Reverse lookup of 127.0.0.1 does not work

[Nick Anderson]

I expected a reverse lookup of 127.0.0.1 to return localhost, instead it returned nothing.

```
root:~# dig -x 127.0.0.1

; <<>> DiG 9.16.1-Ubuntu <<>> -x 127.0.0.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 60017
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;1.0.0.127.in-addr.arpa. IN PTR

;; Query time: 0 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Tue Aug 03 09:25:20 CDT 2021
;; MSG SIZE rcvd: 51

```

If I explicitly lookup using 127.0.0.1 instead of the default 127.0.0.53, I do get a record as expected.

```
root:~# dig -x 127.0.0.1 @127.0.0.1

; <<>> DiG 9.16.1-Ubuntu <<>> -x 127.0.0.1 @127.0.0.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51602
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;1.0.0.127.in-addr.arpa. IN PTR

;; ANSWER SECTION:
1.0.0.127.in-addr.arpa. 10800 IN PTR localhost.

;; Query time: 3 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Aug 03 09:25:15 CDT 2021
;; MSG SIZE rcvd: 74

```

I have an entry in /etc/hosts.

```
root:~# grep 127.0.0.1 /etc/hosts
127.0.0.1 localhost
```

Looks like unbound which seems to work as expected is running on 127.0.0.1:53, and systemd-resolv which is not working is on 127.0.0.53:53.

```
tcp 0 0 127.0.0.1:53 0.0.0.0:* LISTEN 1839/unbound
tcp 0 0 127.0.0.53:53 0.0.0.0:* LISTEN 1511/systemd-resolv

```

I am running 20.04.2:

```
root:~# lsb_release -rd
Description: Ubuntu 20.04.2 LTS
Release: 20.04
```

With systemd version 245.4-4ubuntu3.7

```
root@:~# apt-cache policy systemd
systemd:
  Installed: 245.4-4ubuntu3.7
  Candidate: 245.4-4ubuntu3.11
  Version table:
     245.4-4ubuntu3.11 500
        500 http://us.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages
 *** 245.4-4ubuntu3.7 100
        100 /var/lib/dpkg/status
     245.4-4ubuntu3 500
        500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages
     237-3ubuntu10.50 500
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages

```

[Nick Rosbrook (enr0n)]

This appears to be fixed in newer releases.

[Nick Anderson (nick-anders0n)]

Indeed, I do not see this behavior any longer.

```
$ dig -x 127.0.0.1

; <<>> DiG 9.18.12-0ubuntu0.22.04.2-Ubuntu <<>> -x 127.0.0.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12678
;; flags: qr aa rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;1.0.0.127.in-addr.arpa. IN PTR

;; ANSWER SECTION:
1.0.0.127.in-addr.arpa. 0 IN PTR localhost.
1.0.0.127.in-addr.arpa. 0 IN PTR kubernetes.docker.internal.

;; Query time: 3 msec
;; SERVER: 127.0.0.53#53(127.0.0.53) (UDP)
;; WHEN: Fri Jun 23 11:29:26 CDT 2023
;; MSG SIZE rcvd: 114

$ lsb_release -rd
Description: Ubuntu 22.04.2 LTS
Release: 22.04
```