Adversarial C2 Protocol Implemented in Zig
Calculate IPv4 checksum header
This was causing an issue because virtual networks were dropping packets
without this being set
+36
+36
src/EthIpUdp.zig
+36
src/EthIpUdp.zig
···
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pub fn setPayloadLen(self: *@This(), len: usize) void {
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self.ip.len = @intCast(len + (@bitSizeOf(@TypeOf(self.udp)) / 8) + (@bitSizeOf(@TypeOf(self.ip)) / 8));
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// Zero the checksum field before calculation
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self.ip.header_checksum = 0;
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// Serialize IP header to big-endian bytes
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var ip_bytes: [@bitSizeOf(@TypeOf(self.ip)) / 8]u8 = undefined;
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var w: Writer = .fixed(&ip_bytes);
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w.writeStruct(self.ip, .big) catch unreachable;
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// Calculate checksum over serialized bytes
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self.ip.header_checksum = onesComplement16(&ip_bytes);
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self.udp.len = @intCast(len + (@bitSizeOf(@TypeOf(self.udp)) / 8));
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}
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};
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fn onesComplement16(data: []const u8) u16 {
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var sum: u32 = 0;
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// Process pairs of bytes as 16-bit words
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var i: usize = 0;
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while (i + 1 < data.len) : (i += 2) {
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const word: u16 = (@as(u16, data[i]) << 8) | data[i + 1];
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sum += word;
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}
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// Handle odd byte if present
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if (data.len % 2 == 1) {
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sum += @as(u32, data[data.len - 1]) << 8;
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}
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// Fold 32-bit sum to 16 bits
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while (sum >> 16 != 0) {
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sum = (sum & 0xFFFF) + (sum >> 16);
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}
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// Return ones' complement
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return ~@as(u16, @truncate(sum));
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}
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const std = @import("std");
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const Writer = std.Io.Writer;