Protect sendmail from DoS

[Steven Jones]

Dosing is a pretty rare occurrence, unless you are being specifically
targeted don't worry about it IMHO. DoSing usually means a specific
person or group wants to kill you off, they have decided for what ever
reason they want you dead. There are so many ways that these people can
do it for small users that in effect there is little you can do. If you
could defeat them at the box itself all they have to do is flex a little
bit more muscle and DoS your upstream router(s).

They can attach from literally thousands of smtp drones, or if that is
not enough spam you through open relays with large emails, say 4~10meg a
shot, a few hundred of them coming in will bog down your server.

The problem will be iptables looks at the packets that make up the email
and not the content of the email.

So you need to do any blocking or rate limiting at the smtp level ie per
message and not the IP level which is per packet type thing.

Why are you so concerned about DoS? There are more important things
spend the time learning how to harden the box and tune sendmail, or look
at postfix or qmail.

DoS should not be a huge risk, but if it is then you can do little, but
ride it out.

Regards

Thing

-----Original Message-----
From: Devon Harding [mailto:devonharding at gmail.com] 
Sent: Wednesday, 2 November 2005 2:08 p.m.
To: General Red Hat Linux discussion list
Subject: Re: Protect sendmail from DoS

Is there some way of using something like IPTABLES to block if it sees a
certain amount of connections from a particualar IP? I know Ciphertrust
Ironmail does this.

On 11/1/05, Steven Jones <Steven.Jones at vuw.ac.nz> wrote:
>
> Sendmail has some protection in terms of load limiting, these are a
bit
> high so you can set them lower so the server recovers sooner. This
will
> save your server but in effect it allows DoS sooner.
>
> Possibly you do not understand what a DoS is. DoS is a function of
your
> attacker overloading your network or server's capacity to handle
network
> traffic sent at it.
>
> These days unless you are a big organisation with huge pipes, big
> multiple servers and deep pockets, and someone wants you dead, your
> dead.
>
> If someone wants to take your server out they can, it is simply a
matter
> of logistics, they control 30 or 300 or 3000 or 30000 spam drones of
> hacked broadband connections and the volume these generate is amazing.
>
> I was Dos'd a while back, I was sent 5+gig of volume in 2~3 minutes,
my
> 512k cable modem could not cope so in effect the DoS happened at the
> ISP's end of my pipe, totally outside of my control.
>
> Modern machines, even desktop ones should be able to handle a lot of
> mail, if you are having issues with DoS's then maybe it is something
> else.
>
> Regards
>
> Thing
>
>
>
> -----Original Message-----
> From: Devon Harding [mailto:devonharding at gmail.com]
> Sent: Wednesday, 2 November 2005 10:43 a.m.
> To: General Red Hat Linux discussion list
> Subject: Protect sendmail from DoS
>
> How can I protect my sendmail server against DoS attacks?
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