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

[Dan Streetman]

> dns.pcap Edit (14.5 KiB, application/vnd.tcpdump.pcap)

that's tcpdump output, not actually a pcap; to get an actual pcap use
the tcpdump -w parameter to write the packets to a file.

> Could you please advise how to increase UDP payload size for the local
stub resolver?

resolved adjusts levels automatically.  looking at your dns packet
capture will be the easiest way to tell what the problem is.

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

Status in systemd package in Ubuntu:
  Incomplete

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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