Linux is the generic term for a UNIX-like open source operating system based on the Linux kernel.

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Securing a fresh Ubuntu server [closed]

Say I've got a fresh install of Ubuntu, what steps should I take to secure it for use as a Rails application server?
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6answers
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shutdown: What is difference between “Power Off” and “Halt”?

With your typical Linux shutdown command, what is the difference between halting the system and powering off the system?
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8answers
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How to add a timestamp to bash script log?

I have a constantly running script that I output to a log file: script.sh >> /var/log/logfile I'd like to add a timestamp before each line that is appended to the log. Like: Sat Sep 10 ...
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1answer
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Why does `tar -xvfz` fail, but `tar xvfz` work?

tar -xvfz foo.tar.gz returns an error tar: z: Cannot open: No such file or directory This is completely understandable - the f switch expects a filename, and so needs to be last. However, omitting ...
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6answers
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How do I list loaded Linux module parameter values?

Is there a standard way to list the parameter values of a loaded Linux module? I'm essentially probing for another answer to this Linux kernel module parameters question, because the module I'm ...
33
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7answers
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How to check the physical status of an ethernet port in Linux?

I want to check if a specified ethX is physically up or down. How do I do that with the command line?
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5answers
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Smoothest workflow to handle SSH host verification errors?

This is a simple issue that we all face and probably resolve manually without giving much thought. As servers change, are re-provisioned, or IP addresses reallocated, we receive the SSH host ...
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1answer
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Why does removing an unused IP address from an interface kill connections unrelated to that address

Yesterday I did a quick reinstall of a (physical) server in the datacenter, and since I was short on time and with no easy access to our database I just assigned it an IP that I knew was available and ...
33
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2answers
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Why was I able to delete a file owned by root in my home directory without being root?

So I was doing some maintenance on my server earlier today and noticed I was able to delete a file owned by root in my home directory. I was able to reproduce a sample: [cbennett@nova ~/temp]$ ls ...
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2answers
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Force dig to resolve without using cache

I'm wondering if there is a way to query a DNS server and bypass caching (with dig). Often I change a zone on the DNS server and I want to check if it resolves correctly from my workstation. But since ...
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2answers
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Damaged /vmlinuz and /initrd.img symbolic links after Kernel uninstall

OS: Ubuntu 8.04 LTS Server Edition We just rolled back an kernel update using the following command: sudo apt-get remove linux-image-2.6.24-24-server The uninstallation was successful, but it had ...
33
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3answers
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What's best practice for communication between Amazon EC2 instances?

I've been setting up Amazon EC2 instances for an upcoming project. They are all micro instances, running Ubuntu Server 64bit. Here's what I've setup so far: Web Server -- Apache Database Server ...
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4answers
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EPEL Repo for CentOS 6 causing error

I've got a VM running CentOS 6 (64bit) and I'm attempting to add the EPEL repo like usual to install various packages as I do quite regularly. Today, I'm experiencing some strange errors yet I'm ...
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9answers
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Multiple Linux sysadmins working as root

In our team we have three seasoned Linux sysadmins having to administer a few dozen Debian servers. Previously we have all worked as root using SSH public key authentication. But we had a discussion ...
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4answers
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How to disconnect from lxc-console

I'm working with lxc in Ubuntu 12.04, and it's really great. However, I am unable to disconnect from a lxc-console session after I've connected. I read somewhere that Ctrl-aq will disconnect me from ...
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6answers
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swap partition vs file for performance?

What is better for performance? A partition closer to the inside of the disk will have slower access times, and we must wait for the drive to switch between the OS and swap partitions. On the other ...
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7answers
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filesystem for millions of small files

Which Linux filesystem would you choose for best speed in the following scenario: a hundred million files ~2k file size in average >95% read access pretty random access high concurrency (>100 ...
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10answers
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Why su to root instead of logging in as root?

I've heard often that it is better to su to root rather than log in directly as the root user (and of course people also say that it's even better to use sudo). I've never really understood why one ...
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4answers
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How to convert line breaks in a text file between the Windows and Unix/Linux formats?

How do I convert line breaks in a text file between the Windows and Unix/Linux formats? I have a *nix environment, but that I need to import and export data with the Windows-style line breaks. I ...
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3answers
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Turn off the Linux OOM killer by default?

The OOM killer on Linux wreaks havoc with various applications every so often, and it appears that not much is really done on the kernel development side to improve this. Would it not be better, as a ...
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6answers
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Should websites live in /var/ or /usr/ according to recommended usage?

According to a guide on the Linux directory structure, /usr/ is for application files, and /var/ is for files that change (I assume this means "files that belong to the applications"). Is this ...
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7answers
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Amazon Linux vs. Ubuntu for Amazon EC2 [closed]

I'm setting up my first website on Amazon EC2, and I'm trying to decide which distro to use. I've used Redhat and CentOS in the past, but I have no bias towards any system, I just want to use whatever ...
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6answers
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Merge 2 directory trees in Linux without copying?

I have two directory trees with similar layouts, i.e. . |-- dir1 | |-- a | | |-- file1.txt | | `-- file2.txt | |-- b | | `-- file3.txt | `-- c | `-- file4.txt `-- ...
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5answers
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How passively monitor for tcp packet loss? (Linux)

How can I passively monitor the packet loss on TCP connections to/from my machine? Basically, I'd like a tool that sits in the background and watches TCP ack/nak/re-transmits to generate a report on ...
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6answers
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How to create a symbolic link without using ln?

I deleted a critical symbolic link - libc.so.6. I have the file it should point at, but the basic commands such as ln or wget won't work anymore due to the link missing. However, echo or other Bash ...
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9answers
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How to hide a password passed as command line argument?

