From nanog-bounces@nanog.org Wed Sep 3 11:58:37 2008 From: Alec Berry <alec.berry@restontech.com> Subject: Re: ingress SMTP
Michael Thomas wrote:
I think this all vastly underrates the agility of the bad guys. So lots of ISP's have blocked port 25. Has it made any appreciable difference? Not that I can tell. If you block port 25, they'll just use another port and a relay if necessary.
I'm pretty sure it has, although without aggregate stats from various ISPs it is hard to tell. Since mail transport is exclusively on port 25 (as opposed to mail submission), a bot cannot just hop to another port.
One small data-point -- on a personal vanity domain, approximately 2/3 of all the spam (circa 15k junk emails/month) was 'direct to inbound MX' transmissions. The vast majority of this is coming from end-user machines outside of North America. China, India Thailand, Brazil, Poland, "CZ", and a couple of providers each in Germany and France, appear to be the most prevalent sources _I_ see. The message count would be a fair bit higher, but I have several overseas networks (4 in DE, 2 in TW, 1 in CZ) plus pieces of 2 domestic networks (*da.uu.net, *pub-ip.psi.net) blocked at the firewall. Also firewalled are a couple of dozen IP addresses that have -each- made over 10k attempts to _relay_ mail through me. I'm seeing a significant amount of 'Received' header forgery, apparently intended to fool "dumb" header parsers into believing the direct-to-MX transmission _did_ go through the server associated with the domain used in the '"from: ", "from ", and "Reply-to: " lines. The good news is that only a _really_ dumb parser would be fooled by most of what I'm seeing. :)
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Robert Bonomi wrote:
One small data-point -- on a personal vanity domain, approximately 2/3 of all the spam (circa 15k junk emails/month) was 'direct to inbound MX' transmissions. The vast majority of this is coming from end-user machines outside of North America.
This confirms the limited data I have. I configure my edge firewall (pf) to drop anything to/from the Spamhaus DROP list, as well as sendmail to use their XBL. The DROP list seems like it blocks mostly MX lookups (nice to see the blocking of mail start so early in the process!), so it is hard to say how many SMTP connections never happen (remote server/bot does not know where to connect). The XBL list, which is mostly residential IPs around the world, seems to be the single most effective technique in blocking incoming traffic-- on port 25. Obviously, these connections are coming from ISPs that do *not* block egress TCP 25. Slightly off topic-- I found it quite easy to configure the DROP list to work with pf (or is that the other way around?). I would be happy to share the small Perl script that updates the pf table. When I configured the DROP list on a free public wireless system I maintain, I was amazed at how much egress traffic it blocked-- obviously rogue/bad/evil webservers, IRC hosts, etc. I wonder if anyone else is using it that way? ... alec - -- `____________ / Alec Berry \______________________________ | Senior Partner and Director of Technology \ | PGP/GPG key 0xE8E9030F | | http://alec.restontech.com/#PGP | |-------------------------------------------| | RestonTech, Ltd. | | http://www.restontech.com/ | | Phone: (703) 234-2914 | \___________________________________________/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIv+YdREO1P+jpAw8RAnWzAKDxOmneR6j6DBVyo5/CO1wRYngorQCgo9SJ sArBQqQStX7zIuYK3qo1El0= =C2FM -----END PGP SIGNATURE-----
In article <48BFE61F.8040509@restontech.com> you write:
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Robert Bonomi wrote:
One small data-point -- on a personal vanity domain, approximately 2/3 of all the spam (circa 15k junk emails/month) was 'direct to inbound MX' transmissions. The vast majority of this is coming from end-user machines outside of North America.
This confirms the limited data I have. I configure my edge firewall (pf) to drop anything to/from the Spamhaus DROP list, as well as sendmail to use their XBL. The DROP list seems like it blocks mostly MX lookups (nice to see the blocking of mail start so early in the process!), so it is hard to say how many SMTP connections never happen (remote server/bot does not know where to connect). The XBL list, which is mostly residential IPs around the world, seems to be the single most effective technique in blocking incoming traffic-- on port 25. Obviously, these connections are coming from ISPs that do *not* block egress TCP 25.
You do realise that there a mail clients that check MX records *before* submitting email (or before on sending the email) so that typos get detected in the client before any email is sent from the client. But you would never see those false positives. I know they exist because I've experienced them because I work from home and even though I tunnel email out via the office servers I prefer the typos to be caught locally. I doubt this will change your mind but it might stop someone else from making a bad decision to do what you are doing. Mark
Slightly off topic-- I found it quite easy to configure the DROP list to work with pf (or is that the other way around?). I would be happy to share the small Perl script that updates the pf table. When I configured the DROP list on a free public wireless system I maintain, I was amazed at how much egress traffic it blocked-- obviously rogue/bad/evil webservers, IRC hosts, etc.
I wonder if anyone else is using it that way?
... alec
- -- `____________ / Alec Berry \______________________________ | Senior Partner and Director of Technology \ | PGP/GPG key 0xE8E9030F | | http://alec.restontech.com/#PGP | |-------------------------------------------| | RestonTech, Ltd. | | http://www.restontech.com/ | | Phone: (703) 234-2914 | \___________________________________________/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
iD8DBQFIv+YdREO1P+jpAw8RAnWzAKDxOmneR6j6DBVyo5/CO1wRYngorQCgo9SJ sArBQqQStX7zIuYK3qo1El0= =C2FM -----END PGP SIGNATURE-----
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Mark Andrews wrote:
You do realise that there a mail clients that check MX records *before* submitting email (or before on sending the email) so that typos get detected in the client before any email is sent from the client.
I think you are not familiar with the difference between the DROP list and the XBL. The DROP list is *not* an RBL! I do not allow any traffic at all to or from the DROP list-- including MX lookups. I can't think of any good reasons why I would. The XBL is used only to block mail transport-- it is configured in sendmail, not at the firewall. The scenario you lay out will still work: - - end user on a dial up that happens to be on the XBL (common) - - end user queries MX records, either directly or via their name server - - end user submits mail to their SMTP server (not on the XBL) - - SMTP server transports mail to my system Unless one of those systems mentioned above is a hijacked name server in Kyiv (and thus on the DROP list), everything will work. ... alec - -- `____________ / Alec Berry \______________________________ | Senior Partner and Director of Technology \ | PGP/GPG key 0xE8E9030F | | http://alec.restontech.com/#PGP | |-------------------------------------------| | RestonTech, Ltd. | | http://www.restontech.com/ | | Phone: (703) 234-2914 | \___________________________________________/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIv/dTREO1P+jpAw8RAqiyAKDJt7FbFvplXB1JTe+dKDOOSXUijQCdH/cZ 4m4o9vE5FS96huARs2Rq5yU= =Paen -----END PGP SIGNATURE-----
participants (3)
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Alec Berry -
Mark Andrews -
Robert Bonomi