If there is a DHCP server available, that server responds. When a new host that is configured to use DHCP is brought online, it first asks for an IP via a broadcast (a request sent out that all active devices with an IP on that subnet will see). I'll spare you the background on how DHCP works and summarize a bit. We're going to use DHCP as a good test because it's easy to follow, and readily available to me as I write this. It's called packet sniffing, or packet capture.įirst, let's determine a test service and then look at what a packet capture from that service can tell us.
Fortunately, there is a way to snoop on that traffic and see what's inside of each and every packet. What you'd like to see is exactly what traffic is passing, and what it is saying. Simply knowing that traffic can pass isn't always enough.
On top of that, if you're in a secure, or segmented network, you may run into blocked ports. Wouldn't it be great to figure out where a request is getting bogged down? You can get some of this information in other ways, but actually seeing what's going down the wire can be immensely useful. Name lookups and other dependent services need to function correctly. One service reaches out to another to get some bit of information, then that one reaches out to something else, and so on. Stick around, and we'll talk about a few ways you can snoop on what's running across the wire, or up and down your application stack. Beyond verifying the basics, connectivity, ports, and running services, sometimes you just need to see what's going through the network. You're in the middle of troubleshooting some weird network or service issue, things just aren't right, the ports are open, the service is listening, but something is still off. Linux system administration skills assessment.A guide to installing applications on Linux.Download RHEL 9 at no charge through the Red Hat Developer program.
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