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Questions tagged [linux]

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

196
votes
5answers
334k 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 ...
292
votes
5answers
352k 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/...
44
votes
3answers
7k views

Why is Linux reporting “free” memory strangely?

This is a canonical question about how Unix operating systems report memory usage. Similar Questions: Server refuses to use swap partition Memory Usage in LINUX I have production ...
92
votes
6answers
18k views

Tips for Securing a LAMP Server

This is a Canonical Question about Securing a LAMP stack What are the absolute guidelines for securing a LAMP server?
98
votes
16answers
144k views

Disk full, du tells different. How to further investigate?

I have a SCSI disk in a server (hardware Raid 1), 32G, ext3 filesytem. df tells me that the disk is 100% full. If I delete 1G this is correctly shown. However, if I run a du -h -x / then du tells me ...
249
votes
2answers
66k 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)...
165
votes
7answers
232k views

Setting the hostname: FQDN or short name?

I've noticed that the "preferred" method of setting the system hostname is fundamentally different between Red Hat/CentOS and Debian/Ubuntu systems. CentOS documentation and the RHEL deployment guide ...
212
votes
4answers
158k views

What's the best way of handling permissions for Apache 2's user www-data in /var/www?

Has anyone got a nice solution for handling files in /var/www? We're running Name Based Virtual Hosts and the Apache 2 user is www-data. We've got two regular users & root. So when messing with ...
55
votes
5answers
35k views

How to understand the memory usage and load average in linux server

I am using a linux server which has 128GB of memory and 24 cores. I use top to see how much it is used. Its output is pasted at the end of the post. Here are two questions: (1) I see that each of the ...
26
votes
5answers
9k views

Can I bind a (large) block of addresses to an interface?

I know that the ip tool lets you bind multiple addresses to an interface (eg, http://www.linuxplanet.com/linuxplanet/tutorials/6553/1/). Right now, though, I'm trying to build something on top of IPv6,...
154
votes
3answers
170k views

Meaning of the buffers/cache line in the output of free

Why does my server show total used free shared buffers cached Mem: 12286456 11715372 571084 0 81912 6545228 -/+ buffers/cache: 5088232 7198224 Swap:...
77
votes
15answers
14k views

Hundreds of failed ssh logins

Every night I get hundreds, sometimes thousands, of failed ssh logins on my RedHat 4 server. For firewall reasons from remote sites, I need to run on the standard port. Is there anything I should be ...
33
votes
7answers
36k views

du vs. df difference [duplicate]

I have a fileserver where df reports 94% of / full. But according to du, much less is used: # df -h / Filesystem Size Used Avail Use% Mounted on /dev/sda3 270G 240G 17G 94%...
182
votes
4answers
66k views

LVM dangers and caveats

I have recently started using LVM on some servers for hard drives larger than 1 TB. They're useful, expandable and quite easy to install. However, I could not find any data about the dangers and ...
39
votes
9answers
64k views

Why would a server not send a SYN/ACK packet in response to a SYN packet

Lately, we've become aware of a TCP connection issue that is mostly limited to mac and Linux users who browse our websites. From the user perspective, it presents itself as a really long connection ...
251
votes
12answers
79k 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 ...
66
votes
2answers
171k views

How to do the port forwarding from one ip to another ip in same network?

I would like do some NAT in iptables. So that, all the packets coming to 192.168.12.87 and port 80 will be forwarded to 192.168.12.77 port 80. How to do this with iptables? Or Any other ways to ...
113
votes
7answers
335k views

How can I port forward with iptables?

I want connections coming in on ppp0 on port 8001 to be routed to 192.168.1.200 on eth0 on port 8080. I've got these two rules -A PREROUTING -p tcp -m tcp --dport 8001 -j DNAT --to-destination 192....
70
votes
2answers
27k 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 ...
9
votes
5answers
32k views

Why does Red Hat Linux report less free memory on the system than is actually available?

I have a relatively small home Red Hat Linux server (about 8 GB RAM). I don't use it for much other than running some home grown apps to keep track of various things. The only real things running on ...
10
votes
3answers
16k views

FTP and Apache permission issues

Im having issues as to which user should own my www directory - ftp or apache? When set to the ftp user, the user can add, remoe and easily modify files but php file system actions generate permission ...
196
votes
22answers
84k views

Is it normal to get hundreds of break-in attempts per day?

