To do that a firewall must have transparent proxies and are then called an application firewall. A firewall that can moderate that kind of traffic, need to inspect the traffic stream. Some protocols can in-line signal a port jump and/or create connections one or both ways "at will". UDP and TCP are special because they have 65536 possible src and dst ports that can help connection tracking.
Here are examples of protocols that has that problem:Įven if the traffic is unencrypted it can not be deduced where to NAT a response outside packet, if more than one inside client uses the same protocol to the same outside ip address. When a response outside packet later arrives at the NAT device (firewall), it can not deduce which client to send it to. C1, C2) connect to the same outside server ip address (S) and the traffic is not tcp and udp. NAT - Network address Translationĭue to IPv4 address shortage, the internet society began to use NAT, and therefore the firewall also need to be NAT aware.Ī real problem with NAT is when more than one inside clients (e.g. number of connections attempt - "SYN"-attacks, packet storms.number of connections per (src/dst) ip address.A statefull firewall can additionally moderate trackable traffic by: The better ip firewall - a statefull firewall - can pass packet by packet - and if possible (e.g. The simplest ip firewall - a packet filter firewall - can pass packet by packet or drop them based on:
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