SPAM Prevention Using DNS Solutions

1
SPAM Prevention Using DNS Solutions

• Implementing reverse domain name services (rDNS)
  and planning for SPF Classic
• Presented by Ed Horley
• Date May 2005

2
Overview

• SPAM prevention is the primary reason that rDNS
  and SPF Classic will become de jure within
  approximately 1-2 years (IETF ratified)
• Current methods for SPAM prevention are de facto
  solutions filtering, black/white lists, etc
• Reverse DNS (rDNS) is a great quick check to
  determine if an MTA is being run and maintained
  correctly but it can be spoofed
• Sender Policy Framework (SPF v1) or SPF Classic
  is being used by service providers to confirm
  that the mail servers they are receiving mail
  from are authorized to do so on behalf of the
  sending domain, these records are published by
  the sending domain

3
Overview Continued

• Future additional solutions for SPAM prevention
  are Yahoos DomainKeys, Sender Verification and
  perhaps Microsofts Puzzle Solution (unlikely)
• Sender ID has been rejected by the IETF as a
  proposed standard (de jure) due to inclusion of
  patented technology by Microsoft and Microsoft
  has modified it and resubmitted. It may or may
  not make it through this time depending on the
  dependencies the working committee see on the
  patented or protected intellectual property

4
Current Solutions Overview

• Current de facto solutions
• Blacklists (IP and DNS based)
• rDNS (optional see RFC2505 2.5)
• Anti-spam filtering (Bayesian and others)
• Anti-spam services (Brightmail, Postini,
  Commtouch, etc)
• Hardware appliance filters / services
• Custom built scripts and applications
• Sender Verification (Several different formats
  exist)
• Whitelists (IP and DNS based)
• SPF Classic (optional)

5
Solutions Used Today

• Blacklists
• SpamCop
• MAPS
• ORDB
• SPAMhaus
• Spews
• SURBL
• Mail-abuse
• DSBL
• DNSBL
• DNSRBL

• Client filters
• Audiotrieve InBoxer
• Cloudmark SpamNet
• Lyris MailShield
• McAfee SpamKiller
• Aladdin SpamCatcher
• Sunbelt IHateSpam
• SpamBayes (open source)
• Spam Bully
• MailFrontier Matador
• Cloudmark Spamnet

6
Solutions Used Today

• Server filters
• Exchange IMF (comes bundled with Exchange)
• XWall
• Vircom modusGate
• Sophos PureMessage
• Proofpoint Protection
• SurfControl
• Symantec
• Trend Micro
• GFI MailEssentials
• Sybari Antigen (Microsoft Aquired Feb 2005)
• Network Associates / Mcafee
• SpamAssassin (open source)
• Declude JunkMail

• Hardware Appliances
• Barracuda 300
• BorderWare MXtreme
• CypherTrust IronMail
• IronPort C60
• Mail Foundry
• Sendio ICE Box
• Tumbleweed
• Subscription Services
• Brightmail
• Commtouch
• Greenview Data
• Katharion
• Postini
• PUREmail

7
The Proposed Solutions

• Short term solutions
• Internet Engineering Task Force (IETF) draft
  rfcs
• Sender Policy Framework (SPF Classic)
• Sender ID (SPF Classic PRA) Microsoft draft
  rfc (maybe?)
• DomainKeys (Google and Yahoo have implemented)
• Long term solutions
• Internet Research Task Force (IRTF)
• New version / next generation of SMTP?

8
What to do now?

• SMTP mail gateway filters (hardware or software)
• Consider a commercial service (depends on the
  amount and type of traffic you except to see for
  your environment)
• Software e-mail client filters (Small business
  accounts)
• Blacklists / Whitelists (Enterprise and Service
  Providers)
• rDNS (anyone running an MTA that sends traffic to
  the Internet)
• SPF Classic (anyone running an MTA that sends
  traffic to the Internet)
• DomainKeys (Service Providers)

9
What is rDNS?

• rDNS is an acronym for reverse DNS
• It is a method of name resolution in which an IP
  address is resolved into a domain name
• It is the opposite of the typical resolution
  method of DNS which resolves domain names into IP
  addresses
• It utilizes the existing DNS infrastructure by
  using a special reserved domain name
  in-addr.arpa.
• IP addresses are more specific left to right and
  domain names are more specific right to left,
  therefore the rDNS IP listings are reversed
• Example 63.251.192.20 would have a reverse entry
  of 20.192.251.63.in-addr.arpa.

