Greatest Hits

1559
votes
3answers
1.6m 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 ...
750
votes
18answers
2.5m 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.
2111
543
votes
10answers
1.5m 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@...
2403
votes
30answers
504k 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 ...
670
votes
10answers
841k views

Can scp copy directories recursively?

Currently I can only copy a single .tar file. But how can I copy directories recursively with scp?
1558
1144
votes
39answers
769k 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 -...
1719
831
votes
13answers
1.2m 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 ...
515
votes
31answers
945k 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 ...
438
votes
5answers
1.0m 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?
145
votes
9answers
1.1m views

Command line to list users in a Windows Active Directory group?

Is there a command line way to list all the users in a particular Active Directory group? I can see who is in the group by going to Manage Computer --> Local User / Groups --> Groups and double ...
480
votes
6answers
371k 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/ ...
264
votes
5answers
671k views

How to send ctrl+alt+del using Remote Desktop?

How can I send ctrl+alt+del to a remote computer over Remote Desktop? For example, if I wanted to change the local admin password on a remote PC using a Remote Desktop connection, it would be helpful ...
192
votes
5answers
995k views

Can you pass user/pass for HTTP Basic Authentication in URL parameters?

I believe this is not possible, but someone I know insisted that it works. I don't even know what parameters to try, and I haven't found this documented anywhere. I tried http://myserver.com/~user=...
128
votes
2answers
775k views

What causes the 'Connection Refused' message?

This is a Canonical Question about Connection Refused We see a lot of questions to the effect When I try to connect to a system I get a message Connection refused Why is this ?
176
votes
6answers
1.2m views

What port does SFTP use?

Does SFTP use port 21 or port 22?
595
244
votes
7answers
1.1m views

Check if port is open or closed on a Linux server?

How can I check if a port is listening on a Linux server?
572
votes
16answers
721k 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 ...
349
votes
27answers
336k 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 ...
158
votes
8answers
884k views

Resolve host name from IP address

I'm looking for a command line tool which gets an IP address and returns the host name, for Windows.
533
votes
11answers
513k 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 ...
153
votes
5answers
239k views

What is the difference between authentication and authorization?

Basic question from a novice: What is the difference between authentication and authorization?
286
votes
14answers
170k views

mysqldump throws: Unknown table 'COLUMN_STATISTICS' in information_schema (1109)

Every time I try to make a mysqldump I get the following error: $> mysqldump --single-transaction --host host -u user -p db > db.sql mysqldump: Couldn't execute 'SELECT COLUMN_NAME, ...
273
votes
14answers
834k 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-...
58
votes
8answers
552k views

What does “TTL expired in transit” mean on a ping attempt?

We get the message “TTL expired in transit” when we try to ping to a server in a different network segment. When we run tracert, 4 ip addresses repeat themselves indefinitely: 14 60 ms 59 ms ...
146
votes
3answers
183k views

How can I upgrade to Java 1.8 on an Amazon Linux Server?

I tried sudo yum update but it just keeps java "1.7.0_75". I need 1.8 for it to work with another application but can't figure out how to upgrade it. Do I need to manually install it somehow? There's ...
105
votes
11answers
800k views

How do I force sync the time on Windows Workstation or Server?

What is the command to sync a Windows workstation or server to its configured time source?
203
votes
10answers
349k views

Docker Container time & timezone (will not reflect changes)

Where do Docker containers get their time information? I've created some containers from the basic ubuntu:trusty image, and when I run it and request 'date', I get UTC time. For awhile I got around ...
173
votes
11answers
448k views

“Add correct host key in known_hosts” / multiple ssh host keys per hostname?

