[sr-dev] [kamailio/kamailio] lost: dynamic HELD url resolving (#2574)

### Description According to design logic HELD request need to send a carrier LIS server. Now lost module sends this request to the preconfigured server via `http_client/httpcon` param. I prefer to use dynamic LIS server discovery according to [rfc7216#section-4](https://tools.ietf.org/html/rfc7216#section-4) and [rfc5986#section-4](https://tools.ietf.org/html/rfc5986#section-4). So dynamic LIS discovery works as: 1. Kamailio send reverse `.in-addr.arpa.` or `.ip6.arpa.` DNS request and caller host DNS name; 2. Kamailio send `LIS:HELD` NAPTR request for a resolved caller hostname and LIS server. 3. Kamailio send HELD request for resolved LIS address. So do get working dynamic LIS discovery required to implement two DNS requests. If any DNS request will fail, then `lost_held_query` function returns an error code. To define required dynamic LIS discovery I suggest use an empty string ("") or NULL value ($null) as the first ("con") function param. This feature request for discussion with lost module author (Wolfgang Kampichler @wkampich) and other interest devs. The ticket may be closed at any time. -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: https://github.com/kamailio/kamailio/issues/2574