10
Why you should do rDNS now

• Easy to implement
• Because spammers often use invalid IP addresses
  (bogons) to send e-mails, rDNS will determine the
  authenticity of a domain name compared to the IP
  address from which it is originating
• It is used as one of several de facto methods to
  determine the likelihood of a server being a SPAM
  relay
• Most Internet Service Providers are using this to
  determine legitimate mail sources
• Reduces probability of legitimate mail servers
  being added to a Blacklist

11
What is SPF Classic?

• SPF Classic is used to identify mail servers that
  are explicitly permitted to send mail for a
  particular domain (think outgoing)
• Domain owners identify permitted sending mail
  servers in DNS using TXT records
• SMTP receivers verify the envelope sender address
  against the DNS information and can distinguish
  legitimate mail servers before any message data
  is transmitted
• It is backward compatible with MTAs that are not
  patched with SPF filters or libraries (well,
  actually the old MTA just ignore it if that is
  considered backward compatible!)
• Remember MX records publish which IPs are to
  receive mail (incoming) destined for your domain,
  SPF records says which IPs are allowed to send
  mail (outgoing) on behalf of your domain

12
Why you should do SPF now

• Easy to implement (publish TXT records in DNS)
• It is used by AOL, Symantec, EarthLink, Google
  and more as one of several de facto methods to
  determine trustworthiness of the mail sources
• Most Internet Service Providers are currently or
  starting to use this to determine legitimate mail
  sources
• Will move your mail to priority queues for
  processing for many providers including AOL, you
  can also submit to be added to the Whitelist for
  AOL
• Reduces probability of being added to a Blacklist
• Oct 1st ,2004 Microsoft, MSN and Hotmail will all
  start using Sender ID to prioritize incoming
  e-mail! (Sender ID is backward compatible with
  SPF Classic)

13
What to know about SPF Classic

• Meng Wong created SPF Classic. It used to be
  called Sender Permitted From and was changed to
  Sender Policy Framework
• SPF v1 (Classic) designates specific SMTP servers
  as being authorized to send for a FQDN
• Uses the TXT fields in DNS to publish relevant
  information
• Each sub-domain must be configured specifically
• SPF will become de jure within approximately 1-2
  years most popular filters are flagging this
  already
• Most MTAs support SPF Classic or have plug-ins
  available
• Backward compatible with existing technology
• It breaks e-mail forwarding! You'll have to
  switch from forwarding, where the envelope sender
  is preserved, to remailing, where the envelope
  sender is changed your MTA will have to support
  this

14
What to know about Sender ID

• SPF Classic PRA Sender ID (basically now the
  MTA (think Exchange) checks SPF AND the MUA
  (think Outlook) check the PRA or Purported
  Responsible Address)
• Meng Wong and Microsoft submitted a draft rfc
  merging both solutions and called it Sender ID
  was just turned down as a standard by the IETF
  due to Microsoft patent issues it is back on
  the table again!
• Uses the TXT fields in DNS to publish relevant
  information same as SPF but uses a new version
  number
• Each sub-domain must be configured specifically
  just like SPF
• Microsoft will be updating the MTA/MUAs to
  support PRA (or Sender ID) think Outlook,
  Outlook Express and Exchange
• Backward compatible with existing technology
• It breaks e-mail forwarding! You'll have to
  switch from forwarding, where the envelope sender
  is preserved, to remailing, where the envelope
  sender is changed just like SPF
• SPF v2

15
What to know about PRA

• A purported responsible address is determined as
  the first from the following list of items
• the first Resent-Sender header in the message,
  unless (per the rules of RFC2822) it is preceded
  by a Resent-From header and one or more Received
  or Return-Path headers occur after said
  Resent-From header and before the Resent-Sender
  header (see 3.6.6. of RFC2822 for further
  information on Resent headers),
• the first mailbox in the first Resent-From header
  in the message,
• the Sender header in the message, and
• the first mailbox in the From header in the
  message
• The purported responsible domain of a message is
  defined to be the domain part of the messages
  purported responsible address.

