Check that VPN ports are open

Question

Is there any way to check that UDP ports 500 and 4500 of a VPN server are responding ? The goal is to check if a firewall or something is blocking these ports.

Thanks in advance

score 0 · Accepted Answer · edited May 23 '17 at 12:24

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So, UDP doesn't do acks or connections like tcp does; so the only way to be sure the port is responding is to actually send some data and get a response (there is no requirement that it does respond though).

Since these are specific ports, I assume that there is a specific application/protocol you are looking at. For that you basically need to open the port and either send it some garbage data or some form of identification payload (depending on the protocol).

This previous question outlines what you need to do to handle that.

edited May 23 '17 at 12:24

Community

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answered Jul 25 '13 at 10:35

Oliver Matthews

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Thanks for the quick reply, but I don't have access to the server. In fact I have many VPN servers that may be tested by my c# client application. The only information that will be available to my application is the IP address of the VPN server and these two ports 4500 and 500. – restricteur Jul 25 '13 at 10:57

score 0 · Answer 2 · answered Jul 25 '13 at 14:14

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It seems that I should use ISAKMP protocol to talk to the servers.

There seems to be no way to test that the ports are available without using the protocol used by the server.

answered Jul 25 '13 at 14:14

restricteur

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score 0 · Answer 3 · answered Feb 14 '17 at 05:57

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You may try to use some opensource VPN clients like strongswan and try connect through VPN. Use either IKEv1 or IKEv2 connection, it will try to hit 500/4500 ports for connection. If the response from the server is received then ports are open.

answered Feb 14 '17 at 05:57

Vikash Srivastava

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Check that VPN ports are open

3 Answers3