[Bug 1966037] [NEW] some applications hang for up to 30sec due to /run/systemd/resolve/io.systemd.Resolve

[Jason Haar]

Public bug reported:

I just installed Ubuntu-22.04 last week and have noticed lots of command
line apps seem to hang for 10-30sec before working. Eventually that got
annoying enough that I looked into it.

Anyway, apps like "telnet" (yep, I'm that old) and oddly enough sudo
showed this issue strongly. In the end I ran them with strace and could
see the pause occurred when  /run/systemd/resolve/io.systemd.Resolve was
called. So systemd-resolve was involved.

Then I cranked up "tcpdump -n -i any port 53" and re-ran the commands.
What I found was systemd-resolver was throwing DNS lookups for my
hostname at all the DNS servers I had configured (default on wifi plus
corporate always-on VPN link). Now the thing is our corporate VPN has
SIXTEEN domains in the search domain field... So as systemd-resolver was
doing one "A" plus one "AAAA" lookups for each "hostname.search-domain"
- well there's the hang. In fact 30sec is quite a good response time ;-)

To fix it I simply added my raw (ie non-dot) hostname to /etc/hosts -
completely blocked this self-hostname lookup that must be going on. Now
there are no DNS lookups at all for my hostname in this situation.

All is good. But it does make me wonder if perhaps Ubuntu shouldn't
always put the hostname into /etc/hosts? Even pointing at 127.0.0.99
(for example) might be enough (I haven't actually tested that - I'm
pointing it at my static VPN IP)

Anyway, this is a corner case for sure - but it might shave some delays
off a bunch of folks. This also might seem cosmetic, but I would guess
any application that calls gethostbyname or equivalent on itself is
triggering this.

PS: I had Ubuntu-20.04 before this and didn't have the problem. But
looking at a backup I can see I also had my hostname in the /etc/hosts
file - so this might have affected 20.04 too but I "accidentally" fixed
it early on without realizing it

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1966037

Title:
  some applications hang for up to 30sec due to
  /run/systemd/resolve/io.systemd.Resolve

Status in systemd package in Ubuntu:
  New

Bug description:
  I just installed Ubuntu-22.04 last week and have noticed lots of
  command line apps seem to hang for 10-30sec before working. Eventually
  that got annoying enough that I looked into it.

  Anyway, apps like "telnet" (yep, I'm that old) and oddly enough sudo
  showed this issue strongly. In the end I ran them with strace and
  could see the pause occurred when
  /run/systemd/resolve/io.systemd.Resolve was called. So systemd-resolve
  was involved.

  Then I cranked up "tcpdump -n -i any port 53" and re-ran the commands.
  What I found was systemd-resolver was throwing DNS lookups for my
  hostname at all the DNS servers I had configured (default on wifi plus
  corporate always-on VPN link). Now the thing is our corporate VPN has
  SIXTEEN domains in the search domain field... So as systemd-resolver
  was doing one "A" plus one "AAAA" lookups for each "hostname.search-
  domain" - well there's the hang. In fact 30sec is quite a good
  response time ;-)

  To fix it I simply added my raw (ie non-dot) hostname to /etc/hosts -
  completely blocked this self-hostname lookup that must be going on.
  Now there are no DNS lookups at all for my hostname in this situation.

  All is good. But it does make me wonder if perhaps Ubuntu shouldn't
  always put the hostname into /etc/hosts? Even pointing at 127.0.0.99
  (for example) might be enough (I haven't actually tested that - I'm
  pointing it at my static VPN IP)

  Anyway, this is a corner case for sure - but it might shave some
  delays off a bunch of folks. This also might seem cosmetic, but I
  would guess any application that calls gethostbyname or equivalent on
  itself is triggering this.

  PS: I had Ubuntu-20.04 before this and didn't have the problem. But
  looking at a backup I can see I also had my hostname in the /etc/hosts
  file - so this might have affected 20.04 too but I "accidentally"
  fixed it early on without realizing it

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