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changing the listening ports affects what ...

Posted on 2004-09-04
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Last Modified: 2010-04-19
to be able to cross a firewall to connect to my server at home
i had to change the port number for remote desktop RDP from 3389 to 110 in the registry
an i can remote desktop now to my server

110 is also used by pop3, and i use it now also for RDP now

will this have an influence on pop3

so my question is:

when 2 programs listen to the same port, will this not cause some trouble to those programs and if yes, where

tks a lot
karel
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Question by:karel_jespers
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Dave_Dietz earned 450 total points
ID: 11981679
Yes, this has the potential to cause a problem if you try to run two programs on the same machine that *listen* on the same port.  Unless you are running a POP3 *server* on your machine it shouldn't matter if you set up a different program to use that port for incoming connections.

As an example, there is a (not very popular) chat server that binds to port 443 for clients to connect to.  443 is the standard port used for SSL Web traffic and this causes a problem when you try to use SSL for a webserver on the same machine because both of the programs want to use port 443 exclusively.  Generally the first program to bind a prot wins and any other programs will generally pop up some sort of error about not being able to bind to a specific port.

Some programs are configurable to bind to a given port but only on a certain IP interface which can allow youto run different programs using the same port as long as they will both use a different IP address.

Once again, as long as you only have one program (RDP) that is listening on (binding) a port for exclusinve use you shouldn't have any problems.

Side question: is there any reason you didn't simply open up port 3389 for RDP traffic?

Dave Dietz

Dave Dietz
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by:karel_jespers
ID: 11983432

is there any reason you didn't simply open up port 3389 for RDP traffic?
----------------------------------------------------------------------------------
to be able to cross a firewall
x  ==> firewall ==> homeserver
i have the permissions of the networkadministrator to run everything i want on port 110
but he doesn't want to open de port 3389 on his firewall
so i had to change the listening port

tks dave

for those programs who's ip adress can be configure:
can i then do the following

1) i configure my only networkinterface on my server with 2 ipadresses
2) i configure both program 1 and program 2 to listen to port eg 110
3) on my router i route program1 (port eg 110) communication to networkaddress 1
4) on my router i route program2 (port eg 110) communication to networkaddress 2

can you give me an example or links of such a programs who's listening port + ip adress can be configured (bcs i want to do some testing)

tks karel

 
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LVL 34

Expert Comment

by:Dave_Dietz
ID: 11985027
If your network admin won't open port 3389 I guess that's a good reason to use a different port.....  :-)

1) You can bind as many IP addresses to your NIC as you want as long as you aren't using DHCP to have a dynamic address assigned to you.

2) Yes, on different IPs

3 and 4) generally the way a router/NAT device works is to take any packets arriving at port x on the router/NAT and forward them to port y on the specified internal IP address.  Unless you have a router/NAT with some very advanced features it won't be able to send packets arriving on port 110 to different destination addresses based on what protocol (POP3 vs. RDP) the incoming packets are using.

It sounds like your admin doesn't want to open any non-standard ports.  Might try asking if he (she) will open one or more of the following and then use them as you feel fit:

20   - FTP-data
21   - FTP
25   - SMTP
80   - HTTP
119 - NNTP
443 - HTTPS (SSL)

Dave Dietz
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LVL 6

Author Comment

by:karel_jespers
ID: 11985158
tks a lot for your help
karel
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