Comment 0 for bug 1886128

Description: Ubuntu 18.04.4 LTS
Release: 18.04

systemd-resolve --version

systemd 237
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP
+GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN
-PCRE2 default-hierarchy=hybrid

We met an error: on an attempt to resolve address, the following issue appears:

; <<>> DiG 9.11.3-1ubuntu1.11-Ubuntu <<>> mharder-formrec.cognitiveservices.azure.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 44096
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;mharder-formrec.cognitiveservices.azure.com. IN A

;; Query time: 231 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Tue Apr 28 20:47:14 UTC 2020
;; MSG SIZE rcvd: 72

Let me provide you important notes about the issue:
1) It's not reproducing on Ubuntu 16;
2) Bypassing systemd-resolve - everything works fine;
3) Only the difference between systemd-resolve and END is UDP_PAYLOAD_SIZE

Successful query:

1135 16:27:25.964386 10.1.0.4 168.63.129.16 DNS 128 Standard query 0xc2d4 A mharder-formrec.cognitiveservices.azure.com OPT

Domain Name System (query)
    Transaction ID: 0xc2d4
    Flags: 0x0120 Standard query
        0... .... .... .... = Response: Message is a query
        .000 0... .... .... = Opcode: Standard query (0)
        .... ..0. .... .... = Truncated: Message is not truncated
        .... ...1 .... .... = Recursion desired: Do query recursively
        .... .... .0.. .... = Z: reserved (0)
        .... .... ..1. .... = AD bit: Set
        .... .... ...0 .... = Non-authenticated data: Unacceptable
    Questions: 1
    Answer RRs: 0
    Authority RRs: 0
    Additional RRs: 1
    Queries
        mharder-formrec.cognitiveservices.azure.com: type A, class IN
    Additional records
        <Root>: type OPT
            Name: <Root>
            Type: OPT (41)
            UDP payload size: 4096
            Higher bits in extended RCODE: 0x00
            EDNS0 version: 0
            Z: 0x0000
                0... .... .... .... = DO bit: Cannot handle DNSSEC security RRs
                .000 0000 0000 0000 = Reserved: 0x0000
            Data length: 12
            Option: COOKIE
Unsuccessful query:

1128 16:27:25.713886 10.1.0.4 168.63.129.16 DNS 116 Standard query 0x198d A mharder-formrec.cognitiveservices.azure.com OPT

Domain Name System (query)
    Transaction ID: 0x198d
    Flags: 0x0100 Standard query
        0... .... .... .... = Response: Message is a query
        .000 0... .... .... = Opcode: Standard query (0)
        .... ..0. .... .... = Truncated: Message is not truncated
        .... ...1 .... .... = Recursion desired: Do query recursively
        .... .... .0.. .... = Z: reserved (0)
        .... .... ...0 .... = Non-authenticated data: Unacceptable
    Questions: 1
    Answer RRs: 0
    Authority RRs: 0
    Additional RRs: 1
    Queries
        mharder-formrec.cognitiveservices.azure.com: type A, class IN
    Additional records
        <Root>: type OPT
            Name: <Root>
            Type: OPT (41)
            UDP payload size: 512
            Higher bits in extended RCODE: 0x00
            EDNS0 version: 0
            Z: 0x0000
                0... .... .... .... = DO bit: Cannot handle DNSSEC security RRs
                .000 0000 0000 0000 = Reserved: 0x0000
            Data length: 0
Notable difference:

Success:
            UDP payload size: 4096

Failure:
            UDP payload size: 512
And notable differences in the responses:

Success:
    Flags: 0x8180 Standard query response, No error
        .... ..0. .... .... = Truncated: Message is not truncated

Failure:
    Flags: 0x8380 Standard query response, No error
        .... ..1. .... .... = Truncated: Message is truncated

Interestingly, systemd-resolved is setting the maximum payload size to 512 regardless of whether EDNS0 is configured and regardless of what is sent to it for the payload size.
I tried to found a way to change UDP_PAYLOAD_SIZE,but it seems it is only possible to change it only with direct code modifications.