[lwip-devel] [bug #36899] DNS TTL 0 is cached for a long time

[Måns Andersson]

URL:
  <http://savannah.nongnu.org/bugs/?36899>

                 Summary: DNS TTL 0 is cached for a long time
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: mansandersson
            Submitted on: Fri 20 Jul 2012 10:44:53 AM GMT
                Category: None
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: 1.4.0

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Details:

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.




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