xdp-synproxy: set SYN+ACK with max window size

set window size to 0 for XDP generated SYNACK
seems not common to normal SYNACK which has
larger window size, so set to max window size

Signed-off-by: Vincent Li <vincent.mc.li@gmail.com>

xdp-synproxy/xdp_synproxy.bpf.c

@@ -60,6 +60,7 @@
 #define DEFAULT_WSCALE 7
 #define DEFAULT_TTL 64
 #define MAX_ALLOWED_PORTS 8
+#define MAX_WINDOW_SIZE 65535
 
 #define MAX_PACKET_OFF 0xffff
 
@@ -524,7 +525,7 @@ static __always_inline void tcp_gen_synack(struct tcphdr *tcp_header,
 	swap(tcp_header->source, tcp_header->dest);
 	tcp_header->ack_seq = bpf_htonl(bpf_ntohl(tcp_header->seq) + 1);
 	tcp_header->seq = bpf_htonl(cookie);
-	tcp_header->window = 0;
+	tcp_header->window = bpf_htons(MAX_WINDOW_SIZE);
 	tcp_header->urg_ptr = 0;
 	tcp_header->check = 0; /* Calculate checksum later. */