The port 500/UDP…. is used by…IKE to negotiate the safety parameters necessary for the establishment of IP tunnels. When equipment is behind a translation of addresses, the negotiation can then s, press NAT-T and port 450/UDP, an important point to check when a VPN troubleshooting…
What’s the use of port 500?…
The port 500/UDP…. is used by IKE to negotiate parameters and the authentication of,an IP tunnelsec. When NAT is detected, peers usually switch to…. UDP 4500…. For NAT-T. A tunnel can therefore start on 500 and then depend on 4500 to continue the negotiation and transport IPsec encapsulated….
The diagnosis must look at both peers: identities, cryptographic proposals, authentication and selectors, not just the port state….
Transport Protocol
UDP
Safety precautions
Should we open this port in the firewall?
For a VPN site-to-site or nomade, exposing IKE on the Internet may be necessary: the goal is not to systematically close UDP 500, but to limit peers when their addresses are known, use current cryptographic suites, protect secrets or certificates and log chess…N, open 500/4500 inside all segments without need identified VPN….
How do I check this port?
Reproduce the tunnel, while consulting the IKE newspapers on both ends. A filtered capture on…. udp port 500 or udp port 4500 allows to see if packages arrive and if NAT-T switch occurs….
- Check the reachability of public peer addresses….
- Compare IKE versions, proposals, DH groups and authentication….
- Check the expected identity, certificates or PSKs as appropriate…
- If NAT is present, check also UDP 4500 and the midranges….
FAQ — port 500 / IKE / IPsec….
Why does port 500 work but the IP tunnelsec does not go up?…
IKE may fail on proposals, the authentication, identities or certificates even if UDP 500 packages arrive….
What is the connection between ports 500 and 4500?…
500 is used for IKE; when a NAT is detected, NAT-T usually uses UDP 4500 to continue the tunnel…
Should UDP 500 be allowed from the Internet?….
Yes when a VPN-Shad pair must join your gateway. The rule must remain consistent with the expected architecture and peers….