lwIP  2.0.2
Lightweight IP stack
etharp.c File Reference
#include "lwip/opt.h"
#include "lwip/etharp.h"
#include "lwip/stats.h"
#include "lwip/snmp.h"
#include "lwip/dhcp.h"
#include "lwip/autoip.h"
#include "netif/ethernet.h"
#include <string.h>
#include "path/to/my/lwip_hooks.h"


#define ARP_MAXPENDING   5


enum  etharp_state


void etharp_tmr (void)
void etharp_cleanup_netif (struct netif *netif)
s8_t etharp_find_addr (struct netif *netif, const ip4_addr_t *ipaddr, struct eth_addr **eth_ret, const ip4_addr_t **ip_ret)
u8_t etharp_get_entry (u8_t i, ip4_addr_t **ipaddr, struct netif **netif, struct eth_addr **eth_ret)
void etharp_input (struct pbuf *p, struct netif *netif)
err_t etharp_output (struct netif *netif, struct pbuf *q, const ip4_addr_t *ipaddr)
err_t etharp_query (struct netif *netif, const ip4_addr_t *ipaddr, struct pbuf *q)
err_t etharp_request (struct netif *netif, const ip4_addr_t *ipaddr)

Detailed Description

Address Resolution Protocol module for IP over Ethernet

Functionally, ARP is divided into two parts. The first maps an IP address to a physical address when sending a packet, and the second part answers requests from other machines for our physical address.

This implementation complies with RFC 826 (Ethernet ARP). It supports Gratuitious ARP from RFC3220 (IP Mobility Support for IPv4) section 4.6 if an interface calls etharp_gratuitous(our_netif) upon address change.

Macro Definition Documentation



Re-request a used ARP entry 1 minute before it would expire to prevent breaking a steadily used connection because the ARP entry timed out.


#define ARP_MAXPENDING   5

the time an ARP entry stays pending after first request, for ARP_TMR_INTERVAL = 1000, this is 10 seconds.



Try hard to create a new entry - we want the IP address to appear in the cache (even if this means removing an active entry or so).

Enumeration Type Documentation

◆ etharp_state

ARP states

Function Documentation

◆ etharp_cleanup_netif()

void etharp_cleanup_netif ( struct netif netif)

Remove all ARP table entries of the specified netif.

netifpoints to a network interface

◆ etharp_find_addr()

s8_t etharp_find_addr ( struct netif netif,
const ip4_addr_t ipaddr,
struct eth_addr **  eth_ret,
const ip4_addr_t **  ip_ret 

Finds (stable) ethernet/IP address pair from ARP table using interface and IP address index.

the addresses in the ARP table are in network order!
netifpoints to interface index
ipaddrpoints to the (network order) IP address index
eth_retpoints to return pointer
ip_retpoints to return pointer
table index if found, -1 otherwise

◆ etharp_get_entry()

u8_t etharp_get_entry ( u8_t  i,
ip4_addr_t **  ipaddr,
struct netif **  netif,
struct eth_addr **  eth_ret 

Possibility to iterate over stable ARP table entries

ientry number, 0 to ARP_TABLE_SIZE
ipaddrreturn value: IP address
netifreturn value: points to interface
eth_retreturn value: ETH address
1 on valid index, 0 otherwise

◆ etharp_input()

void etharp_input ( struct pbuf p,
struct netif netif 

Responds to ARP requests to us. Upon ARP replies to us, add entry to cache send out queued IP packets. Updates cache with snooped address pairs.

Should be called for incoming ARP packets. The pbuf in the argument is freed by this function.

pThe ARP packet that arrived on netif. Is freed by this function.
netifThe lwIP network interface on which the ARP packet pbuf arrived.
See also

◆ etharp_output()

err_t etharp_output ( struct netif netif,
struct pbuf q,
const ip4_addr_t ipaddr 

Resolve and fill-in Ethernet address header for outgoing IP packet.

For IP multicast and broadcast, corresponding Ethernet addresses are selected and the packet is transmitted on the link.

For unicast addresses, the packet is submitted to etharp_query(). In case the IP address is outside the local network, the IP address of the gateway is used.

netifThe lwIP network interface which the IP packet will be sent on.
qThe pbuf(s) containing the IP packet to be sent.
ipaddrThe IP address of the packet destination.

◆ etharp_query()

err_t etharp_query ( struct netif netif,
const ip4_addr_t ipaddr,
struct pbuf q 

Send an ARP request for the given IP address and/or queue a packet.

If the IP address was not yet in the cache, a pending ARP cache entry is added and an ARP request is sent for the given address. The packet is queued on this entry.

If the IP address was already pending in the cache, a new ARP request is sent for the given address. The packet is queued on this entry.

If the IP address was already stable in the cache, and a packet is given, it is directly sent and no ARP request is sent out.

If the IP address was already stable in the cache, and no packet is given, an ARP request is sent out.

netifThe lwIP network interface on which ipaddr must be queried for.
ipaddrThe IP address to be resolved.
qIf non-NULL, a pbuf that must be delivered to the IP address. q is not freed by this function.
q must only be ONE packet, not a packet queue!
  • ERR_BUF Could not make room for Ethernet header.
  • ERR_MEM Hardware address unknown, and no more ARP entries available to query for address or queue the packet.
  • ERR_MEM Could not queue packet due to memory shortage.
  • ERR_RTE No route to destination (no gateway to external networks).
  • ERR_ARG Non-unicast address given, those will not appear in ARP cache.

◆ etharp_request()

err_t etharp_request ( struct netif netif,
const ip4_addr_t ipaddr 

Send an ARP request packet asking for ipaddr.

netifthe lwip network interface on which to send the request
ipaddrthe IP address for which to ask
ERR_OK if the request has been sent ERR_MEM if the ARP packet couldn't be allocated any other err_t on failure

◆ etharp_tmr()

void etharp_tmr ( void  )

Clears expired entries in the ARP table.

This function should be called every ARP_TMR_INTERVAL milliseconds (1 second), in order to expire entries in the ARP table.