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osdep/Arp.hpp

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+/*
+ * Copyright (c)2019 ZeroTier, Inc.
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file in the project's root directory.
+ *
+ * Change Date: 2026-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2.0 of the Apache License.
+ */
+/****/
+
+#ifndef ZT_ARP_HPP
+#define ZT_ARP_HPP
+
+#include <stdint.h>
+
+#include <utility>
+
+#include "../node/Constants.hpp"
+#include "../node/Hashtable.hpp"
+#include "../node/MAC.hpp"
+
+/**
+ * Maximum possible ARP length
+ *
+ * ARPs are 28 bytes in length, but specify a 128 byte buffer since
+ * some weird extensions we may support in the future can pad them
+ * out to as long as 72 bytes.
+ */
+#define ZT_ARP_BUF_LENGTH 128
+
+/**
+ * Minimum permitted interval between sending ARP queries for a given IP
+ */
+#define ZT_ARP_QUERY_INTERVAL 2000
+
+/**
+ * Maximum time between query and response, otherwise responses are discarded to prevent poisoning
+ */
+#define ZT_ARP_QUERY_MAX_TTL 5000
+
+/**
+ * ARP expiration time
+ */
+#define ZT_ARP_EXPIRE 600000
+
+namespace ZeroTier {
+
+/**
+ * ARP cache and resolver
+ *
+ * To implement ARP:
+ *
+ * (1) Call processIncomingArp() on all ARP packets received and then always
+ * check responseLen after calling. If it is non-zero, send the contents
+ * of response to responseDest.
+ *
+ * (2) Call query() to look up IP addresses, and then check queryLen. If it
+ * is non-zero, send the contents of query to queryDest (usually broadcast).
+ *
+ * Note that either of these functions can technically generate a response or
+ * a query at any time, so their result parameters for sending ARPs should
+ * always be checked.
+ *
+ * This class is not thread-safe and must be guarded if used in multi-threaded
+ * code.
+ */
+class Arp
+{
+public:
+	Arp();
+
+	/**
+	 * Set a local IP entry that we should respond to ARPs for
+	 *
+	 * @param mac Our local MAC address
+	 * @param ip IP in big-endian byte order (sin_addr.s_addr)
+	 */
+	void addLocal(uint32_t ip,const MAC &mac);
+
+	/**
+	 * Delete a local IP entry or a cached ARP entry
+	 *
+	 * @param ip IP in big-endian byte order (sin_addr.s_addr)
+	 */
+	void remove(uint32_t ip);
+
+	/**
+	 * Process ARP packets
+	 *
+	 * For ARP queries, a response is generated and responseLen is set to its
+	 * frame payload length in bytes.
+	 *
+	 * For ARP responses, the cache is populated and the IP address entry that
+	 * was learned is returned.
+	 *
+	 * @param arp ARP frame data
+	 * @param len Length of ARP frame (usually 28)
+	 * @param response Response buffer -- MUST be a minimum of ZT_ARP_BUF_LENGTH in size
+	 * @param responseLen Response length, or set to 0 if no response
+	 * @param responseDest Destination of response, or set to null if no response
+	 * @return IP address learned or 0 if no new IPs in cache
+	 */
+	uint32_t processIncomingArp(const void *arp,unsigned int len,void *response,unsigned int &responseLen,MAC &responseDest);
+
+	/**
+	 * Get the MAC corresponding to an IP, generating a query if needed
+	 *
+	 * This returns a MAC for a remote IP. The local MAC is returned for local
+	 * IPs as well. It may also generate a query if the IP is not known or the
+	 * entry needs to be refreshed. In this case queryLen will be set to a
+	 * non-zero value, so this should always be checked on return even if the
+	 * MAC returned is non-null.
+	 *
+	 * @param localMac Local MAC address of host interface
+     * @param localIp Local IP address of host interface
+	 * @param targetIp IP to look up
+	 * @param query Buffer for generated query -- MUST be a minimum of ZT_ARP_BUF_LENGTH in size
+	 * @param queryLen Length of generated query, or set to 0 if no query generated
+	 * @param queryDest Destination of query, or set to null if no query generated
+	 * @return MAC or 0 if no cached entry for this IP
+	 */
+	MAC query(const MAC &localMac,uint32_t localIp,uint32_t targetIp,void *query,unsigned int &queryLen,MAC &queryDest);
+
+private:
+	struct _ArpEntry
+	{
+		_ArpEntry() : lastQuerySent(0),lastResponseReceived(0),mac(),local(false) {}
+		uint64_t lastQuerySent; // Time last query was sent or 0 for local IP
+		uint64_t lastResponseReceived; // Time of last ARP response or 0 for local IP
+		MAC mac; // MAC address of device responsible for IP or null if not known yet
+		bool local; // True if this is a local ARP entry
+	};
+
+	Hashtable< uint32_t,_ArpEntry > _cache;
+	uint64_t _lastCleaned;
+};
+
+} // namespace ZeroTier
+
+#endif