Netfilter packet flow

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Netfilter packet flow



### NFQUEUE in Linux Netfilter for Packet Handling and Configuration NFQUEUE is a queue handler that allows packets to be passed from the kernel’s network stack (netfilter) to user space programs such as `iptables` or other custom applications. This mechanism enables deep inspection of packets by external software before deciding whether they should pass through, get modified, logged, dropped, etc. The integration between iptables ruleset management toolchain and nfqueue target makes it possible to define which traffic gets diverted into queues accessible via libnetfilter_queue library functions written primarily in C but also available bindings exist for Python among others languages[^1]. To configure NFQUEUE within an environment: - **Installation**: Ensure necessary packages are installed on your system including development tools like gcc alongside libraries specific to netfilter framework. ```bash sudo apt-get install build-essential libnfnetlink-dev libmnl-dev pkg-config ``` For configuring NFQUEUE with IPTables rule sets: ```bash # Send all TCP SYN packets destined port 80(HTTP) to queue number 1 sudo iptables -A INPUT -p tcp --syn --dport 80 -j NFQUEUE --queue-num 1 ``` User-space application can interact with queued data using APIs provided under `/usr/include/linux/netfilter.h`. A simple example demonstrating how one might set up basic packet processing involves setting up callbacks where each incoming item triggers execution flow defined inside callback function body. ```c #include <stdio.h> #include <stdlib.h> #include <libnetfilter_queue/libnetfilter_queue.h> /* Callback function prototype */ static int cb(struct nfq_q_handle *qh, struct nfgenmsg *nfmsg, struct nfq_data *nfa, void *data); int main(int argc, char **argv){ /* Code omitted */ } // Define what happens when new items arrive at our queue static int cb(struct nfq_q_handle *qh, struct nfgenmsg *nfmsg, struct nfq_data *nfa, void *data){ printf("packet received\n"); return NF_ACCEPT; } ``` This setup facilitates advanced networking tasks beyond traditional firewall capabilities offered directly out-of-the-box without requiring modification to core operating systems components while still maintaining performance benefits associated closely coupled implementations found natively within kernelspace operations [^2].
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