From f2c15c2af30820366fd4f52576dfbe38a430cf3e Mon Sep 17 00:00:00 2001
From: Chris Hiszpanski <chris@hiszpanski.name>
Date: Sat, 11 Jun 2022 17:10:30 -0700
Subject: Toy STUN client

---
 tinystun.c | 112 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 tinystun.c

(limited to 'tinystun.c')

diff --git a/tinystun.c b/tinystun.c
new file mode 100644
index 0000000..3fa703a
--- /dev/null
+++ b/tinystun.c
@@ -0,0 +1,112 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+#include <netinet/in.h>
+#include <sys/socket.h>
+
+/**
+ * Parse reply to STUN binding request
+ *
+ * \param req[in] Original request. Transaction id must match reply.
+ * \param rep[in] Reply. Invalid after this function is called.
+ * \param sz[in] Size of reply (in bytes).
+ *
+ * \return String containing server-reflexive address. Address is in the
+ *   form "ip:port". String overwrites reply. Reply must not be used after
+ *   this function.
+ */
+char* stun_parse_reply(const unsigned char*req, unsigned char* rep, size_t sz)
+{
+	static char addr[23];
+
+	// length requirement
+	if (sz < 32) {
+		return NULL;
+	}
+
+	// type must be binding reply
+	if (rep[0] != 0x01 || rep[1] != 0x01) {
+		return NULL;
+	}
+
+	// magic cookie must match
+	if (rep[4] != 0x21 || rep[5] != 0x12 || rep[6] != 0xa4 || rep[7] != 0x42) {
+		return NULL;
+	}
+
+	// transaction id must match
+	for (int i = 8; i < 20; i++) {
+		if (req[i] != rep[i]) {
+			return NULL;
+		}
+	}
+
+	// first attribute expected to be of type XOR-MAPPED-ADDRESS
+	if (rep[20] != 0x00 || rep[21] != 0x20) {
+		return NULL;
+	}
+
+	// length expected to be 8 bytes
+	if (rep[22] != 0x00 || rep[23] != 0x08) {
+		return NULL;
+	}
+
+	// family expected to be IPv4
+	if (rep[24] != 0x00 || rep[25] != 0x01) {
+		return NULL;
+	}
+
+	// get address and port
+	sprintf(rep, "%d.%d.%d.%d:%d",
+		rep[28] ^ 0x21, rep[29] ^ 0x12, rep[30] ^ 0xa4, rep[31] ^ 0x42,
+		((rep[26] ^ 0x21) << 8) | (rep[27] ^ 0x12));
+
+	return rep;
+}
+
+int rtc_stun_binding_request()
+{
+	int fd;
+	unsigned char req[] = {
+		0x00, 0x01,						// binding request
+		0x00, 0x00,						// length
+		0x21, 0x12, 0xa4, 0x42,			// magic cookie
+		rand(), rand(), rand(), rand(),	// tx id
+		rand(), rand(), rand(), rand(),	// tx id (cont.)
+		rand(), rand(), rand(), rand(),	// tx id (cont.)
+	};
+
+	if (fd = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP), -1 == fd) {
+		perror("socket");
+	}
+
+	struct sockaddr_in sa = {
+		.sin_addr.s_addr = htonl(164 << 24 | 90 << 16 | 156 << 8 | 231),
+		.sin_port = htons(3478),
+		.sin_family = AF_INET
+	};
+
+	if (-1 == sendto(fd, req, sizeof(req), 0, (struct sockaddr*)&sa, sizeof(sa))) {
+		perror("sendto");
+	}
+
+	unsigned char rep[4096];
+	int n;
+	socklen_t sl = sizeof(struct sockaddr_in);
+	if (n = recvfrom(fd, rep, sizeof(rep), 0, (struct sockaddr*)&sa, &sl), -1 == n) {
+		perror("recvfrom");
+	}
+
+	char* a;
+	if (a = stun_parse_reply(req, rep, n), a) {
+		printf("%s\n", a);
+	}
+}
+
+int main()
+{
+	srand(time(0));
+	rtc_stun_binding_request();
+	return 0;
+}
-- 
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