Description
I encountered an error whenever an email is sent to NTFY without a 'Content-Type' header. This error was found in docker image v1.23.0. Error was discovered while setting up email notifications from a pfsense appliance --> ntfy.
I believe that I have found the root cause of the problem, and I have proposed a solution below. I wanted to start off submitting an issue since I don't know go, but if nobody has time to address it, I can see if I can get a dev env up and running to submit a PR with a fix. Just let me know :)
Steps to reproduce:
Create test email file without 'Content-Type' header:
EHLO other.example.com
MAIL FROM: user@other.example.com
RCPT TO: topic@ntfy.example.com
DATA
Subject: Test email
Test message for NTFY
.
QUIT
Send to server with NC:
nc ntfy.example.com 22 < temp.txt
Server response:
220 ntfy.example.com ESMTP Service Ready
250-Hello other.example.com
250-PIPELINING
250-8BITMIME
250-ENHANCEDSTATUSCODES
250-CHUNKING
250-AUTH PLAIN
250 SIZE 1048576
250 2.0.0 Roger, accepting mail from <user@other.example.com>
250 2.0.0 I'll make sure <topic@ntfy.example.com> gets this
354 2.0.0 Go ahead. End your data with <CR><LF>.<CR><LF>
554 5.0.0 Error: transaction failed, blame it on the weather: mime: no media type
221 2.0.0 Bye
Diagnosis
In ntfy/server/smtp_server.go::readMailBody, the problem is with the line:
contentType, params, err := mime.ParseMediaType(msg.Header.Get("Content-Type"))
If 'Content-Type' header isn't set on the message, then msg.Header.Get("Content-Type") with return "". According to the mime library source, the function ParseMediaType returns an error if mediatype argument is "". Approximate stack trace is something like:
src/mime/mediatype.go:143 in ParseMediaType: https://cs.opensource.google/go/go/+/refs/tags/go1.18.2:src/mime/mediatype.go;l=143
err = checkMediaTypeDisposition(mediatype)
if err != nil {
return "", nil, err
}
src/mime/mediatype.go:103 in checkMediaTypeDisposition: https://cs.opensource.google/go/go/+/refs/tags/go1.18.2:src/mime/mediatype.go;l=106
if typ == "" {
return errors.New("mime: no media type")
}
The error there is consistent with the SMTP server log message and error code above.
Proposed solution
Update ntfy/server/smtp_server.go::readMailBody to check if 'Content-Type' is set, and if not, default it to "text/plain" or some other, sensible default. This will ensure that any notification systems who do not set "Content-Type" will still have their notifications accepted.