Online IT checkers and info

DNS settings:

https://mxtoolbox.com
https://dnscheck.pingdom.com
https://dnschecker.org           <—      DMARC, SPF

Mail server check:

https://www.mail-tester.com/
http://www.uceprotect.net/en/rblcheck.php

DNS Server + its settings

https://www.intodns.com

IP Lookup:

http://www.whois-service.ru/lookup/
https://whatismyipaddress.com/ip-lookup
https://ipinfo.io/           <—      also with ASN lookup

Domains: info, subdomains, IPs, history:

https://securitytrails.com/

Current IP:

https://myip.com
https://myip.ru
https://ifconfig.me/
https://checkip.dyndns.org/

Crontab:

https://crontab.guru/every-1-minute
Check crontab execution: grep CRON /var/log/syslog

MAC address Vendor Lookup:

https://www.macvendorlookup.com/
https://macvendors.com/