[lwip-users] [lwip] ARP bug (compare MAC instead of IP)

[leon . woestenberg]

Hello Adam,

in the ARP code, when adding an (MAC, IP) entry, you use IP
addresses to compare. You should, however, use the MAC
addresses to find an old entry.

----
To see what goes wrong, using IP address compare, an example:

Suppose client with MAC 01:02:03:04:05:06 has obtained
IP address 192.168.0.2, and by whatever means, is configured
to use 192.168.0.3 now.

Adding this (MAC, IP) mapping will not find the old entry, and create
a new entry in the ARP cache.  The ARP cache will now hold two
mappings with the same MAC address.
----

static void
add_arp_entry(struct ip_addr *ipaddr, struct eth_addr *ethaddr)
{
  u8_t i, j, k;
  u8_t maxage;

  /* Walk through the ARP mapping table and try to find an entry to
     update. If none is found, the IP -> MAC address mapping is
     inserted in the ARP table. */
  for(i = 0; i < ARPTABLE_SIZE; ++i) {

    /* Only check those entries that are actually in use. */
    if(!ip_addr_isany(&arp_table[i].ipaddr)) {
      /* Check if the source IP address of the incoming packet matches
         the IP address in this ARP table entry. */
      if(ip_addr_cmp(ipaddr, &arp_table[i].ipaddr)) {
<--------------------------- should test MAC bytes here, not IP address
     /* An old entry found, update this and return. */

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