16
What is coming in a few years

• DomainKeys
• A Yahoo! submitted draft rfc
• http//www.ietf.org/internet-drafts/draft-delany-d
  omainkeys-base-00.txt
• Basically public/private keys for authenticating
  client mail and the servers along the path
• Acts as a chain of custody from the source client
  machine to the destination client machine
• Will require a major re-write of all MTAs to
  work 5 to 10 years if at all?
• Backward compatible with existing technology
• Google and Yahoo have already deployed!
• Has promise to be a great standard if adoption is
  quick enough

17
What is coming continued

• Puzzle Solution
• Microsoft proposal
• Assumed for small businesses that cannot afford
  certificate services
• Sending mail server has to perform time consuming
  calculation for each mail sent
• Assumes spammers cannot afford the computational
  costs to send out large bulk mailings or the cost
  of the bulk certificate services
• Will require a major re-write of all MTAs to
  work 5 to 10 years if at all?
• Backward compatible with existing technology
• Solution has serious shortcomings
• Microsoft has little published on this solution

18
Future potential SPAM problems

• Disposable domain names, certificates and
  registrars
• Country Sanctioned Activity (Governments allowing
  for profit activity or turning a blind eye to
  problem spammers) in order to generate s
• Large Zombie Farms controlling clients with legit
  relay access (Think large University or poorly
  managed corporate environments)
• Spyware agents that provide relay capabilities
  similar to Zombie configurations
• IPv6 and Mobile IP devices becoming more
  prevalent
• Potential exploits that could turn large
  peer-to-peer networks into relays

19
How rDNS works
MX mx1.ispA.net -1.1.1.1
MX mx1.ispB.net - 2.2.2.2
ISP A
ISP B
Internet
PTR 1.1.1.1 - mx1.ispA.net PTR 2.2.2.2 -
mx1.ispB.net
20
How to request rDNS for sub /24 address blocks

• You will have to contact your ISP to request rDNS
  delegation do this via e-mail so you have a
  written trail of correspondence
• You will likely have to talk to several
  departments to figure out who can actually do
  this for you, first contact your account manager
• Typically, the DNS group handles the
  sub-delegation but not always sometimes it is
  the networking group
• You will need to be patient but firm inform
  them that you need it for Anti-SPAM reasons for
  your mail server, to be RFC 2505 compliant
• RFC 2317 describes standard methods for rDNS sub
  /24 delegation formats, there is also the DeGroot
  hack from the book "DNS Bind" both work fine

21
Setting up RFC 2317 rDNS Delegation

• Example of 64.94.106.40/29 configuration in the
  providers servers
• ORIGIN 106.94.64.in-addr.arpa.
• zone delegation of 64.94.106.40/29
• 40-47. IN NS ns1.j2global.com.
• 40-47. IN NS ns2.j2global.com.
• 40. IN CNAME 40.40-47.106.94.64.in-addr.arpa.
• 41. IN CNAME 41.40-47.106.94.64.in-addr.arpa.
• 42. IN CNAME 42.40-47.106.94.64.in-addr.arpa.
• 43. IN CNAME 43.40-47.106.94.64.in-addr.arpa.
• 44. IN CNAME 44.40-47.106.94.64.in-addr.arpa.
• 45. IN CNAME 45.40-47.106.94.64.in-addr.arpa.
• 46. IN CNAME 46.40-47.106.94.64.in-addr.arpa.
• 47. IN CNAME 47.40-47.106.94.64.in-addr.arpa.

22
Setting up the rDNS Zone

• Example of 64.94.106.40/29 configuration in your
  servers
• ORIGIN 40-47.106.94.64.in-addr.arpa.
• zone delegation of 64.94.106.40/29
• _at_ IN NS ns1.j2global.com.
• _at_ IN NS ns2.j2global.com.
• _at_ IN TXT "j2 Global Communications, Inc."
• 40 IN PTR 64.94.106.40.efax.com.
• 41 IN PTR 64.94.106.41.efax.com.
• 42 IN PTR 64.94.106.42.efax.com.
• 43 IN PTR 64.94.106.43.efax.com.
• 44 IN PTR 64.94.106.44.efax.com.
• 45 IN PTR 64.94.106.45.efax.com.
• 46 IN PTR 64.94.106.46.efax.com.
• 47 IN PTR 64.94.106.47.efax.com.

