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@peterc
Created March 13, 2023 12:53
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Email list MX record checker
# Read stdin, extract email addresses
# Look up MX records for those addresses
# Return each email address and their first MX record
# (or IP address, as a fallback)
hosts = {}
STDIN.each_line do |l|
# Extract the email and the email domain from each line
email = l[/^([^\s]+\@[^\s+\,]+)\b/, 1]
host = email[/\@(.+)$/, 1]
# If already cached, use what was cached
if hosts[host]
dns = hosts[host]
else
# Do an MX lookup
mx = `dig MX +short #{host}`
dns = if mx =~ /[a-z]+/i
mx.downcase
else
# Technically mail can still work if the host
# has an A record
`dig A +short #{host}` =~ /[a-z]+/i ? a : 'FAIL'
end
# Just get the first entry
dns = dns.split("\n")[0]
hosts[host] = dns
end
puts "#{email},#{dns}"
end
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peterc commented Mar 13, 2023

I only built this for my own use and rate limiting on DNS could be an issue depending on your setup. However, the memoization helps a lot, and you can add @ entries to the dig calls if you need to route the DNS elsewhere. Google's 8.8.8.8 is usually good for hundreds of QPS.

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