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;

src/message.zig

@@ -1,11 +1,3 @@
-/// Type tag for Message union.
-/// This is the first value in the actual packet sent over the network.
-pub const PacketType = enum(u16) {
-    relay = 0x003C,
-    connection = 0x00E9,
-    _,
-};
-
 pub const MessageTypeError = error{
     NotImplementedSaprusType,
     UnknownSaprusType,
@@ -16,16 +8,18 @@ pub const MessageParseError = MessageTypeError || error{
 
 const message = @This();
 
-pub const Message = union(PacketType) {
+pub const Message = union(enum(u16)) {
-    relay: Message.Relay,
+    relay: Message.Relay = 0x003C,
-    connection: Message.Connection,
+    connection: Message.Connection = 0x00E9,
+    _,
 
     pub const Relay = message.Relay;
     pub const Connection = message.Connection;
 
     pub fn toBytes(self: message.Message, buf: []u8) []u8 {
         return switch (self) {
-            inline else => |m| m.toBytes(buf),
+            inline .relay, .connection => |m| m.toBytes(buf),
+            else => unreachable,
         };
     }
 
@@ -36,7 +30,7 @@ pub const relay_dest_len = 4;
 
 pub fn parse(bytes: []const u8) MessageParseError!Message {
     var in: Reader = .fixed(bytes);
-    const @"type" = in.takeEnum(PacketType, .big) catch |err| switch (err) {
+    const @"type" = in.takeEnum(std.meta.Tag(Message), .big) catch |err| switch (err) {
         error.InvalidEnumTag => return error.UnknownSaprusType,
         else => return error.InvalidMessage,
     };
@@ -124,7 +118,7 @@ const Relay = struct {
     /// Asserts that buf is large enough to fit the relay message.
     pub fn toBytes(self: Relay, buf: []u8) []u8 {
         var out: Writer = .fixed(buf);
-        out.writeInt(u16, @intFromEnum(PacketType.relay), .big) catch unreachable;
+        out.writeInt(u16, @intFromEnum(Message.relay), .big) catch unreachable;
         out.writeInt(u16, @intCast(self.payload.len + 4), .big) catch unreachable; // Length field, but unread. Will switch to checksum
         out.writeAll(&self.dest.bytes) catch unreachable;
         out.writeAll(self.payload) catch unreachable;
@@ -178,7 +172,7 @@ const Connection = struct {
     /// Asserts that buf is large enough to fit the connection message.
     pub fn toBytes(self: Connection, buf: []u8) []u8 {
         var out: Writer = .fixed(buf);
-        out.writeInt(u16, @intFromEnum(PacketType.connection), .big) catch unreachable;
+        out.writeInt(u16, @intFromEnum(Message.connection), .big) catch unreachable;
         out.writeInt(u16, @intCast(self.payload.len + 14), .big) catch unreachable; // Saprus length field, unread.
         out.writeInt(u16, self.src, .big) catch unreachable;
         out.writeInt(u16, self.dest, .big) catch unreachable;