-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Zhiyun Qian wrote:
Hi all,
What is the common practice for enforcing port blocking policy (or what is the common practice for you and your ISP)? More specifically, when ISPs try to block certain outgoing port (port 25 for instance), they could do two rules: 1). For any outgoing traffic, if the destination port is 25, then drop the packets. 2). For any incoming traffic, if the source port is 25, then drop the packets.
Note that either of the rule would be able to block outgoing port 25 traffic since each rule essentially represent one direction in a TCP flow. Of course, they could apply both rules. However, based on our measurement study, it looks like most of the ISPs are only using rule 1). Is there any particular reason why rule 1) instead of rule 2)? Or maybe both?
Also, is it common that the rules are based on tcp flags (e.g. SYN, SYN-ACK)? One would think block SYN packet is good enough.
Regards. -Zhiyun
I understand your question, and I believe that you have been given a lot of good answers. However, I believe that, as an ISP, you are asking the wrong question; more precisely, you are only asking part of the real question you should be asking. The more appropriate question should be: "What should be our network filtering policies?" To answer that question, I would start with ingress and egress filtering by IP address, protocol, etc.: 1) Never allow traffic to egress any subnet unless its source IP address is within that subnet range. 2) Never allow traffic to egress any subnet, if that traffic claims to originate from the subnet's network number or broadcast address. 3) Never allow traffic to ingress any subnet, if that traffic is directed to the subnet's network number or broadcast address. 4) Never allow traffic to ingress any network if the source address is bogus. 5) Never allow traffic to ingress or egress any network if it has an protocol not "supported" by your network (e.g., allow only TCP, UDP, ICMP, ESP, AH, GRE, etc.). 6) Never allow traffic to ingress or egress any network if it has an invalid TCP flags configuration. These are the rules I can think of off the top of my head without looking at an actual hardened router. I am sure I am missing some, but these are a good start. My $0.02 worth. Jon - -- Jon R. Kibler Chief Technical Officer Advanced Systems Engineering Technology, Inc. Charleston, SC USA o: 843-849-8214 c: 843-813-2924 s: 843-564-4224 s: JonRKibler e: Jon.Kibler@aset.com e: Jon.R.Kibler@gmail.com http://www.linkedin.com/in/jonrkibler My PGP Fingerprint is: BAA2 1F2C 5543 5D25 4636 A392 515C 5045 CF39 4253 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.8 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkrhFp0ACgkQUVxQRc85QlOYlgCggttgagm2sb90Vg7ntEreFtLr ydAAnjG4zEmkTmLuZpWUey9nNRHZiTLs =VDEG -----END PGP SIGNATURE----- ================================================== Filtered by: TRUSTEM.COM's Email Filtering Service http://www.trustem.com/ No Spam. No Viruses. Just Good Clean Email.