linux kernel not sent packet in netfilter local out dnat

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01-09-2024

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Troubleshooting Linux Kernel DNAT: Why Packets Aren't Being Sent

This article delves into a common issue faced by Linux kernel developers: implementing DNAT (Destination Network Address Translation) in the kernel and encountering problems where packets are not being sent despite the successful DNAT operation.

We will analyze the code provided in the Stack Overflow question and identify potential pitfalls and solutions. This article aims to empower developers with the knowledge needed to troubleshoot and implement DNAT efficiently in their Linux kernel modules.

Problem:

The code snippet from Stack Overflow shows a Linux kernel module that attempts to implement DNAT for TCP packets. The module intercepts packets destined for a specific subnet (10.21.47.0/24) and redirects them to a specific IP address (10.21.177.241:9999). However, while the DNAT is successful (indicated by tcpdump capturing SYN packets), the packets are not being sent to the destination.

Analysis:

The provided code presents several potential issues:

1. Checksum Calculation: The code relies on update_tcp_ip_checksum() to update the TCP and IP checksums after DNAT. This function might be insufficient for accurate checksum recalculation in all scenarios.
2. Routing: The code uses ip_route_me_harder() to route the DNATed packet. While this function is intended to perform routing, it might not be the most suitable for handling DNAT scenarios.
3. Conntrack Integration: The code lacks integration with the Linux conntrack system, which is crucial for managing the state of the TCP connection and handling subsequent packets.

Possible Solutions:

1. Checksum Validation: It is critical to ensure the correct calculation of the IP and TCP checksums after the DNAT operation. If the checksums are incorrect, the receiving system will reject the packet. Consider using the ip_checksum_set() and tcp_checksum_fix() functions for a reliable solution.
2. Conntrack Integration: Incorporate the nf_conntrack module into the DNAT implementation. Conntrack manages connection states and aids in efficiently forwarding subsequent packets associated with a connection. Utilize functions like nf_conntrack_lookup() and nf_conntrack_insert() for managing connection state.
3. Routing with ip_route_input(): Explore the use of ip_route_input() for handling routing after DNAT. This function allows the kernel to select the appropriate interface and route for sending the packet.

Additional Tips:

• Logging: Implement thorough logging in the kernel module to debug the DNAT process. Log details like the packet contents before and after DNAT, the calculated checksums, and the routing results.
• Debugging Tools: Utilize debugging tools like kprobe and kdump to examine the kernel state at different points during the DNAT process. This allows detailed analysis of the packet manipulation and routing decisions.
• Verification: Validate the DNAT implementation using tools like iptables or tcpdump to ensure the DNAT rules are properly configured and the packets are being translated as intended.

Example Code:

While providing a complete code solution is beyond the scope of this article, the following snippet demonstrates a potential improvement by using ip_route_input() for routing and nf_conntrack for connection tracking:

unsigned int hook(void *priv, struct sk_buff *skb, const struct nf_hook_state *state) {
    // ... (previous code) ...

    // DNAT and checksum calculation
    iph->daddr = in_aton("10.21.177.241");
    tcph->dest = htons(9999);
    ip_checksum_set(skb, iph);
    tcp_checksum_fix(skb, iph, tcph);

    // Conntrack integration
    struct nf_conn *ct = nf_conntrack_lookup(state->net, skb, state->sk, NULL);
    if (!ct) {
        ct = nf_conntrack_insert(state->net, skb, state->sk, NULL);
        if (!ct) {
            // Handle conntrack insert error
        }
    }

    // Routing
    int err = ip_route_input(state->net, skb, state->sk);
    if (err < 0) {
        // Handle routing error
    }

    // ... (rest of the code) ...
}

Conclusion:

Successfully implementing DNAT in the Linux kernel requires a thorough understanding of networking fundamentals, checksum calculations, and the Linux kernel's networking stack. This article provided a breakdown of common issues and solutions for troubleshooting DNAT issues in Linux kernel modules. By incorporating the techniques and best practices outlined, developers can enhance their DNAT implementations and ensure efficient packet forwarding.

Remember, this is a complex topic with varying solutions depending on the specific environment and requirements. Always refer to the Linux kernel documentation and relevant resources for detailed information.