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2379
votes
30answers
498k views

Our security auditor is an idiot. How do I give him the information he wants?

A security auditor for our servers has demanded the following within two weeks: A list of current usernames and plain-text passwords for all user accounts on all servers A list of all password ...
1474
votes
3answers
1.5m views

What is a Pem file and how does it differ from other OpenSSL Generated Key File Formats?

I am responsible for maintaining two Debian servers. Every time I have to do anything with security certificates, I Google for tutorials and beat away until it finally works. However, in my searches ...
1087
votes
39answers
700k views

How can I sort du -h output by size

I need to get a list of human readable du output. However, du does not have a "sort by size" option, and piping to sort doesn't work with the human readable flag. For example, running: du | sort -...
801
votes
13answers
1.1m views

How to determine if a bash variable is empty?

What is the best way to determine if a variable in bash is empty ("")? I have heard that it is recommended that I do if [ "x$variable" = "x" ] Is that the correct way? (there must be something more ...
725
votes
17answers
2.3m views

Ping a Specific Port

Just a quick sanity check here. Can you ping a specific port of a machine, and if so, can you provide an example? I'm looking for something like ping ip address portNum.
629
votes
10answers
791k views

Can scp copy directories recursively?

Currently I can only copy a single .tar file. But how can I copy directories recursively with scp?
608
votes
13answers
142k views

How do I deal with a compromised server?

This is a Canonical Question about Server Security - Responding to Breach Events (Hacking) See Also: Tips for Securing a LAMP Server Reinstall after a Root Compromise? Canonical ...
548
votes
16answers
677k views

How do you find what process is holding a file open in Windows?

One thing that annoys me no end about Windows is the old sharing violation error. Often you can't identify what's holding it open. Usually it's just an editor or explorer just pointing to a relevant ...
524
votes
11answers
498k views

In Nginx, how can I rewrite all http requests to https while maintaining sub-domain?

I want to rewrite all http requests on my web server to be https requests, I started with the following: server { listen 80; location / { rewrite ^(.*) https://mysite.com$1 ...
505
votes
31answers
904k views

How do I tell Git for Windows where to find my private RSA key?

My Git setup runs fine on Linux, but when I try to set things up under Windows (using Git for Windows and TortoiseGit), I don't know where to put my private SSH key (or, better still, how to tell ssh ...
491
votes
10answers
1.3m views

How to automate SSH login with password?

How to automate SSH login with password? I'm configuring my test VM, so heavy security is not considered. SSH chosen for acceptable security with minimal configuration. ex) echo password | ssh id@...
472
votes
5answers
213k views

What is the difference between double and single square brackets in bash?

I just wondered what exactly the difference between [[ $STRING != foo ]] and [ $STRING != foo ] is, apart from that the latter is posix-compliant, found in sh and the former is an extension found ...
459
votes
2answers
215k views

What exactly do the colors in htop status bars mean?

By default, htop shows colored status bars for processors, memory, and swap. From left to right, the bars are colored green, blue, yellow, and red depending on some thresholds. What does it mean when ...
453
votes
7answers
74k views

Something is burning in the server room; how can I quickly identify what it is?

The other day, we notice a terrible burning smell coming out of the server room. Long story short, it ended up being one of the battery modules that was burning up in the UPS unit, but it took a good ...
449
votes
8answers
163k views

How does IPv4 Subnetting Work?

This is a Canonical Question about IPv4 Subnets. Related: How does IPv6 subnetting work and how does it differ from IPv4 subnetting? How does Subnetting Work, and How do you do it by ...
437
votes
6answers
328k views

ssh returns “Bad owner or permissions on ~/.ssh/config”

When I try to ssh to another box, I get this strange error $ ssh hostname Bad owner or permissions on ~/.ssh/config But I made sure that I own and have rw permissions on the file: ls -la ~/.ssh/ ...
414
votes
5answers
954k views

What's the default superuser username/password for postgres after a new install?

