate goal is to only go forward with IPv6 sometime in the future.

You’ll see that the format of IPv6 is very different from IPv4. Example: 2001:0:5ef5:79fd:20df:3736:3f57:ffbe. As a developer you need to be aware of the differences if you need to log or validate an IP address or your app needs to show the new format on the screen.

DNS

So how is the name resolved that you enter in the URL text box of your web browser? As we mentioned above the client is configured to point to an initial DNS server. Say that it’s configured to contact nameserverA.isp.com. Therefore the client is going to ask this DNS server to resolve a URL and get the IP address belonging to that URL. The first DNS server probably won’t have this record so it sends a request to the root name servers: do you know where I can find this URL? The root name servers only contain the name server locations for the top level domains: .info, .com, .uk etc. and it’s the only thing it knows. So the root NS responds the first name server, like “no, I only know about top level domains but you can ask the .com name server because I know it has more information.” So nameserverA.isp.com asks the .com name server. The .com name server will have information about where to find the IP addresses of all .com URLs so it tells nameserverA.isp.com to go and ask the cnn.com name server. The cnn.com name server will have all the information about the cnn.com namespace and responds with the IP address.

DNS name resolution

The image is a bit messy so make sure you follow all the arrows based on the description. As you see the configured name server nameserverA.isp.com has a central role in the quest for finding the IP address. It takes a couple of stops before the final answer has been found.

NsLookup

You can use the command line tool called NsLookup to perform DNS queries. Let’s try to look up the IP address of cnn.com:

NsLookup cnn.com

Alternatively you can just type ‘nslookup’, press enter and then you can perform multiple queries:

NsLookup multiple queries

You’ll see that cnn.com returned more than one IP address. It means that we can reach cnn.com using several different IP addresses. Try http://www.microsoft.com and you’ll see that it’s been aliased to akadns.net, which are Akamai addresses. Akamai is a Content Delivery Network solution for faster downloads: Akamai homepage.

When you’re done using nslookup in the multiple query mode you can just type exit to come back to the ‘normal’ command prompt mode. In case you want to change the DNS server for your lookup query then enter the multiple query mode again by typing ‘nslookup’ and run the command ‘server [ip of the dns server you want to ask]’, e.g. server 123.456.678.43 and then ask for http://www.microsoft.com to see if you get the same IP address as in the case of the default DNS server.

The DNS records are cached for a certain period of time in the name servers to speed up the queries. Even your local machine caches this information. In your command window type ping the following 3 URLs using the ping command:

ping http://www.cnn.com
ping http://www.bbc.co.uk
ping http://www.microsoft.com

Then enter the following command: ipconfig /displaydns. This will bring up a list of all records cached on your local machine including the ones you have just pinged:

Ip config display DNS

Check out the Time to Live value. The record microsoft.com is cached for about 3500 seconds on my local machine. Wait a little bit and enter the ipconfig /displaydns command again. You should see that the Time to Live value should decrease. It will eventually reach 0 when the record is cleared from the cache.

Be aware of this caching feature as if you change a DNS record it will take some time to propagate it around the internet. Initially the old record will be returned from the DNS server as it is still in the cache.

Caching also means that if you ask for microsoft.com in your browser multiple times then there’s no need to go through the same name server lookup process over and over again. The immediate name server configured for your computer will have it in its cache and will be able to respond immediately with the correct IP address.

Override DNS in the local host file

It’s possible to override the DNS values on your local machine. This is done in the host file. On Windows machines it is usually located in the C:\Windows\System32\drivers\etc folder. The file is called hosts and you can open and edit it like a normal text file. You can add your ip-name pairs to the file using the following format:

Host file

So the format is: the ip address followed by a tab and then the name. You can even enter localhost IPs where localhost is always 127.0.0.1. You can add multiple names for the same IP as follows:

Host file with multiple names

You can enter the same made up values that I have and save the file. Go back to the command window and type ipconfig /displaydns again. Saving the hosts file will automatically clear the local cache which will be populated with the values in the host file. You should see the values you have just entered in the host file in the command window output. Run a ping command against one of the custom values in the host file, such as ping mysite.com and you’ll see that it will try to reach the IP that you specified. As that IP probably doesn’t exist it will just show a couple of Request timed out values.

Why would you modify the host file? If you migrate a website from one IP address to another, then you probably want to test the new environment in your browser, right? As the URL of the website doesn’t change then it will lead to the old IP address that exists in the name servers. You can then deploy the website to the new environment, override the host file and enter the URL again. You will then be directed to the IP you have specified in the host file. This is a very convenient solution for testing purposes: your clients will not see your beta site as they will still be directed to the old IP. Then when you’re done testing you can propagate the new IP value across the Internet.

Record types

When you type nslookup microsoft.com in the command prompt then it will provide you with one or more records of type A: an A record. An A record turns a name into an IP address. It is the default type of record that nslookup returns. There are other types of records and you can let nslookup return them as well. Run the nslookup command without specifying the name to enter the multiple query mode.

To query name server records you can set the type as follows:

NsLookup name server record

Here you see the name servers that are responsible for the microsoft.com namespace. Here we see 5 name servers. In the name resolution process your computer will pick one of those at random.

You can query mail exchange records (MX) but setting the type as follows:

set type=MX

Then query microsoft.com will give you something like this:

NsLookup mail exchange

If you send an email to Microsoft then you’ll send it to the microsoft-com.mail.protection.outlook.com mail server. That’s who will accept mail for the microsoft.com namespace. It’s possible that there are multiple mail servers in which case the preference parameter tells me in which order I should try to send the email.

Another record type is CNAME which stands for ‘canonical name’, it’s sort of an alias:

set type=CNAME

Then test microsoft.com. You’ll see no CNAME for that:

NSLookup no CNAME

The reason is that we cannot have a CNAME for the root of the domain. However, try http://www.microsoft.com, you’ll get a CNAME:

NsLookup with cname

http://www.microsoft.com is aliased to an Akamai address. This means that when you type http://www.microsoft.com in your web browser and get the IP address for it from the name server lookup then you will be directed to a server owned by the Akamai network.

The last record type to look at is the quad A, or AAAA record type. This is an IPv6 version of an A record so this turns the name into an IPv6 address. If you set the type to CNAME in the command prompt and query a name then you’ll get the AAAA records as well:

AAAA records

This is the case on with Windows 7. If you don’t see this output then test setting the type to AAAA first:

set type=AAAA

and then query a name.

These are the most common record types out there.

About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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