Public bug reported:
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!
** Affects: glibc (Ubuntu)
Importance: Undecided
Status: New
** Patch added: "upstream patch with conflicts resolved for 2.27"
https://bugs.launchpad.net/bugs/1961697/+attachment/5562611/+files/resolv-txnid-collision.patch
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https://bugs.launchpad.net/bugs/1961697
Title:
Transaction ID collisions cause slow DNS lookups in getaddrinfo
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