[sr-dev] [kamailio/kamailio] Kamailio logs errors due to unsupported EDNS0 extension in DNS responses (#2087)

[Florian Floimair]

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### Description

We are running Kamailio in a Microsoft Azure VM. Azure started adding EDNS0 support - in the form of OPT pseudo-records at the end of replies - a few weeks ago and since then we see a lot of ERROR entries in Kamailio log. These do however have no function impact.
>From what I can see they are simply not handled in core/resolv.c (EDNS0 is type 41).
Version in use is latest 5.2 package from Kamailio repo on Debian 9 (Stretch).

Log entries provided below

### Troubleshooting

None, as this has no functional impact aside from filling the logs.

#### Reproduction

I can provide pcap of DNS traffic that contains EDNS0 OPT pseudo-entries on request.

#### Debugging Data

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```

#### Log Messages

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```
Oct  1 14:02:01 kamailio /usr/sbin/kamailio[78878]: ERROR: <core> [core/resolve.c:941]: get_record(): unknown type 41
```
These lines are seen repeatedly only differing in timestamp and PID of kamailio process. In total about 40% of log entries are like the one above. 

#### SIP Traffic

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not related to SIP, but to DNS

### Possible Solutions

Add EDNS0 type 41 in core/resolv.c and don't fire an ERROR in the logs.

### Additional Information

  * **Kamailio Version** - output of `kamailio -v`

```
version: kamailio 5.2.4 (x86_64/linux)
flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES
ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144 MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: unknown
compiled with gcc 6.3.0
```

* **Operating System**:

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```
Debian 9.11
Linux kamailio 4.19.0-0.bpo.5-cloud-amd64 #1 SMP Debian 4.19.37-3~bpo9+1 (2019-05-18) x86_64 GNU/Linux
```


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