Linux · Network Diagnostics

You configured a mail server on a remote host and want to confirm it is actually accepting connections on port 25 — without sending mail. Which old command still earns its keep here?

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More in Network Diagnostics

You want a quick look at the IP addresses under which a Linux box is reachable, but plain hostname only prints the name. Which invocation filters out the relevant addresses, and what is easy to type wrong?

In ip link output the first interface is always lo. What is it, and why does it appear on a machine with no network hardware at all?

Modern Linux names an Ethernet port something like enp0s31f6 instead of the old eth0. What does the ugly name buy you that eth0 didn't?

In ip link output each interface has a 12-hex-digit value after link/ether. What is it and what does it identify?

ip addr on a working interface shows one global IPv6 plus an address starting with fe80. Why can't you reach the box from the internet on the fe80 one?

You run ping with just a target and it never stops. What is its default cadence, and how do you cap it at two probes?

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