Implement ARP subsystem and arp app (AI)
- Created src/arp.h: ARP packet struct, cache entry struct, operation codes, lookup/request/resolve/receive API, sysfs registration - Created src/arp.c: ARP cache with 32 entries, request/reply handling, ARP response to incoming requests for our IP, sysfs /sys/arp/table with formatted IP/MAC/interface/state columns - Created apps/arp/arp.c: reads and displays /sys/arp/table - Kernel calls arp_init() at boot, registered sysfs 'arp' namespace - Tested: clean boot, ARP initialized, arp app in CPIO
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AI
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src/arp.h
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145
src/arp.h
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/**
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* @file arp.h
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* @brief Address Resolution Protocol (ARP) subsystem.
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*
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* Implements ARP (RFC 826) for mapping IPv4 addresses to Ethernet
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* MAC addresses. Maintains an ARP cache and handles ARP requests
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* and replies.
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*
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* The ARP table is exposed via sysfs at /sys/arp for userspace tools.
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*/
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#ifndef ARP_H
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#define ARP_H
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#include <stdint.h>
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/** Maximum number of ARP cache entries. */
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#define ARP_TABLE_SIZE 32
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/** ARP hardware type: Ethernet */
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#define ARP_HW_ETHER 1
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/** ARP operation codes */
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#define ARP_OP_REQUEST 1
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#define ARP_OP_REPLY 2
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/** ARP cache entry states */
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#define ARP_STATE_FREE 0 /**< Unused slot. */
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#define ARP_STATE_INCOMPLETE 1 /**< Request sent, awaiting reply. */
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#define ARP_STATE_RESOLVED 2 /**< MAC address known. */
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/* ================================================================
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* ARP packet (28 bytes for IPv4-over-Ethernet)
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* ================================================================ */
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/**
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* ARP packet structure for IPv4 over Ethernet.
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* All multi-byte fields are in network byte order.
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*/
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typedef struct __attribute__((packed)) arp_packet {
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uint16_t hw_type; /**< Hardware type (1 = Ethernet). */
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uint16_t proto_type; /**< Protocol type (0x0800 = IPv4). */
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uint8_t hw_len; /**< Hardware address length (6). */
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uint8_t proto_len; /**< Protocol address length (4). */
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uint16_t operation; /**< Operation (1=request, 2=reply). */
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uint8_t sender_mac[6]; /**< Sender hardware address. */
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uint32_t sender_ip; /**< Sender protocol address. */
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uint8_t target_mac[6]; /**< Target hardware address. */
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uint32_t target_ip; /**< Target protocol address. */
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} arp_packet_t;
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/** ARP packet size. */
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#define ARP_PACKET_SIZE 28
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/* ================================================================
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* ARP cache entry
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* ================================================================ */
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/**
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* ARP cache entry.
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*/
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typedef struct arp_entry {
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uint32_t ip_addr; /**< IPv4 address (host byte order). */
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uint8_t mac[6]; /**< Resolved MAC address. */
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uint8_t state; /**< ARP_STATE_*. */
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uint8_t iface_idx; /**< Ethernet interface index. */
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uint32_t timestamp; /**< Time when entry was created (tick count). */
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} arp_entry_t;
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/* ================================================================
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* Public API
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* ================================================================ */
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/**
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* Initialize the ARP subsystem.
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* Registers sysfs namespace "arp".
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*/
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void arp_init(void);
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/**
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* Look up an IP address in the ARP cache.
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*
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* @param ip IPv4 address (host byte order).
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* @param mac Output: 6-byte MAC address if found.
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* @return 0 if found and resolved, -1 if not in cache or incomplete.
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*/
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int arp_lookup(uint32_t ip, uint8_t *mac);
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/**
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* Send an ARP request for the given IP address.
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*
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* @param iface_idx Ethernet interface to send on.
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* @param target_ip IP address to resolve (host byte order).
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* @return 0 on success, -1 on failure.
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*/
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int arp_request(uint32_t iface_idx, uint32_t target_ip);
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/**
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* Resolve an IP address to a MAC address.
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*
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* Checks the ARP cache first. If not found, sends an ARP request
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* and returns -1 (caller should retry later).
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*
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* @param iface_idx Ethernet interface index.
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* @param ip IPv4 address (host byte order).
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* @param mac Output: 6-byte MAC address.
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* @return 0 if resolved, -1 if pending.
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*/
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int arp_resolve(uint32_t iface_idx, uint32_t ip, uint8_t *mac);
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/**
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* Process an incoming ARP packet.
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*
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* Called by the Ethernet subsystem when an ARP frame is received.
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*
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* @param data Raw ARP packet.
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* @param len Packet length.
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* @param iface_idx Interface the packet arrived on.
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*/
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void arp_receive(const void *data, uint32_t len, uint32_t iface_idx);
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/**
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* Add a static ARP entry.
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*
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* @param ip IPv4 address (host byte order).
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* @param mac 6-byte MAC address.
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* @param iface_idx Ethernet interface index.
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* @return 0 on success, -1 if table full.
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*/
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int arp_add_static(uint32_t ip, const uint8_t *mac, uint32_t iface_idx);
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/**
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* Get an ARP table entry by index (for enumeration).
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*
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* @param index 0-based index.
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* @return Pointer to entry, or NULL if out of range.
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*/
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const arp_entry_t *arp_get_entry(uint32_t index);
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/**
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* Get the number of active ARP entries.
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*/
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uint32_t arp_get_count(void);
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#endif /* ARP_H */
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