Anti-Spam SMTP Implementation

for anonymous Dial-In

SwiNOG #5, 25.09.2002

Fredy Künzler, Init Seven AG

SPAM is a serious problem.

Better fight it at it‘s origin.

Anonymous Dial-In:

Customer is not known Customer can hide his identity

(Rufnummerunterdrückung) In case of abuse, effort for prevention is high

and unpaid Complaints: Spamcop etc. In case of abuse, it’s only a reaction, and SPAM

has already been distributed

How Spammer abuse anonymous Dial-in (1) New strategy by Spammers, as the classical Spam delivery

technique is not effective anymore (abusing an open SMTP relay). Open SMTP relay servers are disappearing more and more.

Reason: M$ is delivering it’s SMTP servers with default «relay denied» these days.

How Spammer abuse anonymous Dial-in (2) Spam-PC dials over ISDN (Channel Bundling, PPP Multilink)

Spam-PC runs it's own SMTP server. Mail delivery is being done directly to the respective MX server of the recipient. This

technique is common by well known Swiss spammers as M.F. (Nachtsichtgeräte, overpriced digital cameras,

Brockhaus software, GPS equipment etc.) and B.W.L. (Pfefferspray, Graphic gallery software etc.)

as the spamming SMTP server get's the dynamic IP of the provider, complaints will go to the providers abuse desk

Prevention (Solution #1) (1) disallow SMTP traffic (Port 25) leaving own backbone

Implementation (let's assume provider has 192.168.128.0/19 and the dial range is 192.168.136.0/24):

access-list 100 remark SMTP Block of Dial-in-Customersaccess-list 100 permit tcp 192.168.136.0 0.0.0.255 192.168.128.0 0.0.31.255 eq smtpaccess-list 101 deny tcp 192.168.136.0 0.0.0.255 any eq smtpaccess-list 101 permit ip any any

interface FastEthernet 0/0 description connected to Internet (Upstream Provider) ip access-group 100 out

Prevention (Solution #1) (2)Advantage of Solution #1: Spammer cannot do direct MX-Delivery

Disadvantages of Solution #1: User must use provider’s SMTP server, even if he has a GMX or

another freemail address User asks support how to handle «relay denied» error messages Spammer still can abuse providers’s SMTP server, as it must relay

mail traffic from dial-in connections

Prevention (Solution #2) (1) same as solution #1, but with protected SMTP server of the

provider

Dial-Spam-Block is used for SMTP server protection

Dial-Spam-Block counts the number of recipients of mail delivery for each dynamically assigned IP address

Dial-Spam-Block stops relaying mail after limit of recipients has been reached in a certain time frame (default: 50 recipients per 30 minutes allowed)

Prevention (Solution #2) (2) potential Spammer reaches limit within 1 minute and gets a «relay

denied»

counter is reset if dial-in user logs off (radius server sends signal to Dial-Spam-Block)

Dial-Spam-Block currently works with Sendmail (writes into access.db, therefore no sendmail.cf quirk required) and Freeradius. Communication between the two servers is done over SSH.

Prevention (Solution #2) (3) Advantages of Solution #2:

Spammer cannot do direct MX-Delivery SMTP Server of provider is protected

Disadvantages of Solution #2: User must use provider’s SMTP server, even if he has a

GMX or another freemail address User asks support how to handle «relay denied» error

messages

Prevention (Solution #2) (4) Dial-Spam-Block

Concept by Fredy Künzler, Init Seven AG(thanks to the Interconnection people)

Developed by Marco Steinacher, Init Seven AG GPL (General Public License) Runs since March 2002 without problems in the

environment of Init Seven AG. Available at dial-spam-block.sourceforge.net

Prevention (Solution #3) (1) Redirect SMTP traffic from dial-in user to providers SMTP server.

This will ease the configuration of SMTP client. Support must no longer handle «relay denied» questions, as any valid hostname (i.e. mail.bluewin.ch at Init Seven’s dial-in) works up to the limit of the dial-spam-block.

Prevention (Solution #3) (2) Implementation (let’s assume provider has 192.168.128.0/19 and the dial range is 192.168.136.0/24, and the IP address of the SMTP server is 192.168.128.10)

access-list 102 remark SMTP Redirect to SMTP server from Dial-in-Customersaccess-list 102 permit tcp 192.168.136.0 0.0.0.255 any eq smtp

route-map SMTP-Redirect permit 10 match ip address 102 set ip next-hop 192.168.128.10

interface FastEthernet 0/0 description connected to Internet (Upstream Provider) ip policy route-map SMTP-Redirect

Note that the SMTP server must not be in the IP range of dial customers!

Prevention (Solution #3) (3) Configuration of SMTP server:

(requirement: kernel of SMTP server must support iptables)

iptables -A PREROUTING --table nat --protocol tcp --source \ 192.168.136.0/24 --dport 25 -j DNAT --to 192.168.128.10

(Init script for Linux available at www.init7.net/anti-spam/)

Prevention (Solution #3) (4)

*) Protected with Dial-Spam-Block

Prevention (Solution #3) (5)Advantages of Solution #3: Spammer cannot do direct MX-Delivery

SMTP Server of provider is protected

Support is no longer bothered by «relay denied» questions

(Minor) Disadvantage of Solution #3: 'telnet host 25' no longer possible from dial-in range

Solution #3 is operational since August 2002 in the environment of Init Seven AG

Further development (proposed solution #4) Dial-Spam-Block is able to work as a «Teergrube». Instead of a hard

limit as «relay denied after 50 recipients» the SMTP delivery could gradually slow down. For instance the first 20 recipients are delivered instantly, than wait 2 seconds for each address for the next 20 recipients, than wait 5 seconds etc. etc.

Links (1)SMTP-Redirect Implementationwww.init7.net/anti-spam/

Dial-Spam-Blockdial-spam-block.sourceforge.net

Teergrubehttp://www.iks-jena.de/mitarb/lutz/usenet/teergrube.en.html (English)http://www.iks-jena.de/mitarb/lutz/usenet/teergrube.html (German)

Links (2)Freeradiuswww.freeradius.org

Sendmailwww.sendmail.org

Iptableswww.iptables.org

Questions?

Thank you.