23
Checking the rDNS Zone

• Example of checking the 64.94.106.40/29
  configuration
• DiG 2.1 _at_206.13.31.12
  40.106.94.64.in-addr.arpa. PTR
• (1 server found)
• res options init recurs defnam dnsrch
• got answer
• -HEADERid 10
• flags qr rd ra Ques 1, Ans 2, Auth 2,
  Addit 0
• QUESTIONS
• 40.106.94.64.in-addr.arpa, type PTR, class
  IN
• ANSWERS
• 40.106.94.64.in-addr.arpa. 43200 CNAME 40.40-47.10
  6.94.64.in-addr.arpa.
• 40.40-47.106.94.64.in-addr.arpa. 86400 PTR 64.94.1
  06.40.efax.com.
• AUTHORITY RECORDS
• 40-47.106.94.64.in-addr.arpa. 86400 NS ns2.j2globa
  l.com.
• 40-47.106.94.64.in-addr.arpa. 86400 NS ns1.j2globa
  l.com.
• Total query time 48 msec
• FROM us.mirror.menandmice.com to SERVER
  206.13.31.12
• WHEN Tue Jul 20 012009 2004

24
How SPF Classic works
MX mx1.ispA.net -1.1.1.1 TXT "vspf1 a mx -all"
MX mx1.ispB.net - 2.2.2.2 TXT "vspf1 a mx
-all"
ISP A
ISP B
Internet
TXT vspf1 a mx all MX mx1.ispA.net A
mx1.ispA.net - 1.1.1.1
25
SPF Classic Syntax

• Some common SPF options in the TXT field
• a the A record entry for example.com sends
  e-mail on behalf of example.com
• mx the published MX record entries for
  example.com do not only receive e-mail on behalf
  of example.com but send e-mail also
• ptr approve any host that ends in example.com
  as part of its FQDN
• a a list of A record entries that are
  permitted to send e-mail on behalf of example.com
• mx a list of mx record entries that are
  permitted to send e-mail on behalf of example.com
• ip4 a list of ip addresses that are permitted
  to send e-mail on behalf of example.com (CIDR)
• include a different domain that may send
  e-mail on behalf of example.com (relay through
  your service provider)
• -all (fail) everything in the SPF record are
  the ONLY hosts/networks permitted to send
  (strictest policy dont do unless you know all
  the ramifications)
• all (soft fail) everything in the SPF record
  are the ONLY hosts/networks permitted to send
  (middle ground)
• ?all not sure (technically neutral) if
  everything in the SPF record are the ONLY
  hosts/networks permitted to send (start
  publishing with this one first as it is the most
  liberal policy)
• Please see http//spf.pobox.com/mechanisms.html
  for a more detailed description and see
  http//spf.pobox.com/whitepaper.pdf for an
  excellent whitepaper

26
Setting up SPF Classic

• Configuration of example.com SPF
• ORIGIN example.com.
• Leaving out the SOA info for space reasons
• NS records
• _at_ IN NS ns1.example.com.
• _at_ IN NS ns2.example.com.
• MX records
• _at_ IN MX 10 mx1.example.com.
• _at_ IN MX 20 mx2.example.com.
• A records
• mx1 IN A 1.1.1.1
• mx2 IN A 2.2.2.2
• TXT SPF records
• _at_ IN TXT "vspf1 a mx -all"
• mx1 IN TXT "vspf1 a -all"
• mx2 IN TXT "vspf1 a -all"

27
Register your SPF domain

• Once you have configured SPF for your domain you
  should confirm your configuration at
• www.dnsstuff.com
• Then put the logo on your site!

28
Testing SPF Classic

• Testing of example.com SPF
• http//www.dnsstuff.com/pages/spf.htm
• Dummy Sample Output from dnsstuff
• SPF lookup of sender droid_at_example.com. from IP
  1.1.1.1
• SPF string used vspf1 mx -all. ? Obtained the
  TXT record via DNS for example.com
• Processing SPF string vspf1 mx -all. ?
  Checking against the TXT record
• Testing 'mx' on IP1.1.1.1, target domain
  example.com, CIDR 32, defaultPASS. MATCH!
• Testing 'all' on IP1.1.1.1, target domain
  example.com, CIDR 32, defaultFAIL.
• Result PASS

29
Impact on the Internet

• These solutions will help reduce overall
  architecture problems of Authentication,
  Authorization, and Accounting with e-mail (back
  to AAA)
• 68B e-mails daily of which approx. 42.8B are spam
  or 69 spam!1
• Estimated 1,400 annual savings per employee from
  lost productivity currently due to spam2
• 1 The Radicati Group and Brightmail
• 2 - Vircom

