[systemd-devel] Some base questions around systemd-resolved

[struth]

Hello systemd-devel group.
I have just started using systemd-resolved to try and achieve a goal that I
will try to explain.
I know very little about it (web searches so far) so please excuse any
silly questions or trains of thought.
I have a Debian Bullseye client in a Microsoft network that uses a .local
domain.
I know that this is a bad policy, but there is nothing I can do about it. I
have no choice or authority in this matter. This is how they have
configured their whole environment.
I have read here  [ https://github.com/systemd/systemd/issues/8852 ] that
.local can be used.

At times there is complete isolation from the 4 Domain DNS servers and I
want my client machine to still be able to resolve DNS entries
(specifically SRV records with included A records) during this outage.

I thought that systemd-resolved could cache the DNS entries and retain them
until any of the DNS Servers returned to service.
This only seems to happen for a short time after the outage. After some
time ( I don't know how to tell how long) the entries seem to drop from
cache.
I would ideally like the entries to stay in cache until updated from DNS
Server again (once one returns to service).

On the SRV point: How can I be sure that it caches the full result of the
SRV query?
Eg: SRV gives 2 x A-records which then need to resolve to 2xIP-addresses.

I'm not sure of the mailing lists policy for including config samples of
logs, so I will include it here in email and see what happens.
Please excuse if this is too much or too little information.

root at VATCPCOMMLC1:~# cat /etc/systemd/resolved.conf
[Resolve]
DNS= 10.24.1.135 10.24.129.135 10.24.1.136 10.24.129.136
#FallbackDNS=
Domains=itsvic.local
#DNSSEC=no
#DNSOverTLS=no
#MulticastDNS=yes
#LLMNR=yes
#Cache=yes
DNSStubListener=yes
#DNSStubListenerExtra=
#ReadEtcHosts=yes
#ResolveUnicastSingleLabel=no
root at VATCPCOMMLC1:~#

root at VATCPCOMMLC1:~# ls -l /etc/resolv.conf
lrwxrwxrwx 1 root root 39 Jul 30 14:11 /etc/resolv.conf ->
../run/systemd/resolve/stub-resolv.conf
root at VATCPCOMMLC1:~#
root at VATCPCOMMLC1:~# cat ../run/systemd/resolve/stub-resolv.conf
nameserver 127.0.0.53
options edns0 trust-ad
search itsvic.local
root at VATCPCOMMLC1:~#

root at VATCPCOMMLC1:~# cat /etc/nsswitch.conf
# /etc/nsswitch.conf
passwd:         files systemd
group:          files systemd
shadow:         files
gshadow:        files
hosts:          files dns
networks:       files
protocols:      db files
services:       db files
ethers:         db files
rpc:            db files
netgroup:       nis
root at VATCPCOMMLC1:~#
root at VATCPCOMMLC1:~# resolvectl statusresolvectl status
Global
         Protocols: +LLMNR +mDNS -DNSOverTLS DNSSEC=no/unsupported
  resolv.conf mode: stub
Current DNS Server: 10.24.1.135
       DNS Servers: 10.24.1.135 10.24.129.135 10.24.1.136 10.24.129.136
        DNS Domain: itsvic.local
Link 2 (ens192)
Current Scopes: none
     Protocols: -DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported

Link 3 (ens224)
Current Scopes: none
     Protocols: -DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported

Link 4 (bond0)
Current Scopes: LLMNR/IPv4 LLMNR/IPv6
     Protocols: -DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported

root at VATCPCOMMLC1:~# dig srv _sip._tcp.osvsig-mets-prod.voip.itsvic.local
dig srv _sip._tcp.osvsig-mets-prod.voip.itsvic.local
; <<>> DiG 9.16.48-Debian <<>> srv
_sip._tcp.osvsig-mets-prod.voip.itsvic.local
;; global options: +cmd
;; Got answer:
;; WARNING: .local is reserved for Multicast DNS
;; You are currently testing what happens when an mDNS query is leaked to
DNS
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57884
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;_sip._tcp.osvsig-mets-prod.voip.itsvic.local. IN SRV

;; ANSWER SECTION:
_sip._tcp.osvsig-mets-prod.voip.itsvic.local. 3600 IN SRV 20 0 5060
osvn2-mets-prod.voip.itsvic.local.
_sip._tcp.osvsig-mets-prod.voip.itsvic.local. 3600 IN SRV 10 0 5060
osvn1-mets-prod.voip.itsvic.local.

;; Query time: 0 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Tue Jul 30 15:38:47 AEST 2024
;; MSG SIZE  rcvd: 179

Thanks for any help.
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