[Serusers] ENUM issues - req. for discussion

[Klaus Darilion]

Hi all!

Some time of using ser with ENUM revealed several problems which I would 
like to dicuss with you. Be aware - this email is long!

ENUM is a wonderful thing for call routing nevertheless, as it is DNS 
based there are some important things an ENUM aware application has to 
consider:
  1. You never know how long the lookup takes
  2. You never know if the lookup will fail or succeed
  3. If the lookup was successful, you never may trust the result

I will explain this points in detail know:

1+2:
Using enum, the application is giving control to the DNS resolver of the 
OS and the DNS infrastructure. Thus, the ser thread which performs the 
ENUM lookup will be blocked until there is a result from the system's 
DNS resolver.

If DNS is slow, or misconfigured (e.g. a zone is delegated to a 
nameserver which is down), the thread will be blocked for several 
seconds. E.g. if you use debian woody and 2 nameservers in 
/etc/resolv.conf, the timeout is 20 seconds. If you are lucky, the OS 
allows configuration of the DNS timeouts. Nevertheless, you have to 
consider that a ser thread will be blocked up to 20 seconds. This has 
impacts on your configuration:

Typically, you use some kind of the following logic:
if (uri =~ "+[0-9].") {
   if (enum_lookup()) {
     t_relay(); break;
   } else {
     forward to PSTN gateway; break;
   }
}

Thus, the INVITE will be received and the ENUM lookup will be performed. 
If the lookup will take longer than 0.5s, the SIP client will start 
restransmitting the INVITE. Thus, another thread will process this 
INVITE and enother ENUM lookup will be performed. After several seconds, 
all of ser's threads will be blocked with ENUM lookups and your SIP 
proxy will not handle any requests until the DNS query times out. Thus, 
it is very easy to generate a DoS attack against the proxy.
Another funny thing is, that the SIP client will detect a proxy error 
and hangs up, but the INVITEs are still processed in the SIP proxy and 
after the timeout forwarded to the PSTN gateway.

A solution to stop the retransmission is to immediatle 
sl_send_reply("100","Trying"), But this rises another problem. Now, if 
the caller hangs up before the DNS timeout, the SIP client will send 
CANCEL (as it received 100) to the SIP proxy. But the SIP proxy can not 
cancel the transaction, as it is not genereated yet - the INVITE thread 
is still waiting for the ENUM lookup and the transaction will be 
generated after the ENUM lookup (after 20s timeout). Thus, we still end 
up with an INVITE forwarded to the PSTN gateway although the SIP client 
already hang up.

I thought of using t_newtran and t_forward_nonack_uri instead of t_realy 
to generate the transaction before doing the ENUM lookup. Thus, the 
thread which will process the CANCEL should find a transaction and stop 
it. But will this really prevent the INVITE sent to the PSTN gateway, 
once the DNS times out? (not tested)

btw: this problem does not only occour for ENUM lookups, but for all DNS 
lookups (SRV, A, AAAA).

3:
If the ENUM lookup succeeds, you never may trust the result. It may be a 
invalid SIP URI, or a tel: URI, or anything else a user puts into its 
NAPTRs. This may result in a failed transaction, or like revealed at the 
ENUM plugtest in failed accounting. Even worse, maybe it is possible to 
complete crash ser using realy bad formated URIs?

Thus you can't avoid doing some URI checks against the URI received from 
the ENUM lookup. Perfomance issues are no valid arguements! Once I give 
control to external services (DNS, radius, exec), the perfomance 
penalties due to parsing the SIP URI are much more less than due to the 
ENUM lookup.

In case of ser, I would do the URI parsing in the ENUM module, or maybe 
generate a dedicated function/module for checking SIP URIs inside the 
routing logic. Thus, I can also check the result of exec calls.

I'm eagerly waiting for your opinions.

regards,
Klaus