[lwip-users] DNS TTL 0

[Måns Andersson]

Hi
I'm using lwip 1.4.0 with an opendns.com dns server. When OpenDNS can't resolve 
a host they return an ip address pointing to one of their own servers together 
with a TTL of 0. When this happens lwip caches it for DNS_MAX_TTL seconds 
rather than throwing it away. I suggest we change the code the following way:

dns.c:695 (dns_check_entry() state DNS_STATE_DONE)

case DNS_STATE_DONE: {
  /* if the time to live is nul */
  if (--pEntry->ttl == 0) {
    LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", pEntry->name));
    /* flush this entry */
    pEntry->state = DNS_STATE_UNUSED;
    pEntry->found = NULL;
  }
  break;
}

to

case DNS_STATE_DONE: {
  /* if the time to live is nul */
  if (pEntry->ttl == 0 || --pEntry->ttl == 0) {
    LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", pEntry->name));
    /* flush this entry */
    pEntry->state = DNS_STATE_UNUSED;
    pEntry->found = NULL;
  }
  break;
}

By doing this we make sure that the u32 ttl variable does not tip over (ie. --0 
== INT_MAX when we have an unsigned variable) and that we throw away any cache 
that has reached 0 in some way.

// Måns Andersson