[sr-dev] [kamailio/kamailio] dispatcher use only one server from SRV results (#2376)

[sergey-safarov]

### Description

I have configured dispatcher with set:
```
id 1 setid 1 destination "sip:trial.nga911.com" flags 16 priority 0 attrs "" description "trial esrp"
```
Record above from `redis` database

Then kamailio report dispatcher dump
```sh
[root at ip-172-22-6-233 config]# kamctl  dispatcher dump
which: no gdb in (/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin:/)
{
  "jsonrpc":  "2.0",
  "result": {
    "NRSETS": 1,
    "RECORDS":  [{
        "SET":  {
          "ID": 1,
          "TARGETS":  [{
              "DEST": {
                "URI":  "sip:trial.nga911.com",
                "FLAGS":  "AP",
                "PRIORITY": 0
              }
            }]
        }
      }]
  },
  "id": 16982
}
```
Using tcpdump I can see kamailio resolve `A/AAAA` records only for one SRV record.
```sh
[root at ip-172-22-6-233 config]# tcpdump -ni any port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
11:04:23.812111 IP 172.22.6.233.53464 > 172.22.0.2.domain: 30262+ PTR? 233.6.22.172.in-addr.arpa. (43)
11:04:23.812923 IP 172.22.0.2.domain > 172.22.6.233.53464: 30262 1/0/0 PTR ip-172-22-6-233.us-west-1.compute.internal. (99)
11:04:23.813168 IP 172.22.6.233.48244 > 172.22.0.2.domain: 12029+ PTR? 233.6.22.172.in-addr.arpa. (43)
11:04:23.813248 IP 172.22.0.2.domain > 172.22.6.233.48244: 12029 1/0/0 PTR ip-172-22-6-233.us-west-1.compute.internal. (99)
11:04:23.855004 IP 172.22.6.233.45589 > 172.22.0.2.domain: 19167+ A? trial.nga911.com. (34)
11:04:23.856309 IP 172.22.0.2.domain > 172.22.6.233.45589: 19167 0/1/0 (118)
11:04:33.942992 IP 172.22.6.233.56344 > 172.22.0.2.domain: 24272+ SRV? _sip._udp.trial.nga911.com. (44)
11:04:33.944340 IP 172.22.0.2.domain > 172.22.6.233.56344: 24272 2/0/0 SRV proxy-us-west-2-trial-1.nga911.com.:5060 30 30, SRV proxy-us-west-2-trial-0.nga911.com.:5060 30 30 (152)
11:04:33.944580 IP 172.22.6.233.57863 > 172.22.0.2.domain: 27694+ A? proxy-us-west-2-trial-1.nga911.com. (52)
11:04:33.945786 IP 172.22.0.2.domain > 172.22.6.233.57863: 27694 1/0/0 A 172.21.1.32 (68)
```
If host from `A/AAAA` response if failed, then Kamailio do not use other hosts from SRV records.

### Troubleshooting

#### Reproduction

1) configured dispatcher set with `16` flag;
2) SIP URI needs a point to SRV records with two or more backend hosts;
3) start kamailio and check with kamailio use backend;
4) stop used backend;
5) make a call.

#### Debugging Data
none

#### Log Messages
none

#### SIP Traffic
none

### Possible Solutions

not known

### Additional Information

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

```
[root at ip-172-22-6-233 config]# kamailio -v
version: kamailio 5.3.4 (x86_64/linux) 117ff9
flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, 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, TLS_PTHREAD_MUTEX_SHARED
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: 117ff9 
compiled on 15:31:51 May  6 2020 with gcc 8.3.1
```

* **Operating System**:
```
[root at ip-172-22-6-233 config]# cat /etc/os-release 
NAME="CentOS Linux"
VERSION="8 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="8"
PLATFORM_ID="platform:el8"
PRETTY_NAME="CentOS Linux 8 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:8"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-8"
CENTOS_MANTISBT_PROJECT_VERSION="8"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="8"
```


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