![]() The solution to the problem is to set the Accept Policy of the rule to Inbound. In this case, the server eventually exhausts its resources by creating TCP connections for the fake requests. The outbound policy tells the firewall to complete the connection with the server first (verifying it is up) and then complete the connection to the client. If the unfriendly host can change its IP address quickly enough, it can do this very often without a chance for the firewall to differentiate between the attack and ordinary requests.Ī simple SYN flooding attack with faked IP addresses on a firewall with the outbound accept policy: Step 4 – After a certain number of unanswered SYN-ACKs, the firewall recognizes the unfriendly activity and no longer accepts SYNs from the (faked) source IP address.The SYN-ACKs are sent to the fake IP address which does not answer, keeping the connection in a pending state until it times out. Step 3 – The firewall simply lets the SYN packets pass through, using up its own and the protected server’s resources.Step 2 – It then sends as many SYN packets as possible to the protected server.Step 1 – The unfriendly host fakes its IP address and gives itself an address, which is already in use in another network.If you use this outbound TCP accept policy in a firewall rule forwarding traffic to an internal server, you open yourself up to a simple attack: It is therefore fatal if the firewall sends an ACK to the client if the server cannot be reached because then the browser never gets the chance to try the other IP addresses. The browser tries to connect to the first IP address it receives from the DNS server, and, if it is not successful, it tries the next one and so on. This is important for many applications, such as a browser when it tries to connect to a server with many IP addresses for the same hostname (DNS round robin). The main characteristic of the outbound policy is that the client only receives an ACK when the requested server is really up. This example shows how the outbound and inbound accept policies handle TCP connections and which policy to use: Outgoing TCP Connection with Outbound Accept Policy Enabled TCP SYN Flooding Attacks and Countermeasures ![]() These settings are also configured on a per-rule basis. Number of Sessions per Source) to protect against resource exhaustion of the Barracuda CloudGen Firewall. To guard against DoS/DDoS attacks, configure the maximum number of new sessions and the allowed total number of sessions from a single source ( Max. Only after a complete TCP handshake is established, the handshake with the target is processed and traffic will be forwarded to the target address. The firewall rather establishes a complete TCP handshake with the requesting source first, assuring that the requestor is authentic (no IP spoofing) and really intends to establish a TCP session. TCP session requests (SYN packets) are NOT immediately forwarded to the target address even if the session is allowed by the rule set. Inbound Accept Policy – Use the inbound accept policy to protect servers against untrusted networks.The TCP handshake occurs between the source and destination. TCP session requests (SYN packets) are immediately forwarded to the target address if the session is allowed by the rule set. Outbound Accept Policy – Use the outbound accept policy when trusted clients access untrusted networks.Depending on the purpose of the firewall rule, choose one of the two TCP accept policies: You can use different accept policies to change how incoming and outgoing TCP connections are handled on a per-rule basis. In order to establish a TCP connection, the TCP three-way handshake must be completed.
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