if it sees a DNS query go out it records the IP source/destination and the Port source/destination.
#STATEFUL FIREWALL SOFTWARE#
pfSense Plus software does this by default, and can be configured to block traffic. Stateful packet inspection, also referred to as dynamic packet filtering, is a security feature used to invoke fine-grained security policies. They keep track of each connection and allow the traffic to flow through only if there are corresponding entries in it's state table.įor non-TCP protocols eg UDP there are no flags so the stateful firewall sets a timer ie. A stateful firewall is a network-based firewall that individually tracks sessions of network connections traversing it. In essence this is how stateful firewalls work. The TCP FLAGS are SYN/ACK but the firewall has no record of a SYN packet sent from the client. Now lets say the client hasn't sent an intital packet and the server sent a packet with the same info as above. What's more because the firewall expects to see a SYN/ACK from the server because it recorded a SYN from the client. When the server responds the firewall looks up it's state table to see if it has a matching entry for the connection and finds it does.
When the firewall sees the initial packet from the client it records all the info above. Now lets say there is a stateful firewall in between the client and the server. The above is called the TCP 3 way handshake. The client picks a random port eg 33212 and sends a packet to the serverĭestination port = 23 ( 23 is telnet port ) It can really only keep state for TCP connections because TCP uses flags in the packet headers. A stateful firewall keeps track of the "state" of connections based on source/destination IP, source/destination port and connections flags.
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