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jcm
jcm, Certified Networking Engineer
Category: Networking
Satisfied Customers: 84
Experience:  I am a four year experienced Cisco Network Engineer. I possess a CCNA.
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My question is when you have the port triggering feature enabled

Customer Question

My question is when you have the port triggering feature enabled on a network router and say for example if I have port 50 in the triggered range and have port 80 in the forwarded range. My question is, is port 50 going to connect to the remote computer on the internet and send data through port 80, then to the IP address of the computer.
Submitted: 1 year ago.
Category: Networking
Expert:  jcm replied 1 year ago.

jcm :

Hello and welcome to Just Answer! My name is Josh and I would be glad to help you.

jcm :

Port trigger works in a manner where you define a specified destination port (trigger range) and a defined forwarding port. When the router sees traffic destined for that destination port, it opens up forwarding for the forwarding port. In your example when an outbound packet is destined for an IP address on port 50 then the router will forward port 80 inbound from the internet to the same host that sent the original packet destined for port 50.

jcm :

There is no port redirection or translation as your original question implies.

Customer:

So basically if you have port 50 in the triggered range and have port 80 in the forwarded range then I am assuming that port 50 is going contact the remote computer on then internet and it is going to send data through port 80 right.

jcm :

You're assuming that the application on the remote computer on the internet has a need to send traffic back to port 80 but this isn't to misunderstood as the router is going to force traffic back through port 80.

jcm :

So if the application has no need to send traffic back to port 80 then the whole function really is pointless.

Customer:

Please give me a few minutes. Thank you

jcm :

Take your time. I will be here for a little while and feel free to ask any further questions.

jcm :

I've noticed you have been asking a lot of networking questions as of late. Are you studying for a certain career?

Customer:

Yes sir

jcm :

Oh really, and what is that?

Customer:

computer software and repair

jcm :

Cool

Customer:

I am thinking that when you have port triggering enabled on a router that when you have port 50 in the triggered range and have port 80 in the forwarded range that when you get on the internet the triggered range is going to use port 50 under the triggered range to connect to the remote computer on the internet to send data through port 80 to the Ip address of the computer.

jcm :

No that's not the case. It should simply just open up that port.

Customer:

Open the port for what

Customer:

do you mean to open the port for port 80

jcm :

It will open up the "forwarding port" to be accessible inbound from the internet.

jcm :

Yes in your example, port 80

jcm :

But that doesn't mean that all reply traffic is forced through port 80. It just makes it available.

jcm :

So for example if you set the trigger range to have port 53 and the forwarding port to 80 then that would be pointless. This is because port 53 is DNS. By default a DNS server would have no use in being able to connect back to you on port 80. It would continue to operate on the PAT ports.

jcm :

The only reason the "forwarding port" would be of use is if the remote server/PC you are connecting to had a need to come back on that port.

Customer:

So you are saying that if a particular port say for example port 50 wants to send data through port 80, then it will open port 80 back up in order for port 50 to send the data through port 80 right.

jcm :

Yes. But more correctly it would be worded as "particular [application listening on] port 50 wants to send return traffic to port 80 , then it [the router] will already have port 80 opened in order for [the application running on] port 50 to send the data back on port 80"

jcm :

See, usually ports are protocol specific. IE 53 is DNS, 80 is HTTP, etc. So what would be happening in the background on the remote PC is that the application running on port 50 would most likely be sending HTTP traffic back to you for some reason.

Customer:

Let me rephrase the question to make sure that I understand the question

jcm :

OK. Sorry if I'm confusing you. It's kind of a situational question.

Customer:

So for example If I have port 49 in the triggered range and have port 80 in the forward range and when I get ready to access the remote computer on the internet the router is going to use port 49 to open the forward range port for port 80 to allow data to come through port 80 to the Ip address of the computer right.