I just checked my server's /var/log/auth.log and found that I'm getting over 500 failed password/break-in attempt notifications per day! My site is small, and its URL is obscure. Is this normal? ...
85
votes
12answers
18k views

What solutions exist to allow the use of revision control for server configuration files? [closed]

In an environment with multiple system administrators, I see a few advantages to adding the server config files into a revision control system. Most notable are the ability to track changes, who made ...
34
votes
4answers
15k views

Find out which process is changing a file

I'm trying to find a reliable way of finding which process on my machine is changing a configuration file (/etc/hosts to be specific). I know I can use lsof /etc/hosts to find out what processes ...
366
votes
5answers
147k 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 ...
220
votes
15answers
298k 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 ...
70
votes
12answers
9k views

Exchange server replacement that runs on Linux

I've worked as a sysadmin for some years and what I keep coming back to is that users like Microsoft Outlook and want to use its Exchange features. I have tried my fair share of commercial ...
45
votes
3answers
113k views

How to handle relative urls correctly with a reverse proxy

I have a reverse proxy setup as follows in Apache: Server A with address www.example.com/folder is the reverse proxy server. It maps to: Server B with address test.madeupurl.com This kind of works....
40
votes
6answers
50k views

how i can identify which process is making UDP traffic on linux?

my machine is continously making udp dns traffic request. what i need to know is the PID of the process generating this traffic. The normal way in TCP connection is to use netstat/lsof and get the ...
45
votes
13answers
93k views

VMware Linux Server — how can you tell if you are a vm or real hardware?

An interesting question. I have logged into a Linux (most likely SuSE) host. Is there some way that I can tell programmatically that I am a VM host or not? Also assume that the vmtools are not ...
9
votes
3answers
17k views

How to diagnose causes of oom-killer killing processes

I have a small virtual private server running CentOS and www/mail/db, which has recently had a couple of incidents where the web server and ssh became unresponsive. Looking at the logs, I saw that ...
6
votes
5answers
2k views

ssh delay when connecting

When connecting to one specific server (running Debian Lenny) it always takes about 5 seconds before it prompts me to enter a password. After login there is no noticable delay anymore. There is also ...
185
votes
9answers
176k views

How can I run Debian stable but install some packages from testing?

Say you're running a server and you don't want to upgrade to Testing (Squeeze) from Stable (Lenny) to just install a required package or two. What's the best way of installing only certain packages ...
57
votes
17answers
29k views

iptables Tips & Tricks

I'm sure Linux sysadmins are quite familiar with iptables, the userland interface to the netfilter packet-filtering framework. Now, this "Question" is meant to be a Community Wiki for collecting ...
38
votes
8answers
16k views

What tool do you recommend to track changes on a Linux/Unix server

I manage several Linux servers for clients in several roles like email, caching, web serving, filtering, firewalling/routing, and so on. Since I don't own these computers and just provide remote ...
61
votes
5answers
103k views

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 ...
49
votes
4answers
57k views

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 -...
26
votes
7answers
18k views

Avoid linux out-of-memory application teardown

I'm finding that on occasion my Linux box runs out of memory and it starts tearing down random processes to deal with it. I'm curious what administrators do to avoid this? Is the only real ...
30
votes
5answers
24k views

How do I differentiate “fake RAID” from real RAID?

The Ubuntu wiki page on FakeRaid says the following: [A] number of hardware products ... claim to be IDE or SATA RAID controllers... Virtually none of these are true hardware RAID controllers....
25
votes
8answers
6k views

accidently ran “chown www-data:www-data / -R” as root

i just ran this a few seconds ago, i managed to do ctrl-c as soon as i realized what i started doing. so far in the only dir its started going through is /bin im afraid to do anything else, so far ...
87
votes
8answers
179k views

What limits the maximum number of connections on a Linux server?

What kernel parameter or other settings control the maximum number of TCP sockets that can be open on a Linux server? What are the tradeoffs of allowing more connections? I noticed while load testing ...
50
votes
10answers
32k views

Why not block ICMP?

I think I almost have my iptables setup complete on my CentOS 5.3 system. Here is my script... # Establish a clean slate iptables -P INPUT ACCEPT iptables -P FORWARD ACCEPT iptables -P OUTPUT ACCEPT ...
71
votes
2answers
6k 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 ...
58
votes
1answer
18k views

Why are my XFS filesystems suddenly consuming more space and full of sparse files?

I've run XFS filesystems as data/growth partitions for nearly 10 years across various Linux servers. I've noticed a strange phenomenon with recent CentOS/RHEL servers running version 6.2+. Stable ...
29
votes
9answers
53k views

Force local IP traffic to an external interface

I have a machine with several interfaces that I can configure as I want, for instance: eth1: 192.168.1.1 eth2: 192.168.2.2 I would like to forward all the traffic sent to one of these local ...
78
votes
3answers
48k views

Does getting disconnected from an SSH session kill your programs?

So, say I get disconnected from an SSH-session after I've started rsync or cp or any other command that can be long running. Does that command keep running until it's finished after I get disconnected ...
39
votes
5answers
6k views

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 ...
32
votes
9answers
12k views

How do you setup ssh to authenticate using keys instead of a username / password?

How do you setup ssh to authenticate a user using keys instead of a username / password?
23
votes
10answers
46k views

How do I know if I am running 32 or 64-bit Linux?

what would be a command to run in different distros?
29
votes
6answers
4k views

Linux server out of space

I have been asked this question in two consecutive interviews, but after some research and checking with various systems administrators I haven't received a good answer. I am wondering if somebody can ...