.NET Developers’ user guide for troubleshooting networking problems Part 3

This is the last part in the series on basic networking for developers. Let’s look at firewalls first.


Firewalls are a common cause of port connectivity problems. What does a firewall do anyway? A firewall determines which connections are allowed to go through to the operating system and which ones are not. The firewall has a set of rules that state what traffic is allowed through. In the below example port 80 is let in but not port 25:

Firewall stop

Open Windows firewall as follows:

Open Windows firewall

This opens the Windows firewall manager:

Windows firewall managera

You’ll see a ling for Windows Firewall Properties somewhere in the middle of the screen:

Open windows firewall properties

Check the tabs in that window: you’ll see that you can set different options for the domain, private and public profiles which represent different states of Windows. It’s recommended to have the same settings for all 3 profiles unless you want to have different rules for your enterprise and home network. In this window you can turn the firewall on and off where the default is on.

Also by default we block all inbound connections and let all outbound traffic out. So traffic coming into our machine is blocked. You can also set the logging properties:

Open firewall logging settings

By default no dropped or successful connections are logged. If you suspect that your firewall drops data packets coming to your machine then it can be useful to log such events, so change that drop down list to yes. You can also specify the log file on the top of the window.

On the main Firewall screen you’ll see a link to the Inbound rules on the left hand side:

Open firewall inbound rules

You can add new inbound rules using the New Rule link:

Open firewall new inbound rule

You can create a rule by program, port, a predefined set of rules or a custom rule. For a program rule you specify an executable:

Specify Executable For Firewall Inbound Rule

This way we can open up or block the ports a specific program is listening on. Select some executable and click next. In the next screen you can select to open up the ports used by that executable or block them:

Inbound connection either blocked or allowed

Normally you’ll select the Allow option as all inbound traffic is blocked by default anyway. Click next and here you can define which profile to apply the rule to:

Which Windows profile to apply the rule to

As we said normally you’ll apply the same rules to all profiles. Then in the last step of the process you can provide a name for this rule. Give it some name and click finish. The new rule should appear in the list of rules on the main firewall screen.

This way of setting up a rule is useful if you’re not sure which port(s) a process uses. You can instead declare that all ports be opened up that are in use by that application.

Let’s create another rule, this time a port rule. Click the New Rule… link again and select the Port radio button and click next. On the next screen you’ll be able to define the type, i.e. TCP or UDP:

Inbound rule type tcp or udp

You can also define which port to open or close: all ports or just one specific or a range of ports. Let’s specify ’80, 443′ in the Specific local ports text box which will allow HTTP(S) traffic. Click Next and this screen will be familiar: you can allow or block the connection. Click Next. Again, the window will be familiar, you can define in which Windows profiles the rule will apply. In the last screen you can give a name to the rule, just like before. You’ll typically set this rule on your web server. If you don’t open up port 80 on your web server then no-one will be able to access the contents of your number one website.

You can add predefined rules by selecting the Predefined radio button in the very first window of the setup process. Open up the drop down list and you’ll see a whole bunch of predefined rules. These rules represent the Windows services that have been installed on your machine. You’ll see an option called Remote Desktop. This rule allows others to remotely connect to a computer. Click next and you’ll see some information on which port is going to be opened and some other parameters of the rule. If the predefined option needs more than one rule, such as Routing Volume Management, then all of them will be listed here.

The Custom rule type will give you a lot of freedom to define your rules. Click Next and you’ll see the window again where you can pick an application. Click Next to go to the Protocol and Ports window:

Protocol and ports inbound rule

Check out the Protocol type drop down list. Besides TCP and UDP which we discussed here you’ll see a whole range of other protocol types. E.g. the ICMPv4 protocol is used by the ping function by which you can ping a website in the command window. Select that protocol type. You can then click the Customize button where you can specify which ICMP packets to allow:

Customise ICMP packets

Select All ICMP types and click OK. Click Next to go to the Scope window. Here you can specify which local IP address this rule applies to and where we want to allow the traffic from – this is given in the remote IP section in the bottom half of the window. For now select the Any IP address option for both. The last three stages, i.e. Action, Profile and Name will all be familiar by now.

You can always come back later and update your rules. Just left-click a rule in the main window and select Properties from the context menu. This will open the Properties window:

Properties window for updating inbound rules

In this window you can specify a couple of options that were not available during the normal setup process. E.g. you can provide the authorised computers under the Computers tab. You can specify the users that are allowed to access this rule under the Users tab. Under the Scope tab you can define the IP addresses as we saw in the case of the Custom rule. As you see you can get to these options for any rule type but it’s offered during the setup phase only in the case of the Custom rule.

The Scope can be interesting if you want to setup the Remote Desktop predefined rule. You probably don’t want to let any computer get remote access to your computer, right. For any given machine in a network it is most likely enough to let other computers in the same subnet success it. E.g. I can remotely access the web servers that belong to the network of the company I work at. We don’t want anyone else to be able to access those computers.

In that scenario you can specify the correct IPs in the Remote IP address section:

Provide remote ips for remote access

The easiest way to achieve this is by the selecting the Predefined set of computers radio button and marking the local subnet option in the drop down list:

Select local subnet in remote access

You can allow other subnets as well by clicking the Add button again and filling in the IP ranges.

Network Address Translation and private IPs

We mentioned in the previous module of this series that with NAT we can have multiple private IPs corresponding to the same external public IP. We also said that we’re running short of public IP addresses so we can look at them as scarce and expensive commodities. Your ISP will probably only give you a single public IP although you can have several machines online in your home: your laptop, tablet, smart phone, your children’s computers etc. They will all have a private IP. Each private IP will be translated to the public IP in the outgoing traffic. Conversely the public IP will be translated to the correct private IP in the incoming traffic.

It actually makes sense that not all devices need public IPs. Why would anyone need to access your laptop from the public Internet?

The NAT device in the home environment is usually your router. It will translate the sessions back and forth between the internal and external addresses. In case you have a service on your home desktop that you want to make publicly available then the traffic coming to the your public IP will not be routed to the private IP of your desktop unless you set up a specific NAT rule on your routing device. This NAT rule will say that any inbound traffic coming to the external IP address be routed to a specific internal IP address.

Alternatively you can set up a port rule on the NAT device. The port rule will say that any traffic destined for a specific port be routed to an internal IP address. You can set up multiple port rules to direct incoming traffic to the correct private IP.

This can be useful if you want to host a website on your home desktop or you want to be able to remotely access a specific computer in your house.

The easiest way to find your public IP is use one of the many online IP services such as this or this. This sites will show you which IP address you’re coming from.


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

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