From: Matt Corallo <git@bluematt.me>
Date: Wed, 16 Mar 2022 06:31:02 +0000 (+0000)
Subject: Handle initial seq endianness correctly
X-Git-Url: http://git.bitcoin.ninja/index.cgi?a=commitdiff_plain;h=5aacce12cddca4ee3d49ba5eaca1945cc524d5ed;p=tunudptotcp

Handle initial seq endianness correctly
---

diff --git a/main.cpp b/main.cpp
index c116e89..3fa3522 100644
--- a/main.cpp
+++ b/main.cpp
@@ -187,7 +187,7 @@ static std::atomic<uint16_t> local_port(0);
 static uint16_t remote_port;
 static bool are_server;
 
-static void build_tcp_header(unsigned char* buf, uint32_t len, int syn, int synack, in_addr_t src_addr, in_addr_t dest_addr) {
+static int build_tcp_header(unsigned char* buf, uint32_t len, int syn, int synack, in_addr_t src_addr, in_addr_t dest_addr) {
 	buf[0 ] = local_port >> 8;         // src port
 	buf[1 ] = local_port;              // src port
 	buf[2 ] = remote_port >> 8;        // dst port
@@ -233,6 +233,8 @@ static void build_tcp_header(unsigned char* buf, uint32_t len, int syn, int syna
 	uint16_t checksum = tcp_checksum(buf, len + 20 + (longpkt ? 4 : 0), src_addr, dest_addr);
 	buf[16] = checksum;                // Checksum
 	buf[17] = checksum >> 8;           // Checksum
+
+	return 20 + (longpkt ? 4 : 0);
 }
 
 static void build_ip_header(unsigned char* buf, struct pkt_hdr hdr, const in_addr_t& src_addr, const in_addr_t& dest_addr) {
@@ -294,7 +296,7 @@ static int fdr;
 static int fd[TUN_IF_COUNT];
 static struct sockaddr_in dest;
 static in_addr_t src, tun_src, tun_dest;
-static uint64_t tcp_init_magic;
+static uint32_t starting_seq, starting_ack;
 
 #define PENDING_MESSAGES_BUFF_SIZE (0x800)
 #define THREAD_POLL_SLEEP_MICS 250
@@ -363,15 +365,7 @@ static void tcp_to_tun_queue_process() {
 		bool ack = tcp_buf[13] & (1 << 4);
 
 		if (are_server && syn && !ack) {
-			// We're a server and just got a client
-			if (tcp_buf[4 ] != uint8_t(tcp_init_magic >> (7 * 8)) ||
-			    tcp_buf[5 ] != uint8_t(tcp_init_magic >> (6 * 8)) ||
-			    tcp_buf[6 ] != uint8_t(tcp_init_magic >> (5 * 8)) ||
-			    tcp_buf[7 ] != uint8_t(tcp_init_magic >> (4 * 8)) ||
-			    tcp_buf[8 ] != uint8_t(tcp_init_magic >> (3 * 8)) ||
-			    tcp_buf[9 ] != uint8_t(tcp_init_magic >> (2 * 8)) ||
-			    tcp_buf[10] != uint8_t(tcp_init_magic >> (1 * 8)) ||
-			    tcp_buf[11] != uint8_t(tcp_init_magic >> (0 * 8)))
+			if (memcmp(tcp_buf + 4, &starting_seq, 4) || memcmp(tcp_buf + 8, &starting_ack, 4))
 				continue;
 
 			fprintf(stderr, "Got SYN, sending SYNACK\n");
@@ -399,9 +393,9 @@ static void tcp_to_tun_queue_process() {
 			last_ack_recv = std::chrono::steady_clock::now();
 
 		if (are_server && syn && !ack) {
-			build_tcp_header(tcp_buf, 0, 0, 1, src, dest.sin_addr.s_addr);
+			int len = build_tcp_header(tcp_buf, 0, 0, 1, src, dest.sin_addr.s_addr);
 
-			ssize_t res = sendto(fdr, tcp_buf, 20 + 4, 0, (struct sockaddr*)&dest, sizeof(dest));
+			ssize_t res = sendto(fdr, tcp_buf, len, 0, (struct sockaddr*)&dest, sizeof(dest));
 			if (res < 0) {
 				int err = errno;
 				fprintf(stderr, "Failed to send SYNACK with err %d (%s)\n", err, strerror(err));
@@ -511,9 +505,6 @@ int do_init() {
 		local_port = local_port_tmp;
 	}
 
-	uint32_t starting_ack = 0, starting_seq = 0;
-	memcpy(&starting_ack, &tcp_init_magic, 4);
-	memcpy(&starting_seq, ((const unsigned char*)&tcp_init_magic) + 4, 4);
 	highest_recvd_seq = starting_ack;
 	cur_seq = starting_seq;
 
@@ -526,8 +517,8 @@ int do_init() {
 
 	unsigned char buf[1500];
 	if (!are_server) {
-		build_tcp_header(buf, 0, 1, 0, src, dest.sin_addr.s_addr);
-		ssize_t res = sendto(fdr, buf, 20 + 4, 0, (struct sockaddr*)&dest, sizeof(dest));
+		int len = build_tcp_header(buf, 0, 1, 0, src, dest.sin_addr.s_addr);
+		ssize_t res = sendto(fdr, buf, len, 0, (struct sockaddr*)&dest, sizeof(dest));
 		if (res < 0) {
 			int err = errno;
 			fprintf(stderr, "Failed to send initial SYN with err %d (%s)\n", err, strerror(err));
@@ -544,8 +535,8 @@ int do_init() {
 	if (!are_server) {
 		fprintf(stderr, "Got SYNACK, sending ACK and starting tun listen\n");
 
-		build_tcp_header(buf, 0, 0, 0, src, dest.sin_addr.s_addr);
-		ssize_t res = sendto(fdr, buf, 20, 0, (struct sockaddr*)&dest, sizeof(dest));
+		int len = build_tcp_header(buf, 0, 0, 0, src, dest.sin_addr.s_addr);
+		ssize_t res = sendto(fdr, buf, len, 0, (struct sockaddr*)&dest, sizeof(dest));
 		if (res < 0) {
 			int err = errno;
 			fprintf(stderr, "Failed to send initial ACK with err %d (%s)\n", err, strerror(err));
@@ -601,7 +592,9 @@ int main(int argc, char* argv[]) {
 		remote_port = atoi(argv[4]);
 	}
 
-	tcp_init_magic = atoll(argv[5]);
+	uint64_t tcp_init_magic = atoll(argv[5]);
+	starting_seq = htobe32(tcp_init_magic);
+	starting_ack = htobe32(tcp_init_magic >> 32);
 
 	src = inet_addr(argv[7]);