Trying to ssh into a computer I control, I'm getting the familiar message: @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ @@@@@...
65
votes
14answers
475k views

Get list of AD groups a user is a member of

Suppose I have the user id of a user in Active Directory. I'd like to get a list of all AD groups in which that user is currently a member of. How can I do this from the Windows command line? I've ...
25
votes
9answers
283k views

Span on multiple monitors when logged into a Citrix session via Citrix Receiver (or Citrix server)

I am using the Citrix Receiver in Windows 8, and on Windows XP on another PC. I often log into work from home and am unable to span the session to dual monitors like one can easily do when using "...
225
votes
16answers
690k views

Why do I get “Permission denied (publickey)” when trying to SSH from local Ubuntu to a Amazon EC2 server?

I have an instance of an application running in the cloud on Amazon EC2 instance, and I need to connect it from my local Ubuntu. It works fine on one of local ubuntu and also laptop. I got message "...
283
votes
6answers
559k 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 ...
505
114
votes
4answers
897k views

How to check if a port is blocked on a Windows machine?

On the Windows platform, what native options to I have to check if a port (3306, for example) on my local machine (as in localhost), is being blocked?
243
votes
13answers
500k views

How to Unban an IP properly with Fail2Ban

I'm using Fail2Ban on a server and I'm wondering how to unban an IP properly. I know I can work with IPTables directly: iptables -D fail2ban-ssh <number> But is there not a way to do it with ...
88
votes
6answers
715k views

How to determine the hostname from an IP address in a Windows network?

My LAN has 50 Windows hosts. At the Windows command line I try ping to get the IP address of a running Windows machine. The question is how to get hostname of a specific IP address in the same ...
27
votes
4answers
218k views

Outlook 2013: how to reset to “first run” status?

TL;DR Is there a way via script, powershell, reg delete, via telekinesis, whatever to reset Outlook 2013 as if no profiles ever existed and it was running for the first time ever? Still working ...
210
votes
5answers
422k views

Nginx reverse proxy + URL rewrite

Nginx is running on port 80, and I'm using it to reverse proxy URLs with path /foo to port 3200 this way: location /foo { proxy_pass http://localhost:3200; ...
172
votes
11answers
413k views

hosts file ignored, how to troubleshoot?

The hosts file on Windows computers is used to bind certain name strings to specific IP addresses to override other name resolution methods. Often, one decides to change the hosts file, and discovers ...
151
votes
2answers
320k views

What is Active Directory Domain Services and how does it work?

This is a Canonical Question about Active Directory Domain Services (AD DS). What is Active Directory? What does it do and how does it work? How is Active Directory organized: Forest, Child Domain, ...
153
votes
5answers
484k views

VirtualBox: How to set up networking so both host and guest can access internet and talk to each other

I was wondering if someone could give me a simple guide on how to set up virtual networking in VirtualBox (4.0.2) so that the following scenarios work: Both Host and Guest can access the Internet ...
254
votes
15answers
405k 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 ...
280
votes
11answers
400k 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 * ...
230
votes
7answers
302k views

How to force or redirect to SSL in nginx?

I have a signup page on a subdomain like: https://signup.example.com It should only be accessible via HTTPS but I'm worried people might somehow stumble upon it via HTTP and get a 404. My html/...
337
votes
6answers
459k 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/...
94
votes
5answers
508k views

Windows Server restart / shutdown history

How can I easily see a history of every time my Windows Server has restarted or shutdown and the reason why, including user-initiated, system-initiated, and system crashed? The Windows Event Log is ...
242
votes
6answers
418k views

How can I find out what AD groups I'm a member of?

I'm running a Windows XP desktop in a corporate environment. How can I find out what AD groups I belong to?
136
votes
9answers
809k views

What port should I open to allow remote desktop?

What port(s) should I open/NAT to allow me to use Remote Desktop?
483
votes
3answers
234k 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 ...
284
votes
20answers
400k 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 ...
117
votes
8answers
598k views

How can I figure out my LDAP connection string?

We're on a corporate network thats running active directory and we'd like to test out some LDAP stuff (active directory membership provider, actually) and so far, none of us can figure out what our ...

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