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

[sergey-safarov]

> How about expanding the discovery procedure with a NAPTR request sent to the provider's domain (as found in From and/or P-A-I) in case (1) fails

Yes, looks as it will be a good extension.
Domain from `From` header may be used only if domain not available in `P-A-I` header.

Here examples of `From` and `P-A-I` headers that we can receive
**Example 1** - domain matched in `From` and `P-A-I` headers
```
From: <sip:+18556667788 at 206.147.246.118:5060>;tag=gK0e0d09a4
P-Asserted-Identity: <sip:+18556667788 at 206.147.246.118:5060>
```

**Example 2** - private IP address in `From` header and no domain in `P-A-I` header. `P-A-I` contains tel and sip uri.
```
From: <sip:+16692481432 at 198.226.45.84>;tag=gK04090b5e
P-Asserted-Identity: <tel:+16692481432>, <sip:+16692481432 at vzimstest.com>
```

**Example 3** - IPv6 address in `From` header and no domain in `P-A-I` header
```
From: "9255111003"<sip:9255111003@[2605:97c0:2058:3120:1::1]:50001>;tag=75fdbcf61a;epid=SC597fbd
P-Asserted-Identity: <tel:9255111003>
```
**Example 4** - no domain in `From` and `P-A-I` headers
```
From: <tel:+13559598013>;tag=SDtl3t701-5f7f754a-5f89c26229204b60-Noo-lucentNGFS-000264
P-Asserted-Identity: <tel:+11005534340>
```

Think not required to make double reverse and NAPTR DNS lookups if the domain from `P-A-I` or `From` headers equals to caller network IP address.

Could you look at this example.
```
P-Asserted-Identity: <tel:+16692481432>, <sip:+16692481432 at vzimstest.com>
```
IP address does exist here, put present caller domain. So we can skip reverse lookup and execute NAPTR.


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