I sent that? One morning, I got a bounce message from my own mail sendmail server, saying that it failed to deliver a message I never sent. That’s red alert. It means that someone managed to provoke my mail server to send an outbound message. It’s red alert, because my mail server effectively relays spam [...]
A short one: What to do if unmount is impossible with a # umount /path/to/mount umount: /path/to/mount: target is busy but grepping the output of lsof for the said path yields nothing. In other words, the mount is busy, but no process can be blamed for accessing it (even as a home directory). If this [...]
Introduction Firejail is a lightweight security utility which ties the hands of running processes, somewhat like Apparmor and SELinux. However it takes the mission towards Linux kernel’s cgroups and namespaces. It’s in fact a bit of a container-style virtualization utility, which creates sandboxes for running specific programs: Instead of a container for an entire operating [...]
Introduction Still in 2020, it seems like Microsoft lives up to its reputation: Being arrogant, thinking that anyone in business must be a huge corporate, and in particular ending up completely ridiculous. Microsoft’s mail servers, which accept on behalf of Hotmail, MSN, Office 365, Outlook.com, or Live.com users are no exception. This also affects companies [...]
It started with an error message This post is a bit of a coredump of myself attempting to resolve a sudden web server failure. And even more important, understand why it happened (check on that) and try avoiding it from happening in the future (not as lucky there). I’ve noticed that there are many threads [...]
There are many ways to ask a Linux machine how it’s doing. I’ve collected a few of them, mostly for my own reference. I guess I’ll add more items as I run across new ones. General Info inxi -Fxxxz (neat output, but makes the system send me security “password required” alert mails because of attempts [...]
What’s this? These are somewhat random jots I made while setting up an authoritative BIND server, so that a simple VPS machine can function standalone. Well, almost standalone, as it takes some help from a slave DNS to supply the second DNS entry. But even if that slave goes away suddenly, the show will go [...]
But why? Fact number one: Running your own mail server is the most likely cause for messing up, and that can mean an intrusion to the server or just turning it into a public toilet for spam. Nevertheless, if mail delivery is important to you, there’s probably no way around. And I’m not talking about [...]
Introduction This is an explicit walkthrough on how a domain name is resolved. Doing the recursion manually, that is. And then some remarks on the mess with DNS glue records. Getting the root servers $ dig NS . ; <<>> DiG 9.11.3-1ubuntu1.2-Ubuntu <<>> NS . ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: [...]
Intro Whether you just want your non-Gmail personal email to get through, or you have a website that produces transactional emails (those sent by your site or web app), there’s a long fight with spam filters ahead. The war against unsolicited emails will probably go on as long as email is used, and it’s an [...]