Calculate IPv4 checksum header

This was causing an issue because virtual networks were dropping packets
without this being set

src/EthIpUdp.zig

@@ -49,9 +49,45 @@ pub const EthIpUdp = packed struct(u336) { // 42 bytes * 8 bits = 336
 
     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;