62
votes
11answers
19k views

Why drop caches in Linux?

In our servers we have a habit of dropping caches at midnight. sync; echo 3 > /proc/sys/vm/drop_caches When I run the code it seems to free up lots of RAM, but do I really need to do that. ...
62
votes
3answers
4k views

Rsync triggered Linux OOM killer on a single 50 GB file

I have a single 50 GB file on server_A, and I'm copying it to server_B. I run server_A$ rsync --partial --progress --inplace --append-verify 50GB_file root@server_B:50GB_file Server_B has ...
62
votes
4answers
81k views

Running Cron every 2 hours [duplicate]

I have the cron job as shown below, and wanted it to run every 2 hours, but it keeps running every 2 minutes. Can someone tell me where I'm going wrong? * */2 * * * /path-to-script
62
votes
18answers
68k views

Why hasn't rsync caught on in the Windows world? [closed]

It seems that rsync is the de-facto standard for efficient file backup and sync in Unix/Linux. Does anyone have any thoughts on why it wouldn't have caught on in the Windows world? Why hasn't it ...
62
votes
11answers
155k views

How do I find if there is a rogue DHCP server on my Network?

What's the best approach towards determining if I have a rogue DHCP server inside my network? I'm wondering how most admins approach these kinds of problems. I found DHCP Probe through searching, ...
62
votes
3answers
33k views

What is the difference between Load Balancer and Reverse Proxy?

I'm not clear about the difference between Load Balancer and Reverse Proxy. They both seems having same behavior: distributing incoming requests to backend servers.
62
votes
3answers
19k views

Can I create a top level TLD? (.michael for instance)

It appears that ICANN is allowing the creation of top level domains. Instead of 'registering' a domain, you would essentially be signing up to be a registrar (you'd be giving out registrations on ...
61
votes
6answers
50k views

Is X.Y.Z.0 a valid IP address?

Are IP addresses with a 0 in the last octet valid? 10.6.43.0 In my case, I have the the following netmask 255.255.252.0 What about a 0 for the other octets?
61
votes
10answers
87k views

How to tell if a Linux system is big endian or little endian?

I know that certain processors are Big Endian and others are Little Endian. But is there a command, bash script, python script or series of commands that can be used at the command line to determine ...
61
votes
7answers
6k views

DNS failing to propagate worldwide

I haven't changed anything related to the DNS entry for serverfault.com, but some users were reporting today that the serverfault.com DNS fails to resolve for them. I ran a justping query and I can ...
61
votes
6answers
22k views

Why use Chef/Puppet over shell scripts?

New to Puppet and Chef tools. Seems like the job that they are doing can be done with shell scripting. Maybe it was done in shell scripts until these came along. I would agree they are more readable. ...
61
votes
13answers
86k views

What's the best way to check if a volume is mounted in a Bash script?

What's the best way to check if a volume is mounted in a Bash script? What I'd really like is a method that I can use like this: if <something is mounted at /mnt/foo> then <Do some ...
61
votes
2answers
5k views

How should an IT department choose a standard Linux distribution?

There is a lot of community feeling about what Linux distributions are appropriate for production server environments and which aren't, however, a lot of this feeling seems religiously based, and ...
61
votes
5answers
19k views

Active Directory explained

If you had to explain Active Directory to someone how would you explain it?
61
votes
2answers
85k views

SFTP logging: is there a way?

I'm wondering if there is a way to log commands received by the server. It can be all SSH commands, as long as it includes information on commands related to file transfer. I'm having issues with an ...
61
votes
5answers
14k views

How can I implement ansible with per-host passwords, securely?

I would like to use ansible to manage a group of existing servers. I have created an ansible_hosts file, and tested successfully (with the -K option) with commands that only target a single host ...
61
votes
12answers
95k views

Subversion error: (405 Method Not Allowed) in response to MKCOL

I am getting the following error while trying to commit a new directory addition. svn: Commit failed (details follow): svn: Server sent unexpected return value (405 Method Not Allowed) in response to ...
61
votes
2answers
16k views

Why is it so difficult to upgrade between major versions of Red Hat and CentOS?

"Can we upgrade our existing production EL5 servers to EL6?" A simple-sounding request from two customers with completely different environments prompted my usual best-practices answer of "yes, but ...
60
votes
11answers
107k views

IIS 7.5 (Windows 7) - HTTP Error 401.3 - Unauthorized

I'm trying to test my ASP.Net website on localhost and I'm getting this error: HTTP Error 401.3 - Unauthorized You do not have permission to view this directory or page because of the access control ...
60
votes
22answers
10k views

How the heck is http://to./ a valid domain name?

Apparently it's a URL shortener. It resolves just fine in Chrome and Firefox. How is this a valid top-level domain? Update: for the people saying it's browser shenanigans, why is it that: ...
60
votes
19answers
20k views

How do you document a network?

I'm not sure how to ask this question, since I'm not in the field. Say you're a network admin and you leave your job. How does the new guy know where to start?
60
votes
2answers
18k views

Colors in bash after piping through less?

When I have dircolors defined life is full of... color. When I pipe ls through less to scroll around I lose the colors. Any suggestions?
60
votes
6answers
14k views

Why can't MX records point to an IP address?

I understand you should not point a MX record at an IP address directly, but should instead point it to an A record, which, in turns, points to the IP address of your mail server. But, in principle, ...
60
votes
2answers
4k views

Does memtest86+ test memory which is used to run itself?