I have just installed postgres 8.4 on Ubuntu 9.10 and it has never asked me to create a superuser. Is there a default superuser and its password? If not, how do I create a new one?
369
votes
21answers
61k views

How do you search for backdoors from the previous IT person?

We all know it happens. A bitter old IT guy leaves a backdoor into the system and network in order to have fun with the new guys and show the company how bad things are without him. I've never ...
363
votes
5answers
149k views

Anyone else experiencing high rates of Linux server crashes during a leap second day?

*NOTE: if your server still has issues due to confused kernels, and you can't reboot - the simplest solution proposed with gnu date installed on your system is: date -s now. This will reset the ...
336
votes
27answers
313k views

How can I get the size of an Amazon S3 bucket?

I'd like to graph the size (in bytes, and # of items) of an Amazon S3 bucket and am looking for an efficient way to get the data. The s3cmd tools provide a way to get the total file size using s3cmd ...
336
votes
20answers
314k views

How to run a server on port 80 as a normal user on Linux?

I have Googled about a solution for quite some time, but couldn't find an answer. I am on Ubuntu Linux and want to run a server on port 80, but due to security mechanism of Ubuntu, I get the ...
329
votes
6answers
430k views

What permissions should my website files/folders have on a Linux webserver?

This is a Canonical Question about File Permissions on a Linux web server. I have a Linux web server running Apache2 that hosts several websites. Each website has its own folder in /var/www/. /var/...
318
votes
2answers
222k views

How do I change my private key passphrase?

I have an existing public/private key pair. The private key is password protected, and the encryption may be either RSA or DSA. These keys are the kind you generate with ssh-keygen and generally store ...
314
votes
27answers
65k views

100% uptime for a web application

We received an interesting "requirement" from a client today. They want 100% uptime with off-site failover on a web application. From our web application's viewpoint, this isn't an issue. It was ...
281
votes
160answers
294k views

Useful Command-line Commands on Windows

The aim for this Wiki is to promote using a command to open up commonly used applications without having to go through many mouse clicks - thus saving time on monitoring and troubleshooting Windows ...
276
votes
45answers
67k views

What are the obstacles to providing reliable Internet access and Wi-Fi at large tech conferences? [closed]

Every tech conference I've ever been to, and I've been to a lot, has had absolutely abysmal Wi-Fi and Internet access. Sometimes it's the DHCP server running out of addresses. Sometimes the backhaul ...
276
votes
4answers
155k views

How do I select which Apache MPM to use?

This is a Canonical Question about selecting the right Apache httpd MPM. I'm a little confused between the different MPMs offered by Apache - 'worker', 'event', 'prefork', etc. What are the major ...
272
votes
8answers
546k views

How to bind MySQL server to more than one IP address?

Is there a secret way to bind MySQL to more than one IP address? As far as I can see the bind-address parameter in the my.cnf does not support more than one IP and you can't have it more than once.
270
votes
18answers
374k views

Can I automatically add a new host to known_hosts?

Here's my situation: I'm setting up a test harness that will, from a central client, launch a number of virtual machine instances and then execute commands on them via ssh. The virtual machines will ...
270
votes
6answers
303k views

How to assign permissions to ApplicationPoolIdentity account

In IIS 7 on Windows Server 2008, application pools can be run as the "ApplicationPoolIdentity" account instead of the NetworkService account. How do I assign permissions to this "...
268
votes
5answers
101k views

Redirect, Change URLs or Redirect HTTP to HTTPS in Apache - Everything You Ever Wanted to Know About Mod_Rewrite Rules but Were Afraid to Ask

This is a Canonical Question about Apache's mod_rewrite. Changing a request URL or redirecting users to a different URL than the one they originally requested is done using mod_rewrite. This includes ...
267
votes
15answers
25k views

I've inherited a rat's nest of cabling. What now?

You know, you see pictures like below and sort of chuckle until you actually have to deal with it. I have just inherited something that looks like the picture below. The culture of the organization ...
266
votes
17answers
234k views

Showing total progress in rsync: is it possible?

