Add broadcast initial interest using raw sockets

Use this from the relay message

src/Client.zig

@@ -2,6 +2,7 @@ const base64Enc = std.base64.Base64Encoder.init(std.base64.standard_alphabet_cha
 const base64Dec = std.base64.Base64Decoder.init(std.base64.standard_alphabet_chars, '=');
 
 var rand: ?Random = null;
+var socket: ?gcat.nic.RawSocket = null;
 
 const max_message_size = 2048;
 
@@ -12,13 +13,61 @@ pub fn init() !void {
         break :blk seed;
     });
     rand = prng.random();
-    try network.init();
+
+    socket = try .init("enp7s0");
 }
 
 pub fn deinit() void {
-    network.deinit();
+    socket.?.deinit();
 }
 
+/// Used for relay messages and connection handshake.
+/// Assumes Client .init has been called.
+fn broadcastInitialInterestMessage(msg_bytes: []align(@alignOf(SaprusMessage)) u8) !void {
+    var packet_bytes: [max_message_size]u8 = comptime blk: {
+        var b: [max_message_size]u8 = @splat(0);
+
+        // Destination MAC addr to FF:FF:FF:FF:FF:FF
+        for (0..6) |i| {
+            b[i] = 0xff;
+        }
+
+        // Set Ethernet type to IPv4
+        b[0x0c] = 0x08;
+        b[0x0d] = 0x00;
+
+        // Set IPv4 version to 4
+        b[0x0e] = 0x45;
+
+        // Destination broadcast
+        for (0x1e..0x22) |i| {
+            b[i] = 0xff;
+        }
+
+        // Set TTL
+        b[0x16] = 0x40;
+
+        // Set IPv4 protocol to UDP
+        b[0x17] = 0x11;
+
+        // Set interest filter value to 8888.
+        b[0x24] = 0x22;
+        b[0x25] = 0xb8;
+        break :blk b;
+    };
+    var msg: *SaprusMessage = try .bytesAsValue(msg_bytes);
+    try msg.networkFromNativeEndian();
+    defer msg.nativeFromNetworkEndian() catch unreachable;
+
+    // The byte position within the packet that the saprus message starts at.
+    const saprus_start_byte = 42;
+    @memcpy(packet_bytes[saprus_start_byte .. saprus_start_byte + msg_bytes.len], msg_bytes);
+
+    try socket.?.linkLayer().send(packet_bytes[0 .. saprus_start_byte + msg_bytes.len]);
+}
+
+// fn broadcastSaprusMessage(msg_bytes: []align(@alignOf(SaprusMessage)) u8) !void {}
+
 fn broadcastSaprusMessage(msg_bytes: []align(@alignOf(SaprusMessage)) u8, udp_port: u16) !void {
     if (false) {
         var foo: gcat.nic.RawSocket = try .init("enp7s0"); // /proc/net/dev
@@ -63,7 +112,7 @@ pub fn sendRelay(payload: []const u8, dest: [4]u8) !void {
     relay.dest = dest;
     _ = base64Enc.encode(relay.getPayload(), payload);
 
-    try broadcastSaprusMessage(msg_bytes, 8888);
+    try broadcastInitialInterestMessage(msg_bytes);
 }
 
 fn randomPort() u16 {