Re: systemd-resolve SERVFAIL on lookups found by upstream DNS server

[Mantas Mikulėnas <grawity@xxxxxxxxx>]

On Fri, Apr 16, 2021 at 7:41 PM Francesco Belladonna <francesco@xxxxxx> wrote:

Greetings,
I’ve been trying to debug why systemd-resolve is not able to perform nslookup static-exp1.licdn.com.
Altering /etc/resolv.conf to point directly to the DNS server (or my router in this case) solves the problem, which seems to suggest the problem is isolated to systemd-resolve.
The problem is identical on both my laptops which are running 2 different O.S. (Kubuntu 18.04 and Fedora 33).
The entire DNS configuration is provided by the router acting as DHCP server.

The system I’m performing my tests is Kubuntu, where the systemd version is:

systemd --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

SYSTEMD_LOG_LEVEL is set to debug.
Is there any other useful tool I can use to debug this further?
The problematic domain is static-exp1.licdn.com which is the CDN for LinkedIn. I have no idea why this specific domain is affected.

Most likely because it has one of those interesting CNAME chains:

$ rdt static-exp1.licdn.com
static-exp1.licdn.com = 2-01-2c3e-003d.cdx.cedexis.net
   2-01-2c3e-003d.cdx.cedexis.net = li-prod-static.azureedge.net
      li-prod-static.azureedge.net = li-prod-static.afd.azureedge.net
         li-prod-static.afd.azureedge.net = star-azureedge-prod.trafficmanager.net
            star-azureedge-prod.trafficmanager.net = dual.t-0009.t-msedge.net
               dual.t-0009.t-msedge.net = t-0009.t-msedge.net
                  t-0009.t-msedge.net = Edge-Prod-LON21r3.ctrl.t-0009.t-msedge.net
                     edge-prod-lon21r3.ctrl.t-0009.t-msedge.net = standard.t-0009.t-msedge.net
                        standard.t-0009.t-msedge.net = 13.107.213.19, 13.107.246.19, 2620:1ec:46::19, 2620:1ec:bdf::19

(Though sometimes it's shorter, pointing at epsiloncdn instead of Azure. It depends on where you're making the query from.)

I think this was fixed in git a few weeks ago. There's already an Ubuntu bug report for the same issue: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1921636

--

Mantas Mikulėnas

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