I need to get an SSL certificate from some mail servers. I know I need to execute the following steps. Do a DNS mx query on the domain name (dig google.com mx).Use the information to download an SSL certificate (openssl s_client -showcerts -connect google.com:443). But I am not sure what information I should pick up from step 1 and how it should be used in step 2
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Log in to answerBest Answer · By JanBask Cyber Security Expert
Answered on Mar 16, 2022
For extracting certificate from TLS server, you should pick the domain of one MX record from the answer section of your DNS query and feed it to openssl:
$ dig gmail.com mx
[...]
;; ANSWER SECTION:[...]
$ openssl s_client -connect gmail-smtp-in.l.google.com:25 -starttls smtp
[...]Note: it's not a safe assumption that the web server (if there is any) running on the same machine as the SMTP server uses the same certificate. see also https://serverfault.com/questions/131627/how-to-inspect-remote-smtp-servers-tls-certificate#131628
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