Someone in this thread mentioned using "socat - tcp:host:port" as a replacement for "telnet host port" for manually testing plaintext services.
I just tried this, and learned that the two are not equivalent. qpsmtpd is quite insistent that all of its input lines be terminated by CR-LF, not by LF. =============================================================== hobbit:~$ socat - tcp:localhost:25 220 greg.wooledge.org ESMTP qpsmtpd 1.00 ready; send us your mail, but not your spam. HELO WORLD 250 greg.wooledge.org Hi localhost [127.0.0.1]; I am so happy to meet you. MAIL FROM:<[email protected]> 250 <[email protected]>, sender OK - how exciting to get mail from you! RCPT TO:<[email protected]> 250 <[email protected]>, recipient ok DATA 354 go ahead From: Greg To: Greg Subject: hi hi . 421 See http://smtpd.develooper.com/barelf.html =============================================================== telnet does not have that problem. So, I searched through the manual a bit more, and it turns out there's an option for that: =============================================================== hobbit:~$ socat - tcp:localhost:25,crnl 220 greg.wooledge.org ESMTP qpsmtpd 1.00 ready; send us your mail, but not your spam. HELO WORLD 250 greg.wooledge.org Hi localhost [127.0.0.1]; I am so happy to meet you. MAIL FROM:<[email protected]> 250 <[email protected]>, sender OK - how exciting to get mail from you! RCPT TO:<[email protected]> 250 <[email protected]>, recipient ok DATA 354 go ahead From: Greg To: Greg Subject: hi Date: Wed, 08 Jul 2026 13:47:36 -0400 hi . 250 Queued! 1783534744 qp 143613 <> =============================================================== Anyone looking to replace telnet with socat for this purpose will probably need to use that option, on some or all services.
