1 files changed, 57 insertions, 0 deletions
diff --git a/src/EthIpUdp.zig b/src/EthIpUdp.zig
new file mode 100644
index 0000000..472e421
--- /dev/null
+++ b/src/EthIpUdp.zig
@@ -0,0 +1,57 @@
+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));
+        self.udp.len = @intCast(len + (@bitSizeOf(@TypeOf(self.udp)) / 8));
+    }
+};
+
+const std = @import("std");
+const Writer = std.Io.Writer;