30
Resource Links

• rDNS
• http//www.ietf.org/rfc/rfc2317.txt
• http//www.ietf.org/rfc/rfc2505.txt
• http//www.arin.net/registration/lame_delegations/
  index.html
• http//kbase.menandmice.com/view.html?rec31
• http//www.microsoft.com/windows2000/techinfo/resk
  it/en-us/default.asp?url/windows2000/techinfo/res
  kit/en-us/cnet/cncf_imp_dewg.asp
• http//dedicated.pacbell.net/custcare/dns_workshee
  t.html
• DNS tools
• http//www.dnsstuff.com/
• http//us.mirror.menandmice.com/cgi-bin/DoDig
• http//network-tools.com/
• http//www.squish.net/dnscheck/
• http//www.dns.net/dnsrd/tools.html
• http//www.dnsreport.com/
• http//www.samspade.org/t/
• General references
• http//www.spamanatomy.com/

31
Resource Links

• SPF
• http//spf.pobox.com/howworks.html
• http//spf.pobox.com/rfcs.html
• http//spf.pobox.com/wizard.html
• http//www.ietf.org/internet-drafts/draft-mengwong
  -spf-01.txt
• http//www.dnsstuff.com/pages/spf.htm
• Microsofts PRA (E-mail Caller ID)
• http//www.microsoft.com/downloads/details.aspx?Fa
  milyID9a9e8a28-3e85-4d07-9d0f-6daeabd3b71bdispla
  ylangen
• Sender ID the merged PRA and SPF
• http//www.microsoft.com/presspass/press/2004/may0
  4/05-25SPFCallerIDPR.asp
• http//www.microsoft.com/presspass/press/2004/jun0
  4/06-24SIDSpecIETFPR.asp
• http//www.microsoft.com/mscorp/twc/privacy/spam_s
  enderid.mspx
• Yahoo! DomainKeys
• http//antispam.yahoo.com/domainkeys
• http//www.ietf.org/internet-drafts/draft-delany-d
  omainkeys-base-00.txt
• http//boycott-email-caller-id.org/

32
A look at some Service Providers records

• aol.com. 300 IN TXT
  "vspf1 ip4152.163.225.0/24 ip4205.188.139.0/24
  ip4205.188.144.0/24 ip4205.188.156.0/23
  ip4205.188.159.0/24 ip464.12.136.0/23
  ip464.12.138.0/24 ptrmx.aol.com ?allaol.com.
  300 IN TXT "spf2.0/pra
  ip4152.163.225.0/24 ip4205.188.139.0/24
  ip4205.188.144.0/24 ip4205.188.156.0/23
  ip4205.188.159.0/24 ip464.12.136.0/23
  ip464.12.138.0/24 ptrmx.aol.com ?all
• cisco.com. 86400 IN TXT
  "vspf1 ptr"
• earthlink.net. 1800 IN TXT
  "vspf1 ip4207.217.120.0/23 ip4207.69.200.0/24
  ip4209.86.89.0/24 ?all
• efax.com. 86400 IN TXT
  "vspf1 ptr ?all"
• google.com. 300 IN TXT
  "vspf1 ptr ?all
• hotmail.com. 3600 IN TXT
  "vspf1 includespf-a.hotmail.com
  includespf-b.hotmail.com includespf-c.hotmail.co
  m includespf-d.hotmail.com all
• microsoft.com. 3600 IN TXT
  "vspf1 mx redirect_spf.microsoft.com"
• msn.com. 900 IN TXT
  "vspf1 includespf-a.hotmail.com
  includespf-b.hotmail.com includespf-c.hotmail.co
  m includespf-d.hotmail.com all
• netzero.net. 600 IN TXT
  "vspf1 ptruntd.com ptrnetzero.net ptrjuno.com
  ?all

33
Questions and Answers
34
About Ed Horley

• Ed Horley is a Sr. Network Engineer for j2 Global
  Communications, better known as eFax. Ed
  currently designs, supports and maintains j2's
  58 international and domestic collocation sites
  along with j2's core data center IP
  infrastructure. He is experienced in e-commerce
  web content delivery, large scale e-mail
  delivery, firewalls, IPSec VPN's, and specializes
  in routing and switching and DNS issues.
• Ed is a Cisco Certified Network Professional
  (CCNP), a Microsoft Certified Professional (MCP)
  and a Microsoft Most Valuable Professional (MVP).
  Ed graduated from the University of the Pacific
  in 1992 with a BS in Civil Engineering.
• When he is not playing on network gear you can
  find him out on the lacrosse field as an Umpire
  for Women's Lacrosse. He is currently married to
  his wonderful wife Krys and has two children,
  Briana and Aisha. He lives and works in Walnut
  Creek, CA.

35
Contact Info

• Ed Horley ehorley_at_gmail.com