I have searched for this option already, but have only found solutions that involve custom patching. The fact that it does not show in --help and no more info can be found probably indicates the ...
263
votes
16answers
379k views

How do I get the current Unix time in milliseconds in Bash?

How do I get the current Unix time in milliseconds (i.e number of milliseconds since Unix epoch January 1 1970)?
263
votes
13answers
87k views

Can I nohup/screen an already-started process?

I'm doing some test-runs of long-running data migration scripts, over SSH. Let's say I start running a script around 4 PM; now, 6 PM rolls around, and I'm cursing myself for not doing this all in ...
262
votes
12answers
804k views

What's the command-line utility in Windows to do a reverse DNS look-up?

Is there a built-in command line tool that will do reverse DNS look-ups in Windows? I.e., something like <toolname> w.x.y.z => mycomputername I've tried: nslookup: seems to be forward look-...
258
votes
2answers
74k views

Why is “chmod -R 777 /” destructive?

This is a Canonical Question about File Permission and Why 777 is "destructive". I'm not asking how to fix this problem, as there are a ton of references of that already on Server Fault (reinstall OS)...
256
votes
6answers
490k views

Why is my crontab not working, and how can I troubleshoot it?

This is a Canonical Question about using cron & crontab. You have been directed here because the community is fairly sure that the answer to your question can be found below. If your question is ...
255
votes
18answers
314k views

Copying a large directory tree locally? cp or rsync?

I have to copy a large directory tree, about 1.8 TB. It's all local. Out of habit I'd use rsync, however I wonder if there's much point, and if I should rather use cp. I'm worried about permissions ...
250
votes
12answers
99k views

Is it bad to redirect http to https?

I just installed an SSL Certificate on my server. It then set up a redirect for all traffic on my domain on Port 80 to redirect it to Port 443. In other words, all my http://example.com traffic is ...
247
votes
14answers
33k views

What is this very short power cable called?

I have a couple of networking components in my rack that take giant AC adapters ("power bricks") that don't fit neatly into my rackmount PDU. I have one "thingy" that is shown below, and I need to ...
246
votes
7answers
53k views

What’s the point in having “www” in a URL?

Other than for historical reasons, is there is reason to have “www” in a URL? Should I create a permanent redirect from www.xyz.com to xyz.com, or from xyz.com to www.xyz.com? Which one would you ...
245
votes
5answers
267k views

What's the difference between IP address 0.0.0.0 and 127.0.0.1?

I know that 127.0.0.1 ~ 127.255.255.254 are the loopback IP addresses for most modern operating systems, and these IP addresses can be used to refer to our own computer. But what's 0.0.0.0? It seems ...
242
votes
10answers
337k views

Displaying a remote SSL certificate details using CLI tools

In Chrome, clicking on the green HTTPS lock icon opens a window with the certificate details: When I tried the same with cURL, I got only some of the information: $ curl -vvI https://gnupg.org * ...
241
votes
4answers
81k views

Why do I need Nginx and something like Gunicorn?

I'm looking for an overly simplified answer to the following question. I'm trying to build a foundational understanding of how Nginx works alongside something like Gunicorn. Do I need both Nginx and ...
239
votes
13answers
140k views

Environment variables of a running process on Unix?

I need to troubleshoot some problems related to environment variables on a Unix system. On Windows, I can use a tool such as ProcessExplorer to select particular a process and view values of each ...
239
votes
4answers
149k views

How can I see which flags Nginx was compiled with?

How can I check what modules have been added to an nginx installation?
239
votes
3answers
165k views

Moving an already-running process to Screen

My office job routinely sees me connected to a Linux box via VNC. Sometimes I start a remote job on the console, and realize later that it runs much longer than expected. (Should have started that ...
238
votes
10answers
142k views

How to tell which local branch is tracking which remote branch in Git?

I would like to know if there's a way to tell which local branch is tracking which remote branch in Git. I'm using one remote server, which is named "origin".
238
votes
15answers
376k views

Finding out what user Apache is running as?

I want to secure a file upload directory on my server as described beautifully here, but I have one problem before I can follow these instructions. I don't know what user Apache is running as. I've ...

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