[Bug 1961697] Re: Transaction ID collisions cause slow DNS lookups in getaddrinfo

[Launchpad Bug Tracker]

Status changed to 'Confirmed' because the bug affects multiple users.

** Changed in: glibc (Ubuntu)
       Status: New => Confirmed

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https://bugs.launchpad.net/bugs/1961697

Title:
  Transaction ID collisions cause slow DNS lookups in getaddrinfo

Status in glibc package in Ubuntu:
  Confirmed

Bug description:
  When resolving DNS names with getaddrinfo(), I have seen this hang for
  5 seconds and then retry and succeed. The issue is that glibc will
  issue a both an A and AAAA query on the same socket, and in some
  circumstances they can be sent with the same DNS transaction ID as
  well.

  I verified this with a packet capture; in the packet capture, I saw
  the A and AAAA queries for a name be made with the same DNS
  transaction ID, get responses, do nothing for five seconds, and then
  send the same DNS query again. On the glibc side, I confirmed that
  it's blocked waiting for the DNS response by interrupting it with gdb,
  even though the packet capture shows the response has well and truly
  arrived. I've attached a packet capture & a backtrace of the glibc
  hang.

  I believe this is the same issue reported in these places:
      * In RHEL: https://bugzilla.redhat.com/show_bug.cgi?id=1904153
      * Also RHEL: https://bugzilla.redhat.com/show_bug.cgi?id=1903880
      * Upstream: https://sourceware.org/bugzilla/show_bug.cgi?id=26600

  The environment I noticed this bug in was:
      * Docker for Mac on an arm64 m1 Macbook
      * Docker for Mac Linux kernel version is 5.10.76-linuxkit
      * Linux is also arm64, not emulated
      * Container with the buggy DNS environment is Ubuntu bionic (also arm64, not emulated)
      * Glibc 2.27-3ubuntu1.4

  However one of the redhat reporters noticed this issue in m6 series
  EC2 instances in AWS.

  A patch has been provided upstream for this issue:
  https://sourceware.org/pipermail/libc-alpha/2020-September/117547.html

  I applied the upstream patch to glibc 2.27-3ubuntu1.4 and rebuilt the
  package, and the problem went away. I've attached the exact patch I
  applied, since I had to work through some conflicts.

  So, I think that patch just needs to be backported to Bionic and (I
  think) Focal as well. Is that reasonable?

  Thanks!

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