The drawbacks of local packet captures

Probably the most common way of capturing network data is not a decision between SPAN or TAP – it is Wireshark simply being installed on one of the computers that need to be analyzed. While this an easy way to capture network packets it is also an easy way to get “wrong” results, because there are a lot of side effects when capturing packets directly on a computer. I discussed a lot of these side effects in my Sharkfest 2013 talk “PA-14: Top 5 False Positives” already, but let’s go check them out again.

IPv6 DHCP flood

A few days ago I took a capture for some reason and saw something unexpected that had nothing to do with what I wanted to check: there were tons and tons of DHCPv6 packets trying to renew an IPv6 address in a never ending stream of packets, and really fast, too.

A creative way of refusing connections

A few days ago, Olli, one of our team members, sent me a funny trace that he’d taken while configuring the security settings on a Netoptics Bypass kit. This device has an SNMP and HTTP management service, and when he disabled the HTTP service he verified if the setting was accepted (like you should). Usually, there would either be a Reset packet coming back when a SYN is sent, or no answer at all. To his surprise, the device behaved a little different than expected.