[Dshield] SMTP Failure

Johannes B. Ullrich jullrich at euclidian.com
Sun Jun 20 16:49:12 GMT 2004

> Jun/20/2004 11:00:59     SMTP: unexpected reply: 451 See 
> http://pobox.com/~djb/docs/smtplf.html.           

Welcome to RFC hell and the wonderful world of qmail. The
URL above explains things actually pretty well.

Various operating systems use different methods to indicate
the end of a line. There are essentially two ASCII characters

# 10 - Line Feed. (LF, \n)
# 13 - Carriage Returns. (CR \r)

In Unix, 'LF' is typically used by itself to indicate the
end of a line, while other operating systems use 'CR+LF'.

E-Mail is supposed to be cross platform. In order for mail
servers that run on various operating systems to interpret
headers correctly, they have to know how to recognize the
end of the line.

This problem has been solved by RFC 822 in 1982. I believe
this is now also known as IETF standard 822.bis, which
provides it more weight (there are many RFCs which are
"Requests For Comments". Some of them are popular enough
to be used as standards, while others are more or less

Now most mail servers don't care. They will use either 
CR, or CR+LF. However, 'qmail' is a mail server that
is particular picky about following standards. Many people
like qmail, because it has a very good security history
(but enough bugs, and a very unresponsive maintainer, so
most people run heavily patched versions of qmail)

In short: Your ISP is using qmail as a mail server, there is
nothing wrong with that. qmail chooses to interpret the RFCs
very strictly. Again: Nothing wrong with that. The fault is
with DLink for implementing their mail sending option sloppy.

Full text of RFC822:  


Visiting SANSFIRE?? Stop by at IPNet and say Hi ;-)
Johannes Ullrich                     jullrich at euclidian.com
contact: http://johannes.homepc.org/contact.htm

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