I'm running a software daemon that requires for certain actions to enter a passphrase to unlock some features which looks for example like that: $ darkcoind masternode start <mypassphrase> ...
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7answers
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How to get e-mail from (failed) cron-jobs in Ubuntu?

I create cron-jobs in Ubuntu by placing the executable in one of /etc/cron.{daily,hourly,monthly,weekly}. There are lots of directories starting with cron: kent@rat:~$ ls -ld /etc/cron* drwxr-xr-x 2 ...
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3answers
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How to get TX/RX bytes without ifconfig?

Since ifconfig is apparently being deprecated in major Linux distributions, I thought I'd learn something about the ip tool that's supposed to be used instead of ifconfig. And here I ran into a ...
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3answers
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How do I do 'mount --bind' in /etc/fstab?

I'm trying to figure out how to add "mount --bind /proc/ /chroot/mysql/proc" to /etc/fstab, but how can I to do this?
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3answers
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How can I allow one user to su to another without allowing root access?

I'd like to allow certain users to su to another user account without having to know that account's password, but not allow access to any other user account (i.e. root). For instance, I'd like to ...
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6answers
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How to determine the “age” of a linux system since installation?

I thought that I could easily check the timestamp of particular files. Then I realized that it wouldn't be so easy when I saw timestamps like 1991.
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How to correct Postfix' 'Relay Access Denied'?

This morning, in order to correct a problem with a name mismatch in the security certificate, I followed the recommended steps from How to fix mail server SSL?, but now, when attempting to send an ...
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Where is the statement of deprecation of ifconfig (on linux)?

A lot of people is stating that the ifconfig command is deprecated in favor of the ipone (on linux at least). This is often used as an argumentation to switch from ifconfig to ip (see some comment ...
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iptables port redirect not working for localhost

I want to redirect all traffic from port 443 to the internal port 8080. I'm using this config for iptables: iptables -t nat -I PREROUTING --source 0/0 --destination 0/0 -p tcp \ --dport 443 ...
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5answers
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Job scheduling using crontab, what will happen when computer is shutdown during that time?

I schedule some tasks using crontab. What will happen if my computer is shutdown or turned off during the time when cron was scheduled to do something? Does each missed cron job run after computer ...
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5answers
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How useful is mounting /tmp noexec?

Many people (including the Securing Debian Manual) recommend mounting /tmp with the noexec,nodev,nosuid set of options. This is generally presented as one element of a 'defense-in-depth' strategy, by ...
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3answers
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Tuning Linux IP routing parameters — secret_interval and tcp_mem

We had a little failover problem with one of our HAProxy VMs today. When we dug into it, we found this: Jan 26 07:41:45 haproxy2 kernel: [226818.070059] __ratelimit: 10 callbacks suppressed Jan 26 ...
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1answer
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Is it possible to influence the way CPUs are enumerated under Linux?

I have a HP DL380 G7 with 2 mismatched CPUs in it. One is a quad core CPU with faster cores, and one is a 6 core CPU with slower cores. On this box I run an application that due to licensing reasons ...
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5answers
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How to add message that will be read with dmesg?

I am trying to write some custom messages in my dmesg output. I tried: logger "Hello" but this does not work. It exits without error, but no "Hello" appears int the output of: dmesg I am using a ...
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7answers
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Can I run a cron job more frequently than every minute?

Is it possible to run a cron job every 30 seconds without a sleep command?
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7answers
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Reasons to disable / enable SELinux

In the line of this question on StackOverflow and the completely different crowd we have here, I wonder: what are your reasons to disable SELinux (assuming most people still do)? Would you like to ...
29
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3answers
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How do I extract login history?

I need to know the login history for specific user (i.e. login and logout time), How do I extract this history for a specific date range in Linux ?
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4answers
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How to make a global ~/.vimrc?

Right now, I make everyone do ~/.vimrc and put their settings there. How can I make a global, default .vimrc for new users?
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6answers
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protocol version mismatch — is your shell clean?

When following the instructions to do rsync backups given here: http://troy.jdmz.net/rsync/index.html I get the error "protocol version mismatch -- is your shell clean?" I read somewhere that I ...
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7answers
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What happens if I delete lost+found

When you create some Linux filesystems like ext3 a 'lost+found' directory is created. According to this files will be placed there if files were damaged from some kind of system crash. What happens ...
29
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7answers
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Linux: how to restore config file using apt-get/aptitude?

I've occasionally lost my config file "/etc/mysql/my.cnf", and want to restore it. The file belongs to package mysql-common which is needed for some vital functionality so I can't just purge ...
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How can a Linux Administrator improve their shell scripting and automation skills?

In my organization, I work with a group of NOC staff, budding junior engineers and a handful of senior engineers; all with a focus on Linux. One interesting step in the way the company grows talent is ...
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4answers
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Can you have more than one ~/.ssh/config file?

We have a bastion server that we use to connect to multiple hosts, and our .ssh/config has grown to over a thousand lines (we have hundreds of hosts that we connect to). This is beginning to get a ...
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8answers
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Why rsync is faster than NFS?

Few days ago I noticed something rather odd (at least for me). I ran rsync copying the same data and deleting it afterwards to NFS mount, called /nfs_mount/TEST. This /nfs_mount/TEST is ...
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How do I redirect subdomains to a different port on the same server?

I have some subdomains I want to redirect to specific ports on the same server. Say I have dev.mydomain.com I want dev.mydomain.com to transparently redirect to mydomain.com:8080 and I want to ...