pdns-recursor: make the record cache forward-aware

[Luis Filipe Domingues]

We are using ubuntu 16.04 as our dns recursoron production. We installed the pdns-recursor package from ubuntu repositories, pdns-recursor (4.0.0~alpha2-2ubuntu0.1) and we faced an issue that is really similar to this one: https://github.com/PowerDNS/pdns/pull/4377.

Here it is the version installed:
```
# pdns_recursor --version
Aug 30 06:39:50 PowerDNS Recursor 4.0.0-alpha2 (C) 2001-2016 PowerDNS.COM BV
Aug 30 06:39:50 Using 64-bits mode. Built using gcc 5.4.0 20160609.
Aug 30 06:39:50 PowerDNS comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it according to the terms of the GPL version 2.
Aug 30 06:39:50 Features: openssl lua
Aug 30 06:39:50 Configured with: " '--build=x86_64-linux-gnu' '--prefix=/usr' '--includedir=${prefix}/include' '--mandir=${prefix}/share/man' '--infodir=${prefix}/share/info' '--sysconfdir=/etc' '--localstatedir=/var' '--disable-silent-rules' '--libdir=${prefix}/lib/x86_64-linux-gnu' '--libexecdir=${prefix}/lib/x86_64-linux-gnu' '--disable-maintainer-mode' '--disable-dependency-tracking' '--sysconfdir=/etc/powerdns' '--enable-reproducible' '--with-lua' 'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fPIE -fstack-protector-strong -Wformat -Werror=format-security' 'LDFLAGS=-Wl,-Bsymbolic-functions -fPIE -pie -Wl,-z,relro -Wl,-z,now -latomic' 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2' 'CXXFLAGS=-g -O2 -fPIE -fstack-protector-strong -Wformat -Werror=format-security -DPACKAGEVERSION='\''"4.0.0~alpha2-2ubuntu0.1.Ubuntu"'\'''"
```

And here is the output of a dig request:

```
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 16855
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;<Some server internaly>. IN A

;; Query time: 654 msec
;; SERVER: 10.176.228.2#53(10.176.228.2)
;; WHEN: Fri Aug 30 08:13:13 CEST 2019
;; MSG SIZE rcvd: 68
```

Without changing anything to the configuration, we installed the upstream 4.0.X series version.

Here you find the output of the version number, and the same dig request as above:

```# pdns_recursor --version
Aug 30 07:03:37 PowerDNS Recursor 4.0.9 (C) 2001-2017 PowerDNS.COM BV
Aug 30 07:03:37 Using 64-bits mode. Built using gcc 5.4.0 20160609 on Nov 6 2018 16:30:52 by root@505b66e59573.
Aug 30 07:03:37 PowerDNS comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it according to the terms of the GPL version 2.
Aug 30 07:03:37 Features: openssl lua
Aug 30 07:03:37 Configured with: " '--host=' '--build=' '--prefix=/usr' '--sysconfdir=/etc/powerdns' '--mandir=${prefix}/share/man' '--infodir=${prefix}/share/info' '--libdir=${prefix}/lib/' '--libexecdir=${prefix}/lib' '--with-lua' '--with-protobuf=yes' '--enable-systemd' '--with-systemd=/lib/systemd/system' 'build_alias=' 'host_alias=' 'CFLAGS=-g -O2 -fPIE -fstack-protector-strong -Wformat -Werror=format-security' 'LDFLAGS=-Wl,-Bsymbolic-functions -fPIE -pie -Wl,-z,relro -Wl,-z,now' 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2' 'CXXFLAGS=-g -O2 -fPIE -fstack-protector-strong -Wformat -Werror=format-security'"

```

```
; <<>> DiG 9.10.3-P4-Ubuntu <<>> @10.176.228.2 storage-dc1r03n12.fuel.eu-zrh.hub.k.grp
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50592
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;<Some server internaly>. IN A

;; ANSWER SECTION:
<Some server internaly>. 0 IN A 10.X.X.X

;; Query time: 164 msec
;; SERVER: 10.176.228.2#53(10.176.228.2)
;; WHEN: Fri Aug 30 08:53:48 CEST 2019
;; MSG SIZE rcvd: 84
```

Please backport bugfix for pdns_recursor, or update the distributed package to the last 4.0.X version.

Version of ubuntu:
# cat /etc/os-release
NAME="Ubuntu"
VERSION="16.04.5 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.5 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial

Regards,
Luis Domingues