VpnHood.Net.Packets

A fast, low-allocation IP packet library for .NET: parse and build IPv4, IPv6, TCP, UDP, ICMPv4 and ICMPv6 packets in place, and compute their Internet checksums with SIMD. It is the packet layer of VpnHood, and works on its own for any .NET app that reads or writes raw packets: a TUN device, a raw socket, a capture file.

dotnet add package VpnHood.Net.Packets

How it works

• A packet is a view over its bytes: IpV4Packet, IpV6Packet, TcpPacket, UdpPacket and the ICMP packets read and write the buffer in place, with no copy.
• SourceAddressValue and DestinationAddressValue read a packet's addresses as IpAddressValues, with no object made, for code that runs per packet; SourceAddress and DestinationAddress make an IPAddress on first read and keep it.
• PacketBuilder parses packets and builds new ones in memory rented from a pool. Dispose a packet to return its memory.
• A packet is made on a PacketMemory. A Memory<byte>, an array or an ArraySegment<byte> converts to one you keep; one made from an IMemoryOwner<byte>, or with PacketMemory.FromRentedArray, gives the packet that memory, which it returns when disposed. A rented array needs no owner object.
• BuildIp, BuildUdp, BuildTcp, BuildIcmpV4, BuildIcmpV6 and BuildDns compute no checksum: call UpdateAllChecksums() before sending. The echo, reset and ICMP error builders compute them unless told not to.
• An IPv6 packet reads through its Hop-by-Hop, Destination Options, Fragment and Authentication headers to the upper layer.

Quick start

Each example below runs as a test in CI (PacketsReadmeExampleTest.cs). Their namespaces are System.Net, VpnHood.Net.Packets, VpnHood.Net.Packets.Extensions and VpnHood.Net.Toolkit.Net.

Build a UDP packet

using var ipPacket = PacketBuilder.BuildUdp(
    new IPEndPoint(IPAddress.Parse("10.0.0.2"), 5353),
    new IPEndPoint(IPAddress.Parse("10.0.0.1"), 53),
    "hello"u8);

ipPacket.UpdateAllChecksums(); // BuildUdp computes no checksum itself
var bytes = ipPacket.Buffer; // the packet's own bytes, ready for a TUN device

Parse a packet

// Parse copies the received bytes into pooled memory; PacketBuilder.Attach would read them in place
using var ipPacket = PacketBuilder.Parse(received);
Console.WriteLine($"{ipPacket.SourceAddress} -> {ipPacket.DestinationAddress}, {ipPacket.Protocol}");

if (ipPacket.Protocol == IpProtocol.Udp && !ipPacket.IsFragment) {
    var udp = ipPacket.ExtractUdp(); // a view over the packet's bytes, not a copy
    Console.WriteLine($"UDP {udp.SourcePort} -> {udp.DestinationPort}, {udp.Payload.Length} bytes, " +
                      $"checksum valid: {udp.IsChecksumValid(ipPacket)}");
}

Answer a ping

using var request = PacketBuilder.BuildIcmpEchoRequest(
    IPAddress.Parse("10.0.0.2"), IPAddress.Parse("10.0.0.1"), "ping"u8,
    identifier: 1, sequenceNumber: 1); // checksums computed

// answer it as a host would: swap the addresses, keep the identifier, sequence number and data
var echo = request.ExtractIcmpV4();
using var reply = PacketBuilder.BuildIcmpEchoReply(
    request.DestinationAddress, request.SourceAddress, echo.Payload.Span,
    echo.Identifier, echo.SequenceNumber);

License

LGPL-2.1-only. See the VpnHood repository.