save an SMTP session from telnet in a log with a shell script

Question

I am creating a script to test an email by HELO ..

Directly in telnet console, the commands run fine, but in a script, I can not get the output.

In bash:

$ telnet alt1.aspmx.l.google.com 25
HELO verify-email.org
MAIL FROM: <[email protected]>
RCPT TO: <[email protected]>
quit

results in:

root@san [~]# telnet alt1.aspmx.l.google.com 25
Trying 2a00:1450:4010:c09::1a...
Connected to alt1.aspmx.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP qm6si6508388lbb.110 - gsmtp
HELO verify-email.org
250 mx.google.com at your service
MAIL FROM: <[email protected]>
250 2.1.0 OK qm6si6508388lbb.110 - gsmtp
RCPT TO: <[email protected]>
550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. Learn more at
550 5.1.1  https://support.google.com/mail/answer/6596 qm6si6508388lbb.110 - gsmtp
quit
221 2.0.0 closing connection qm6si6508388lbb.110 - gsmtp
Connection closed by foreign host.

---

In a script:

cat << EOF | telnet alt1.aspmx.l.google.com 25
HELO verify-email.org
MAIL FROM: <[email protected]>
RCPT TO: <[email protected]>
quit
EOF

---

OR

{ echo "HELO verify-email.org"; sleep 1; echo "MAIL FROM: <[email protected]>"; sleep 1; echo "RCPT TO: <[email protected]>" ; sleep 1 ;  echo quit } | telnet alt1.aspmx.l.google.com 25

---

OR

#!/usr/bin/expect -f

spawn  alt1.aspmx.l.google.com 25
send '"HELO verify-email.org\r"'
send '"MAIL FROM: <[email protected]>\r"'
send '"RCPT TO: <[email protected]>\r"'
send '"quit\r"'

---

OR

sh aa 1> aa.txt 2>&1

OR

sh aa &> aa.txt

---

brings no results.

Answer 1

In general, you probably don't want to pipe things into telnet. A reasonable alternative is netcat (which is nc on most systems).

That said, I wrote a tiny bash HTTP client a while back, relying on bash's internal support for making TCP socket connections. An SMTP client is a little more complex, but still fairly easy. SMTP is nice. You can load up a bunch of commands, then just read multiple response lines all at once.

#!/usr/bin/env bash

target="$1"
address="$2"

success=""

# Open a two-way socket connection on fd/3
exec 3<>/dev/tcp/$target/25

# Stand back, we're doing SMTP!
printf "HELO $HOSTNAME\r\n" >&3
printf "MAIL FROM: <$USER@$HOSTNAME>\r\n" >&3
printf "RCPT TO: <$address>\r\n" >&3
printf "QUIT\r\n" >&3

# Now that we've had our say, it's time to listen.
while read -u 3 code message; do
  echo ">> <$code> ${message%$'\r'}"    # Debugging output
  case "$code" in
    2??) success="${success:-true}" ;;  # Only set variable if it's not already set
    5??) success=false ;;               # (i.e. false overrides all other responses)
  esac
done

# Close connections, clean up after ourselves
exec 3<&-; exec 3>&-

if [ -z "$success" ]; then
        echo "NOTICE: Not sure we reached an SMTP server..."
        exit 1
elif $success; then
        echo "NOTICE: All's well, $target accepts mail for $address"
else
        echo "NOTICE: I detected a failure."
        exit 1
fi

Note the Parameter Expansion of ${message%$'\r'}, which strips the last character from the line if it is a CR. This is done because SMTP responses use \r\n as newlines, whereas your script probably considers the \r as merely part of the line (or the $message variable).