Modifying forwarded packet contents using nftables or iptables queues

NFT
1

I’m currently involved in a project that requires frequent modifications to the contents of packets during forwarding. I’m exploring whether it’s feasible to use nftables or iptables queues in a userspace application for this purpose, similar to a man-in-the-middle setup.

I’ve noticed that the available documentation typically covers only accepting or dropping packets, rather than altering them. There are also suggestions that the queue library merely copies packets from kernel space, which might prevent modifications. I’m curious if there’s a workaround or a known technique that can be used to achieve this.

I would appreciate any advice or experiences on this matter. Thank you!

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hey there, i’ve dealt with similar stuff before. have u considered using libpcap? it’s pretty good for packet manipulation. u can capture packets, modify em, and send em back out. it’s more flexible than nftables/iptables for what ur trying to do. just a thought!

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hey strummingmelody! i’m super curious about your project. have you looked into using the netfilter hooks directly? it’s a bit more low-level than nftables or iptables, but it might give you the control you need for modifying packet contents on the fly.

i remember tinkering with something similar a while back. it’s definitely tricky, but there’s some cool stuff you can do with it. what kind of modifications are you trying to make to the packets? anything specific?

also, have you considered writing a custom kernel module? it’s a bit more work upfront, but it could give you way more flexibility for packet manipulation. just a thought!

let me know how it goes. i’d love to hear more about what you’re working on!

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While nftables and iptables queues are powerful tools, they’re not ideally suited for packet modification. For your use case, I’d recommend looking into netfilter’s NFQUEUE target combined with libnetfilter_queue. This setup allows you to intercept packets in userspace, modify them, and then reinject them into the network stack.

Here’s a high-level approach:

  1. Use iptables to route packets to NFQUEUE
  2. In your userspace application, use libnetfilter_queue to receive packets
  3. Modify the packet contents as needed
  4. Use libnetfilter_queue to send the modified packet back to the kernel

Be aware that this method can introduce latency, especially with high traffic volumes. You might need to optimize your code and possibly use multiple queues for better performance. Also, ensure you have proper error handling to avoid dropping packets accidentally.