I have trouble with getting mailcow to work due to dns lookup issues. Anti virus and anti spam services are not able to retreive their signature updates.

I did an update.sh of mailcow so there i am on the latest version.

jaco@ /opt/mailcow-dockerized $ docker --version
Docker version 19.03.12, build 48a66213fe
jaco@ /opt/mailcow-dockerized $ docker-compose --version
docker-compose version 1.26.1, build f216ddbf

from the mailcowdockerized_clamd-mailcow_1 container logs

Wed Jul  1 12:40:45 2020 -> Reading CVD header (daily.cvd): Wed Jul  1 12:40:55 2020 -> !remote_cvdhead: Download failed (6) Wed Jul  1 12:40:55 2020 -> ! Message: Couldn't resolve host name
Wed Jul  1 12:40:55 2020 -> ^Failed to get daily database version information from server: https://database.clamav.net
Wed Jul  1 12:40:55 2020 -> !check_for_new_database_version: Failed to find daily database using server https://database.clamav.net.
Wed Jul  1 12:40:55 2020 -> Giving up on https://database.clamav.net...
Wed Jul  1 12:40:55 2020 -> !Update failed for database: daily
Wed Jul  1 12:40:55 2020 -> ^fc_update_databases: fc_update_database failed: HTTP GET failed (11)
Wed Jul  1 12:40:55 2020 -> !Database update process failed: HTTP GET failed (11)
Wed Jul  1 12:40:55 2020 -> !Update failed.

and in the cli

root@95f895ad7475:/# nslookup database.clamav.net
Server:         127.0.0.11
Address:        127.0.0.11#53

** server can't find database.clamav.net: REFUSED

root@95f895ad7475:/# nslookup unbound
Server:         127.0.0.11
Address:        127.0.0.11#53

Non-authoritative answer:
Name:   unbound
Address: 172.11.1.254
Name:   unbound
Address: fd4d:6169:6c63:6f77::2

root@95f895ad7475:/# cat /etc/resolv.conf 
nameserver 127.0.0.11
options ndots:0
root@95f895ad7475:/#

root@95f895ad7475:/# nslookup database.clamav.net 172.11.1.254
Server:         172.11.1.254
Address:        172.11.1.254#53

** server can't find database.clamav.net: REFUSED

How comes the dns server is not set to unbound in that container ?

From the cli of mailcowdockerized_unbound-mailcow_1

bash-5.0# nslookup database.clamav.net 127.0.0.1
Server:         127.0.0.1
Address:        127.0.0.1:53

Non-authoritative answer:
database.clamav.net     canonical name = database.clamav.net.cdn.cloudflare.net
Name:   database.clamav.net.cdn.cloudflare.net
Address: 104.16.218.84
Name:   database.clamav.net.cdn.cloudflare.net
Address: 104.16.219.84

Non-authoritative answer:
database.clamav.net     canonical name = database.clamav.net.cdn.cloudflare.net
Name:   database.clamav.net.cdn.cloudflare.net
Address: 2606:4700::6810:da54
Name:   database.clamav.net.cdn.cloudflare.net
Address: 2606:4700::6810:db54

bash-5.0# nslookup database.clamav.net 172.11.1.254
Server:         172.11.1.254
Address:        172.11.1.254:53

** server can't find database.clamav.net: REFUSED

** server can't find database.clamav.net: REFUSED

bash-5.0# netstat -nlp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp        0      0 0.0.0.0:53              0.0.0.0:*               LISTEN      -
tcp        0      0 127.0.0.11:33941        0.0.0.0:*               LISTEN      -
tcp        0      0 127.0.0.1:8953          0.0.0.0:*               LISTEN      -
tcp        0      0 :::53                   :::*                    LISTEN      -
udp        0      0 0.0.0.0:53              0.0.0.0:*                           -
udp        0      0 127.0.0.11:36201        0.0.0.0:*                           -
udp        0      0 :::53                   :::*                                -
Active UNIX domain sockets (only servers)
Proto RefCnt Flags       Type       State         I-Node PID/Program name    Path

the corresponding part from docker-compose.yml

    clamd-mailcow:
      image: mailcow/clamd:1.36
      restart: always
      dns:
        - ${IPV4_NETWORK:-172.22.1}.254
      environment:
        - TZ=${TZ}
        - SKIP_CLAMD=${SKIP_CLAMD:-n}
      volumes:
        - ./data/conf/clamav/:/etc/clamav/
      networks:
        mailcow-network:
          aliases:
            - clamd