How to read a pcap file generated by tcpdump that contains large UDP packets and reassemble the IP fragmented packets? - networking

Question

I would like to read a pcap file generated by tcpdump that contains large UDP packets that have undergone IPV4 fragmentation. The original packets are of a size of around 22000 bytes.
In C++, I would use libtins with its IPV4Reassembler. Is there a way that I can do something similar in Rust?
Currently in Rust here is what I have written so far: a highly incomplete first-pass attempt (using crate pnet):
use pnet::packet::{
ethernet::{EtherTypes, EthernetPacket},
ip::IpNextHeaderProtocols,
ipv4::Ipv4Packet,
udp::UdpPacket,
Packet,
};
struct Ipv4Reassembler {
cap: pcap::Capture<pcap::Offline>,
}
impl Iterator for Ipv4Reassembler {
type Item = Vec<u8>;
fn next(&mut self) -> Option<Self::Item> {
let mut payload = Vec::<u8>::new();
while let Some(packet) = self.cap.next().ok() {
// todo: handle packets other than Ethernet packets
let ethernet = EthernetPacket::new(packet.data).unwrap();
match ethernet.get_ethertype() {
EtherTypes::Ipv4 => {
let ipv4_packet = Ipv4Packet::new(ethernet.payload()).unwrap();
// dbg!(&ipv4_packet);
// todo: discard incomplete packets
// todo: construct header for reassembled packet
// todo: check id, etc
let off: usize = 8 * ipv4_packet.get_fragment_offset() as usize;
let end = off + ipv4_packet.payload().len();
if payload.len() < end {
payload.resize(end, 0);
}
payload[off..end].clone_from_slice(ipv4_packet.payload());
if ipv4_packet.get_flags() & 1 == 0 {
return Some(payload);
}
}
_ => {}
}
}
None
}
}
fn main() {
let pcap_path = "os-992114000702.pcap";
let reass = Ipv4Reassembler {
cap: pcap::Capture::from_file(&pcap_path).unwrap(),
};
for payload in reass {
let udp_packet = UdpPacket::new(&payload).unwrap();
dbg!(&udp_packet);
dbg!(&udp_packet.payload().len());
}
}
In C++ here is the code I would use (using libtins):
#include <tins/ip_reassembler.h>
#include <tins/packet.h>
#include <tins/rawpdu.h>
#include <tins/sniffer.h>
#include <tins/tins.h>
#include <tins/udp.h>
#include <iostream>
#include <string>
void read_packets(const std::string &pcap_filename) {
Tins::IPv4Reassembler reassembler;
Tins::FileSniffer sniffer(pcap_filename);
while (Tins::Packet packet = sniffer.next_packet()) {
auto &pdu = *packet.pdu();
const Tins::Timestamp &timestamp = packet.timestamp();
if (reassembler.process(pdu) != Tins::IPv4Reassembler::FRAGMENTED) {
const Tins::UDP *udp = pdu.find_pdu<Tins::UDP>();
if (!udp) {
continue;
}
const Tins::RawPDU *raw = pdu.find_pdu<Tins::RawPDU>();
if (!raw) {
continue;
}
const Tins::RawPDU::payload_type &payload = raw->payload();
std::cout << "Packet: " << payload.size() << std::endl;
// do something with the reassembled packet here
}
}
}
int main() {
const std::string pcap_path = "os-992114000702.pcap";
read_packets(pcap_path);
}
g++ -O3 -o pcap pcap.cpp -ltins
It seems that one solution is to implement RFC815 but I am not sure how to do that in Rust. I have found:
this old pull request to smolltcp but it appears to have been abandoned.
Fuschia reassembly.rs but I have no idea how to use this outside of Fuschia.