How Fetchmail made me a spammer

[Frank Shute]

On Thu, Jan 14, 2010 at 10:16:56AM +0100, Benjamin Lutz wrote:
>
> Last night, I got an email from one of my users for whom I handle emails. He 
> said that friend sent him a large email, which was rejected because of its 
> size; and that his friend now gets a notice to that effect every minute.
> 
> What had happened?
> 
> 1) The friend sent a 20MB Email to my user's public email account.
> 2) Fetchmail downloads that 20MB email from the public POP3 server.
> 3) Fetchmail tries to pass the email to the local postfix server.
> 4) Postfix refuses the email with a permanent 552 error because
>    it's larger than 10MB.
> 5) Fetchmail generates and sends a rejection notice, but does not
>    delete the 20MB Email from the POP3 server because the
>    "softbounce" option is still the default.
> 6) Fetchmail sleeps 60 seconds.
> 7) Continue at step 2).
> 
> The damage done:
> - roughly 20GB of bandwidth wasted by downloading the 20MB email over
>   and over.
> - an estimated 1000 rejection notices sent to the poor guy who originally
>   sent the 20MB email (well, that should teach him not to send big mails! :)
> - personal embarassment.
> 
> The lessons learned:
> - I need better monitoring. I already monitor postfix's queue size and
>   get alerts if it goes above a certain size, but in this case, the email
>   in question never ended up in the queue. Monitoring bandwidth usage at
>   the firewall and mails-per-hour at the mail server (which includes error
>   notices) should let me detect sooner that something is amiss next time.
> - Postfix's default 10MB size limit seems outdated seeing how internet
>   connections have become faster; I've upped it to 50MB.
> - Fetchmail's defaults are dangerous. The softbounce option, which is the
>   default (the manpage claims it'll be disabled by default with the next
>   version,) can generate large amounts of spam.
> 
> Cheers
> Benjamin

Benjamin,

You might want to give getmail a try. In the getmail conf you can
limit the size of emails it fetches. I'm not sure you can do that with
fetchmail.

As you say, the guy who sent a >10MB email was rather silly, although
I don't know what the "standard Windows user" uses for file transfer
other than email. It's not like they usually have a web server set up.

Regards,

-- 

 Frank

 Contact info: http://www.shute.org.uk/misc/contact.html