Differences

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--- advanced-firewall [2026/07/21 15:23] – pedro
+++ advanced-firewall [2026/09/30 12:59] (current) – [NAT] rs232
@@ Line 21: / Line 21: @@
  \\ **Enable TCP SYN cookies:  **uses "SYN cookies" to protect the router from SYN Flood attacks.
-It encodes information from the SYN packet into the (SYN/ACK) response. It is a standard method for preventing SYN floods. However, it has limitations that may cause issues with some old TCP/IP stacks.
+This uses a standard method for preventing SYN floods. It encodes information from the SYN packet into the (SYN/ACK) response. Its limitations may cause issues with some old TCP/IP stacks. \\
  \\
@@ Line 31: / Line 31: @@
 **Allow DHCP Spoofing: **makes FreshTomato accept/process packets from DHCP servers with different IP than the one it advertises.
-Such behaviour is often categorized as a DHCP spoofing attack, but rarely, might be legitimate. Using this lowers security.\\  \\
+Such behaviour is often categorized as a DHCP spoofing attack, but rarely, might be legitimate. Using this lowers security.
-**Smart MTU black hole detection:** this feature detects ICMP black holes and smartly tries to adjust the path MTU.
+ \\
-For details on MTU and black holes, see this blog post: [[https://blog.cloudflare.com/path-mtu-discovery-in-practice/|The CloudFlare Blog: Path MTU Discovery in Practice]]
+**Smart MTU black hole detection:** detects ICMP black holes and smartly tries to adjust the path MTU.
+For details on MTU and black holes, see this blog post: [[https://blog.cloudflare.com/path-mtu-discovery-in-practice/|The CloudFlare Blog: Path MTU Discovery in Practice]] \\
+\\
 **TCP MSS Clamping:**
-TCP MSS Clamping adjusts the Maximum Segment Size of forwarded TCP connections on WAN and VPN interfaces to match the discovered path MTU.
+This adjusts the Maximum Segment Size of forwarded TCP connections on WAN and VPN interfaces to match the discovered path MTU.
-This helps avoid MTU-related connection problems, especially on PPPoE links, VPN tunnels, IPv6 tunnels, or networks where ICMP fragmentation-needed messages are blocked.
+This helps avoid MTU-related connection problems, especially on PPPoE links, VPN tunnels, IPv6 tunnels, or networks with blocked ICMP fragmentation-needed messages.
 Disabling this option reduces the number of firewall rules and may slightly improve performance on low-end routers, but can cause connection stalls or TLS handshake timeouts on links with reduced MTU.
-Recommended setting: enabled, unless MTU/MSS is handled manually or the network is known to work reliably without MSS clamping.
+Recommended setting: enabled, unless MTU/MSS is handled manually or the network works reliably without MSS clamping.
 ===== NAT =====
-**NAT loopback:** lets LAN devices access each other via the router's WAN interface.
+**NAT loopback:** The NAT Loopback (Hairpinning) logic is implemented in the firewall subsystem (firewall.c) using iptables rules applied to the PREROUTING and POSTROUTING chains of the nat table.
-Also known as "Hairpinning", it's often used when connecting to the DDNS domain of your router via the LAN. These days, this setting is almost never needed. Also, it can create speed bottlenecks.
+\\
- \\
+. Disabled \\
+Under the Hood (iptables): FreshTomato omits loopback rules in the NAT chains.
+\\
+Behavior: When a local LAN client sends traffic destined for the router’s public WAN IP address (or DDNS domain name), the router receives it, sees that there is no active rule translating internal LAN traffic back into the LAN, and simply drops or rejects the traffic.
+\\
+Pros:
+- Lowest router CPU load and max throughput (zero extra NAT inspection or packet rewriting for LAN traffic).
+- Forces proper network boundary segregation.
+\\
+Cons:
+- LAN devices cannot access local servers using public domain names or public IP addresses nor the router admin interface on the WAN.
+\\
+\\
+. Forwarded Only (default) \\
+Under the Hood (iptables): FreshTomato generates rules that apply loopback translation only to traffic aimed at ports that are explicitly defined in the Port Forwarding rules.
+\\
+Matches LAN traffic destined for WAN_IP:PORT only if PORT exists in nat PREROUTING / port-forward tables. These are essentially ports you find defined under Port Forwarding.
+\\
+Behavior: If you forward port 8443 to an internal server at 192.168.1.10:443, a LAN device reaching out to [https://yourdomain.com:8443](https://yourdomain.com:8443) gets rewritten and looped back inside the LAN. However, if you try to reach [https://yourdomain.com:443](https://yourdomain.com:443) (a port with no active forward rule), it is rejected/dropped.
+\\
+Pros:
+- Security & Control: Prevents local LAN traffic from hitting closed or un-forwarded router ports via the external WAN identity.
+- Performance: Limits loopback processing overhead to explicitly required ports only.
+\\
+Cons:
+- Will not work for dynamic or non-forwarded services (e.g., UPnP-opened ports or DMZ configurations depending on implementation). It will not work for the router administration GUI called on its WAN interface from a LAN device.
+\\
+\\
+. All \\
+Under the Hood (iptables): FreshTomato inserts a blanket PREROUTING NAT rule matching all traffic coming from the LAN interface aimed at the WAN interface IP address, regardless of whether a port forward rule exists.
+\\
+Behavior: Any LAN connection sent to the WAN IP (only to the WAN IP, not the Internet!) is looped back to the router or LAN destination.
+\\
+Pros:
+- "It just works" convenience—applies to UPnP mapped ports, DMZ hosts, and all services without needing individual port forward rules. This is the only mode supporting the WAN router administration called from the LAN.
+\\
-  * All
+Cons:
-  * Forwarded Only
+- Performance Bottlenecks (minor): Every single piece of LAN-to-"WAN-IP" traffic runs through blanket SNAT/DNAT rule-matching cycles, bypassing hardware NAT/CTF (Cut-Through Forwarding) acceleration on supported chipsets. NOTE: this is to the WAN-IP not the Internet!
-  * Disabled
+- Unintended Loops/Routing Anomalies (rare): Can cause severe instability or packet loops when using dual routers, wireless bridges, or complex multi-subnets.
  \\

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