Security Basics mailing list archives
RE: PortFast Question
From: "Steve Fletcher" <safletcher () insightbb com>
Date: Mon, 27 Sep 2004 13:22:57 -0500
Actually, it sounds like the problem is that portfast is NOT enabled on. The reason that some devices have problems when this is not enabled is due to the system expecting the network connection up sooner. With portfast disabled, the switch will check for a loop BEFORE it enables the port. This increases the time before the port is enabled, which causes problems with some devices. When portfast IS enabled, the port is turned on and the device is allowed access to the network, while in the background switch continues to check for loops. If a loop is then found, the port is shutdown. Also, because of the way portfast works, it IS possible to connect another switch into a port with portfast enabled. However, it is NOT RECOMMENDED. If a switch is connected to a portfast enabled port and a loop is created, this will cause problems on the network until the loop is detected and the offending port is shutdown. Therefore, any port that a switch is connected to should have portfast disabled. As for security issues, I don't know of any security issues that should arise from the use of portfast. The only possibility is for a limited time Denial of Service attack by creating a loop, but once the switch discovers the loop, it will take care of itself. Now, disabling spanning tree altogether is another story. Doing that would allow someone to create a loop that will last until you can find it and shutdown the offending port. Hope this helps. Steve Fletcher MCSE (NT4/Win2k), MCSE: Security (Win2k), HP Master ASE, CCNA, Security+ safletcher () insightbb com -----Original Message----- From: Stephen W. Corey - 5535 [mailto:swc () wardandsmith com] Sent: Monday, September 27, 2004 7:03 AM To: security-basics () securityfocus com Subject: RE: PortFast Question We run portfast on all Catalyst ports that connect to a "non-switch" device, like PCs, servers, routers, etc. From what I saw, it works by not listening for MAC addresses as long before going to "active" state. I have never heard of any security issues by doing this. I believe Cisco still recommends this mode for optimum performance. You can always use Nessus (or some other up to date vuln scanner) to see if anything can be exploited. As for why it happens, here's my thought. Because it's speeding up a "natural" switch port process, weird things can happen. Depending on how the device (i.e. PC hardware) acts on layer 2, it may need the "full" startup procedure to be run. To me, portfast is a non-standard shortcut, and it may not work in every situation. As you probably read, you can't plug a portfast port into a switch, so there could easily be other devices it's incompatible with (Cisco can't test everything). -----Original Message----- From: Josh Sukol [mailto:secnews () gmail com] Sent: Friday, September 24, 2004 10:05 AM To: security-basics () securityfocus com Subject: PortFast Question I am running a small network using four Cisco Catalyst 2950 switches. I am in the process of configuring a new software package that uses some proprietary hardware that connects to the network via Ethernet. When plugged into the network the device would connect for a minute or two and than connectivity would drop (i.e. ping would fail, and the light on the switch would turn from green to amber) This pattern continued for as long as the device was plugged into the network. The cabling was checked and tested with other equipment and there were no other problems. After trying several other things I eventually started changing the ethernet port settings on the switch itself and found that by enabling portfast the device functioned fine. I have found very little information about port fast security issues. I was able to find and did read up on PortFast BPDU guard and potential DoS using malformed packets. Are there any other security issues that effect me enabling Portfast on specific ports that connect back to a single device? Are there any other ways to solve this problem that might allow me to sidestep this potential security issues all together? - Slightly Off Topic - If anyone knows why this behavior occurs and why enabling portfast fixes the connectivity issue I would be very interested to a hear an explanation. Thanks in advance for the wisdom! ------------------------------------------------------------------------ --- Computer Forensics Training at the InfoSec Institute. All of our class sizes are guaranteed to be 12 students or less to facilitate one-on-one interaction with one of our expert instructors. Gain the in-demand skills of a certified computer examiner, learn to recover trace data left behind by fraud, theft, and cybercrime perpetrators. Discover the source of computer crime and abuse so that it never happens again. http://www.infosecinstitute.com/courses/computer_forensics_training.html ------------------------------------------------------------------------ ----
Current thread:
- PortFast Question Josh Sukol (Sep 24)
- Re: PortFast Question John R. Morris (Sep 28)
- <Possible follow-ups>
- RE: PortFast Question Stephen W. Corey - 5535 (Sep 27)
- RE: PortFast Question Steve Fletcher (Sep 28)
- RE: PortFast Question LordInfidel (Sep 28)
- RE: PortFast Question JGrimshaw (Sep 28)
- Re: PortFast Question Maarten Claes (Sep 29)
- Re: PortFast Question Sec News (Sep 28)
- Re: PortFast Question Chris Moody (Sep 30)
- RE: PortFast Question David Gillett (Sep 29)
- RE: PortFast Question JGrimshaw (Sep 28)
- RE: PortFast Question Scherer, Brian (Sep 28)
- RE: PortFast Question LordInfidel (Sep 29)