Feat: socket thread self check

src/selfcheck.h

@@ -0,0 +1,39 @@
+#ifndef SELFCHECK_H
+#define SELFCHECK_H
+
+#define SELFCHECK_CHALLENGE_LEN   16
+#define SELFCHECK_RECV_TIMEOUT_S  2
+#define SELFCHECK_FAIL_THRESHOLD  3
+#define SELFCHECK_DRIFT_THRESHOLD 10
+#define SELFCHECK_COOLDOWN_S      60
+
+/*
+ * Init challenge buffer and start UDP socket listener thread.
+ * The socket discards any datagram and always
+ * responsds with the challenge bytes.
+ *
+ * Returns 0 on success.
+ * Returns -1 on failure.
+ */
+int selfcheck_start_listener(int port);
+
+/*
+ * Checks if the random challenge bytes are received
+ * when sending an empty datagram to the domain and port.
+ *
+ * Returns 0 if the response matches.
+ * Returns -1 on DNS failure, send failure, or timeout.
+ * Returns -2 if the response did not match.
+ */
+int selfcheck_domain(const char *domain, int port);
+
+/*
+ * Same as selfcheck_domain but sends directly to `ip` without DNS.
+ *
+ * Returns 0 if the response matches.
+ * Returns -1 on send failure or timeout.
+ * Returns -2 if the response did not match.
+ */
+int selfcheck_ip_str(const char *ip, int port);
+
+#endif /* SELFCHECK_H */