Customer:

But if there is no more data coming from port 49 then it will close port 80 right.

jcm :

Ok let me explain this another way.

jcm :

Look at port 49 and 80 as two completely seperate things to the router.

jcm :

The router doesn't care if/who/what comes back on port 80. It also doesn't direct any specific traffic back on port 80. It simply opens that port up to be forwarded from the internet as if you were to go in and put a manual port forward in.

jcm :

The traffic coming back from port 49 may or may not use port 80. Most of the time with port triggering the traffic that comes in to the forwarding port (80) isn't sourced from the same port as the triggered port (49)

jcm :

Lets say you have a PC on your lan with 192.168.1.10 and your router's ISP assigned WAN ip is 2.2.2.2. So you could have a connection being initiated outbound from 192.168.1.10 to 1.1.1.1:49 on the internet. Your router's triggering picks this up and opens 2.2.2.2:80 -> 192.168.1.10:80

jcm :

That's it.

jcm :

The traffic *could* come back from 1.1.1.1 to --> 2.2.2.2:80 but it doesn't have to.

jcm :

The source IP *could* also be 1.1.1.1:49 but most of the time it isn't.

jcm :

So let's say you need a server at 3.3.3.3 to be able to get back to you on 2.2.2.2:80. However, you can't keep port 80 forwarded all the time for security reasons. Well you know that whenever your PCs go outbound to destination port 49 that's a legit reason tp let someone in on port 80. Hence, a good case for port triggering.

Customer:

I am still not understanding where you are coming from but let me type from the book and you might get a understanding of what I am talking about here.

jcm :

My point is that there is no correlation or direct tie back that port 49 is coming back on port 80.

jcm :

It *can* but it isn't forced to and doesn't have to.

Customer:

It saids in the book Port triggering opens a port when a pc on the network initiate communication through another port. For example, in figure 9-37b, computer C sends data to port 50 to a computer on the internet. The router is configured to open port 80 for communication from this remote computer. Port 80 is closed until this trigger occurs.

jcm :

Yes that statement is true BUT that doesn't mean that the return traffic will come from port 50. It will just come from the internet computer's IP address. Most likely sourced from a NAT port.

jcm :

That also doesn't restrict the port forward on 80 to *only* allow computer C to send traffic back. In that time span where the port trigger is opened it is quite possible and likely that any computer can come back on 80.

jcm :

I meant *internet computer, not computer C

jcm :

That also doesn't restrict the port forward on 80 to *only* allow that specific internet computer to send traffic back. In that time span where the port trigger is opened it is quite possible and likely that any computer can come back on 80.

jcm :

It's very important to know that the original communication from your local PC behind the router to the internet computer on port 50 isn't being "forced over" or "carried over" to port 80 on your router.

Customer:

So basically it is going to open the port for communication right.

jcm :

Just in general. Yes.

jcm :

Just like if you go in and put in the port forward manually.

jcm :

No special carry-over or transition from the original outgoing connection to port 50. That port 50 trigger is only there so the router knows WHEN to open port 80 up for forwarding not WHAT to move over to port 80.

Customer:

Do you mean to forward data request to port 80 right.

jcm :

When it opens port 80 up it just listens for any traffic destined for it's WAN ip on port 80 and then forwards it to the local PC.

Customer:

Are you talking about on the port triggering

jcm :

Same for port triggering and standard port forwarding

jcm :

Only difference is that your outbound connection to port 50 is what sets it up instead of you doing it manually.

Customer:

Thank you for your help Josh and I will ask more question If I have any. Thanks for your time.

jcm :

No prob. Feel free to rate my service :D

jcm, Certified Networking Engineer
Category: Networking
Satisfied Customers: 84
Experience: I am a four year experienced Cisco Network Engineer. I possess a CCNA.
jcm and 6 other Networking Specialists are ready to help you
Expert:  jcm replied 1 year ago.
Hey man, if you have any further questions let me know. If I confused you or anything let me know. I'd like to get that rating up to "Excellent" if possible.

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jcm
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I am a four year experienced Cisco Network Engineer. I possess a CCNA.