Re: sendmail 8.9.3 patches to curb RCPT harvesters

[peterw () CLARK NET (Peter W)]

Tim Pierce wrote:

> On Thu, Mar 11, 1999 at 07:31:21PM -0500, Peter W wrote:

> > Per Joseph's suggestion. Use these patches against sendmail 8.9.3 and add
> >
> > O RCPTFailDelay=30
> >
> > to sendmail.cf to make sendmail sleep() for 30 seconds before reporting any
> > "550" errors. Set the value to 0 for "normal"  behavior.
>
> According to the reports I'm seeing, GeoList Pro does not wait for a
> response from the server -- instead, it streams the RCPT TO commands
> continuously and then reads the results at the end of transmission.
> If that is the case, it doesn't sound like this patch will have any
> effect.

(1) It will slow down your server's responses to GeoList, et al. Set the delay
to 60 seconds and a 5000 word attack could take 83 hours to send the responses
they need (assuming they don't run parallel attacks, and sendmail already has
means to limit such attacks).

(2) sendmail logs the RCPT failure notices via syslog as soon as it sends them
to the client.

Which is why I said the changes would "frustrate current RCPT abuse tools, give
admins more time to notice the failures in the maillog and react". If the
attacker thinks it's worthwhile to wait for the response  codes, and you're not
watching your log files, then you're right, these changes won't help you much.

Also I said "current... tools" because a more intelligent harvester could
compare the delay rates of "250" and "550" responses and learn when to time out
and assume the RCPT failed. To get around this you'd need to make a simple
change so that sendmail had a deliberate delay before *all* RCPT responses; that
doesn't seem warranted yet, and would slow legitimate mail delivery. See my
previous post for comments on RFC 1123 and deferring verification.

-Peter

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