1 files changed, 109 insertions, 0 deletions
diff --git a/src/EthIpUdp.zig b/src/EthIpUdp.zig
new file mode 100644
index 0000000..251ed64
--- /dev/null
+++ b/src/EthIpUdp.zig
@@ -0,0 +1,109 @@
+// Copyright 2026 Robby Zambito
+//
+// This file is part of zaprus.
+//
+// Zaprus is free software: you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the Free Software
+// Foundation, either version 3 of the License, or (at your option) any later
+// version.
+//
+// Zaprus is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+// A PARTICULAR PURPOSE. See the GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along with
+// Zaprus. If not, see <https://www.gnu.org/licenses/>.
+
+pub const EthIpUdp = packed struct(u336) { // 42 bytes * 8 bits = 336
+    // --- UDP (Last in memory, defined first for LSB->MSB) ---
+    udp: packed struct {
+        checksum: u16 = 0,
+        len: u16,
+        dst_port: u16,
+        src_port: u16,
+    },
+
+    // --- IP ---
+    ip: packed struct {
+        dst_addr: u32,
+        src_addr: u32,
+        header_checksum: u16 = 0,
+        protocol: u8 = 17, // udp
+        ttl: u8 = 0x40,
+
+        // fragment_offset (13 bits) + flags (3 bits) = 16 bits
+        // In Big Endian, flags are the high bits of the first byte.
+        // To have flags appear first in the stream, define them last here.
+        fragment_offset: u13 = 0,
+        flags: packed struct(u3) {
+            reserved: u1 = 0,
+            dont_fragment: u1 = 1,
+            more_fragments: u1 = 0,
+        } = .{},
+
+        id: u16,
+        len: u16,
+        tos: u8 = undefined,
+
+        // ip_version (4 bits) + ihl (4 bits) = 8 bits
+        // To have version appear first (high nibble), define it last.
+        ihl: u4 = 5,
+        ip_version: u4 = 4,
+    },
+
+    // --- Ethernet ---
+    eth_type: u16 = std.os.linux.ETH.P.IP,
+    src_mac: @Vector(6, u8),
+    dst_mac: @Vector(6, u8) = @splat(0xff),
+
+    pub fn toBytes(self: @This()) [336 / 8]u8 {
+        var res: [336 / 8]u8 = undefined;
+        var w: Writer = .fixed(&res);
+        w.writeStruct(self, .big) catch unreachable;
+        return res;
+    }
+
+    pub fn setPayloadLen(self: *@This(), len: usize) void {
+        self.ip.len = @intCast(len + (@bitSizeOf(@TypeOf(self.udp)) / 8) + (@bitSizeOf(@TypeOf(self.ip)) / 8));
+
+        // Zero the checksum field before calculation
+        self.ip.header_checksum = 0;
+
+        // Serialize IP header to big-endian bytes
+        var ip_bytes: [@bitSizeOf(@TypeOf(self.ip)) / 8]u8 = undefined;
+        var w: Writer = .fixed(&ip_bytes);
+        w.writeStruct(self.ip, .big) catch unreachable;
+
+        // Calculate checksum over serialized bytes
+        self.ip.header_checksum = onesComplement16(&ip_bytes);
+
+        self.udp.len = @intCast(len + (@bitSizeOf(@TypeOf(self.udp)) / 8));
+    }
+};
+
+fn onesComplement16(data: []const u8) u16 {
+    var sum: u32 = 0;
+
+    // Process pairs of bytes as 16-bit words
+    var i: usize = 0;
+    while (i + 1 < data.len) : (i += 2) {
+        const word: u16 = (@as(u16, data[i]) << 8) | data[i + 1];
+        sum += word;
+    }
+
+    // Handle odd byte if present
+    if (data.len % 2 == 1) {
+        sum += @as(u32, data[data.len - 1]) << 8;
+    }
+
+    // Fold 32-bit sum to 16 bits
+    while (sum >> 16 != 0) {
+        sum = (sum & 0xFFFF) + (sum >> 16);
+    }
+
+    // Return ones' complement
+    return ~@as(u16, @truncate(sum));
+}
+
+const std = @import("std");
+const Writer = std.Io.Writer;