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Verify reverse DNS of a mail server IP (FCrDNS)

harmlessDNSdebian, ubuntu
Adjust the values, the command updates live
user@server
ptr=$(dig +short -x 203.0.113.10 | head -n 1); echo "PTR: $ptr"; dig +short "$ptr" A

Looks up the PTR hostname of the IP and then queries its A record. Only if the original IP appears there is the reverse DNS entry considered confirmed, which many mail servers require. Ideally the hostname also matches the HELO name of the mail server.

Note: For IPv6 addresses, query AAAA instead of A at the end.

Also searched as

  • check fcrdns
  • reverse dns does not match hostname
  • emails rejected because of ptr

Related one-liners

All in DNS

Look up the reverse DNS (PTR) of an IP address

$dig +short -x 203.0.113.10
harmlessDNS

Check a domain's DKIM key in DNS

$dig +short default._domainkey.example.com TXT
harmlessDNS

Look up the MX records of a domain

$dig +short example.com MX
harmlessDNS

Show the DMARC policy of a domain

$dig +short _dmarc.example.com TXT
harmlessDNS

Show the SPF record of a domain

$dig +short example.com TXT | grep -i "v=spf1"
harmlessDNS

Check DNS propagation across public resolvers

$for ns in 1.1.1.1 8.8.8.8 9.9.9.9 208.67.222.222; do echo "$ns: $(dig +short @$ns example.com A | tr '\n' ' ')"; done
harmlessDNS

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