diff --git a/xdp-synproxy/xdp_synproxy.bpf.c b/xdp-synproxy/xdp_synproxy.bpf.c
index b5b08971..14dfa869 100644
--- a/xdp-synproxy/xdp_synproxy.bpf.c
+++ b/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. */