[Bug 1886128] Re: systemd-resolved does not resolve address due to udp payload size.

[Dan Streetman]

Unfortunately, this is a bug in upstream systemd.

For some reason, resolved's current upstream code clamps the 'best'
server protocol level at 512-byte-sized EDNS0 if DNSSEC is disabled.
Since the default is for DNSSEC to be disabled, this means by default,
resolved will restrict its udp edns0 packets sent to upstream
nameservers at 512 bytes, as you indicated in the bug description.

normally, this is ok because most nameservers support TCP, and resolved
always falls back to TCP when it finds a truncated response.  However,
your upstream nameserver seems to either not support TCP DNS, or its
port 53 is firewalled for TCP.

This will take some time to open an upstream bug and investigate
patching. If you open an upstream bug please add a comment with the
link.

** Changed in: systemd (Ubuntu)
     Assignee: (unassigned) => Dan Streetman (ddstreet)

** Changed in: systemd (Ubuntu)
   Importance: Undecided => Medium

** Changed in: systemd (Ubuntu)
       Status: Confirmed => In Progress

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https://bugs.launchpad.net/bugs/1886128

Title:
  systemd-resolved does not resolve address due to udp payload size.

Status in systemd package in Ubuntu:
  In Progress

Bug description:
  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.

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