One of the common server failure scenarios is bad DRAM, sometimes even when ECC memory is used. memtest86+ is one of the most useful tools to diagnose DRAM problems. As it loads itself at the start ...
60
votes
10answers
17k views

Unique Features of bash compared to zsh

I have been a zsh user for quite some time (before that tcsh and before that csh). I am quite happy with it, but was wondering if there are any compelling features of bash that do not exist in zsh. ...
60
votes
3answers
410k 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?
59
votes
10answers
4k views

How to extinguish a small fire in a server rack to minimize damage to surrounding equipment?

Suppose I have a rack with several servers and other stuff. One of servers overheats severely and either starts smoking or catches fire while there's a serviceman nearby. If anything similar happens ...
59
votes
4answers
243k 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 ...
59
votes
4answers
31k views

How do I edit git's history to correct an incorrect email address/name [closed]

When I started using git I just did a git init and started calling add and commit. Now I am starting to pay attention and I can see that my commits are showing up as cowens@localmachine, rather than ...
59
votes
7answers
8k views

What are the drawbacks of running a database inside a virtual machine? How do I overcome them? [closed]

Running anything inside a virtual machine will have some level of performance hit, but how much does it really impact the performance of a database system? I found this academic reference paper with ...
59
votes
8answers
67k views

Is a wildcard CNAME DNS record valid?

I know it's valid to have a DNS A record that's a wildcard (e.g. *.mysite.com). Is it possible/valid/advised to have a wildcard CNAME record?
58
votes
4answers
15k views

Is there a difference between how two ampersands and a semi-colon operate in bash?

If I wanted to run two separate commands on one line, I could do this: cd /home; ls -al or this: cd /home && ls -al And I get the same results. However, what is going on in the ...
58
votes
17answers
13k views

Do you skip a rack unit between servers?

It seems like there's a lot of disagreement in mindsets when it comes to installing rackmount servers. There have been threads discussing cable arms and other rackmount accessories, but I'm curious: ...
58
votes
9answers
150k 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! @ ...
58
votes
3answers
6k views

Is a VM with 2 CPUs really faster than one with 4 CPUs?

Our IT created a VM with 2 CPUs allocated rather than the 4 I requested. Their reason is that the VM performs better with 2 CPUs rather than 4 (according to them). The rationale is that the VM ...
57
votes
8answers
91k views

How can I kill all stopped jobs?

When I try to exit from my Linux server I get the message: There are stopped jobs. : Is there a single command to kill these?
57
votes
5answers
19k views

How does Ubuntu keep track of the “System restart required” flag in motd?

I have a few Ubuntu servers (8.10, 9.10) that is set to automatically install security updates. Some times these updates requires a restart of the system, and this string is shown in motd: *** System ...
57
votes
6answers
67k views

When does /tmp get cleared?

I'm taking to putting various files in /tmp, and I wondered what the rules on deleting them are? I'm imagining it's different for different distributions, and I'm particularly interested in Ubuntu ...
57
votes
6answers
71k views

What does “Warning: untrusted X11 forwarding setup failed: xauth key data not generated” mean when ssh'ing with -X?

When I use ssh -X on my Mac (running OS X 10.6.7) to connect to my Ubuntu (11.04) box, I get the following warning: Warning: untrusted X11 forwarding setup failed: xauth key data not generated ...
57
votes
6answers
12k views

Heartbleed: are services other than HTTPS affected?

The OpenSSL 'heartbleed' vulnerability (CVE-2014-0160) affects webservers serving HTTPS. Other services also use OpenSSL. Are these services also vulnerable to heartbleed-like data leakage? I'm ...
57
votes
13answers
93k views

Windows 2008: WinSXS directory growing uncontrollably, blocking server

I run a (remotely hosted) virtual Server with Windows 2008 Server for a client. Initially, it had 10 GB of space. During the course of a few weeks - during which nothing was done on the machine ...
57
votes
19answers
9k views

Best Practise and Solutions for Sharing Passwords [closed]

We have various passwords that need to be known to more than one person in our company. For example, the admin password to our internet routers, the password for our web-host, and also a few "non-IT" ...
56
votes
15answers
10k views

Are IP addresses “trivial to forge”?

I was reading through some of the notes on Google's new public DNS service: Performance Benefits Security Benefits I noticed under the security section this paragraph: Until a standard ...
56
votes
8answers
82k views

Filename length limits on linux?

Are there any filename or path length limits on Linux?
56
votes
5answers
7k views

I accidentaly forbid SSH connection to a remote server… What's next?

Let's say it again, we all make mistakes, and I have just made one. A brief history: I was doing some stuff on a VPS (Debian) I'm renting, when I noticed some strange behaviour. Using the netstat ...
56
votes
7answers
71k views

mysqldump to a tar.gz

Usually after dumping a MySQL database with mysqldump command I immediately tar/gzip the resultant file. I'm looking for a way to do this in one command: So from this: mysqldump dbname -u root -p ...
56
votes
3answers
93k views

What is the difference between unicast, anycast, broadcast and multicast traffic?

I have never had the privilege of working in an environment that required complicated routing or if it did require it, it was handled upstream of me. I've always used very simple static routing ...
56
votes
12answers
265k views

How to start/stop iptables on Ubuntu?

How can I start/stop the iptables service on Ubuntu? I have tried service iptables stop but it is giving "unrecognized service". Why is it doing so? Is there any other method?
56
votes
3answers
11k views

How to handle security updates within Docker containers?

When deploying applications onto servers, there is typically a separation between what the application bundles with itself and what it expects from the platform (operating system and installed ...
55
votes
8answers
84k views

How can I monitor hard disk load on linux

Is there a good command line utility to monitor hard disk load on linux? Something like top but then monitoring disk activity i.s.o. cpu usage. More specifically, I suspect that for some (heavy load) ...

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