Greatest Hits

705 votes
9 answers
2.1m 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@...
  • 10k
1752 votes
3 answers
2.0m 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 ...
801 votes
18 answers
2.7m 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.
  • 8,135
1377 votes
39 answers
1.1m 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 -...
  • 17.1k
765 votes
9 answers
1.0m views

Can scp copy directories recursively?

Currently I can only copy a single .tar file. But how can I copy directories recursively with scp?
  • 8,361
2455 votes
31 answers
520k 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 ...
  • 24.1k
947 votes
15 answers
1.3m 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 ...
  • 22.5k
549 votes
32 answers
1.1m 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 ...
469 votes
5 answers
1.2m 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?
  • 6,141
153 votes
9 answers
1.2m 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 ...
user avatar
327 votes
5 answers
861k 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 ...
  • 3,373
561 votes
6 answers
485k 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/ ...
  • 14.5k
460 votes
22 answers
312k 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, ...
  • 5,679
69 votes
11 answers
211k views

How do I check if Log4j is installed on my server?

I have read about security vulnerabilities related to Log4j. How do I check if Log4j is installed on my server? My specific servers use Ubuntu 18.04.6 LTS. I have installed many third-party packages ...
  • 841
238 votes
5 answers
1.2m 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=...
  • 5,780
619 votes
18 answers
867k 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 ...
  • 9,849
283 votes
7 answers
1.3m 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?
385 votes
27 answers
433k 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 ...
187 votes
9 answers
1.1m 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.
  • 1,993
183 votes
6 answers
1.3m views

What port does SFTP use?

Does SFTP use port 21 or port 22?
  • 3,751
148 votes
2 answers
925k 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 ?
  • 115k
369 votes
7 answers
848k 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 ...
206 votes
11 answers
586k 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! @ @@@@@...
61 votes
14 answers
403k views

What causes SSH error: kex_exchange_identification: Connection closed by remote host?

I setup a SSH server online that is publicly accessible by anyone. Therefore, I get a lot of connections from IPs all over the world. Weirdly, none actually try to authenticate to open a session. I ...
  • 713
551 votes
11 answers
569k 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 ...
  • 8,362
122 votes
13 answers
1.1m 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?
  • 82.8k
296 votes
5 answers
629k 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; ...
  • 3,125
390 votes
14 answers
655k 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 * ...
  • 12.8k
252 votes
11 answers
529k 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 ...
  • 2,663
125 votes
35 answers
458k views

SSH Suddenly returning Invalid format

So a while ago I set up a server on AWS, and used their generated SSH key. I saved the key to Lastpass, and have successfully retrieved it from there before, and got it working. However, after trying ...
  • 1,353
300 votes
14 answers
957k 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-...
  • 3,135
76 votes
15 answers
628k 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 ...
  • 1,924
152 votes
5 answers
241k views

What is the difference between authentication and authorization?

Basic question from a novice: What is the difference between authentication and authorization?
user avatar
58 votes
8 answers
631k 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 ...
  • 685
281 votes
15 answers
654k 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 ...
  • 3,073
190 votes
6 answers
657k 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 ...
  • 6,757
146 votes
3 answers
188k 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 ...
  • 1,463
205 votes
13 answers
524k 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 ...
  • 5,115
273 votes
15 answers
501k 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 ...
user avatar
123 votes
4 answers
991k 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?
25 votes
9 answers
315k 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 "...
  • 1,924
228 votes
17 answers
716k views

Permission denied (publickey). SSH from local Ubuntu to Amazon EC2 server

I have an instance of an application running in the cloud on an Amazon EC2 instance, and I need to connect to it from my local Ubuntu. It works fine on one local ubuntu and also laptop. I got this ...
  • 2,421
241 votes
8 answers
844k views

How do I add Access-Control-Allow-Origin in NGINX?

How do I set the Access-Control-Allow-Origin header so I can use web-fonts from my subdomain on my main domain? Notes: You'll find examples of this and other headers for most HTTP servers in the ...
  • 3,319
192 votes
9 answers
903k views

How to check sshd log?

I have Ubuntu 9.10 installed with sshd and I can successfully connect to it using login and password. I have configured an RSA key login and now have "Server refused our key" as expected. Ok, now I ...
  • 3,475
561 votes
3 answers
304k 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 ...
  • 6,051
90 votes
6 answers
755k 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 ...
  • 1,607
366 votes
6 answers
542k 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/...
  • 13.2k
27 votes
4 answers
220k 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 ...
  • 32.5k
345 votes
18 answers
543k 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)?
  • 3,732
161 votes
4 answers
214k views

Getting "Cannot ioctl TUNSETIFF tun: Operation not permitted" when trying to connect to OpenVPN

I'm trying to setup an OpenVPN Access Server in AWS using the market place AMI, but I;m struggling to connect to it. The access server is up and running. I've also added a user with Auto-Login and ...