Frequent Questions

632 votes

13 answers

164k views

How do I deal with a compromised server?

This is a Canonical Question about Server Security - Responding to Breach Events (Hacking) See Also: Tips for Securing a LAMP Server Reinstall after a Root Compromise? Canonical ...

gunwin

6,420

asked Jan 2, 2011 at 21:31

45 votes

5 answers

27k views

Can you help me with my software licensing issue?

This is a Canonical Question about Licensing. Questions on licensing are off-topic on Server Fault. If your question has been closed as a duplicate of this question, then this is because we want to ...

Ben Pilbrow

12.1k

asked Dec 22, 2010 at 18:07

144 votes

4 answers

43k views

Can you help me with my capacity planning?

This is a canonical question about capacity planning Related: How do you do load testing and capacity planning for web sites? How do you do load testing and capacity planning for databases? I have a ...

voretaq7

80.1k

asked Apr 30, 2012 at 19:20

285 votes

5 answers

115k views

Redirect, Change URLs or Redirect HTTP to HTTPS in Apache - Everything You Ever Wanted to Know About mod_rewrite Rules but Were Afraid to Ask

This is a Canonical Question about Apache's mod_rewrite. Changing a request URL or redirecting users to a different URL than the one they originally requested is done using mod_rewrite. This includes ...

Kyle Brandt

84k

asked Dec 20, 2010 at 16:59

389 votes

7 answers

943k 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 ...

Eric Leschinski

4,301

asked Nov 17, 2012 at 4:51

476 votes

8 answers

186k views

How does IPv4 Subnetting Work?

This is a Canonical Question about IPv4 Subnets. Related: How does IPv6 subnetting work and how does it differ from IPv4 subnetting? How does Subnetting Work, and How do you do it by ...

Community wiki

20 revs, 11 users 40%
Kyle Brandt

97 votes

8 answers

195k views

How to send emails and avoid them being classified as spam?

This is a canonical question about how to handle email sent from your server being misclassified as spam. For additional information you may find these similar questions helpful: Best ...

kcode

1,825

asked Jul 31, 2009 at 5:53

125 votes

5 answers

76k views

How do you do load testing and capacity planning for web sites?

This is a canonical question about capacity planning for web sites. Related: Can you help me with my capacity planning? How do you do load testing and capacity planning for databases? What are some ...

gWaldo

12k

asked Jan 16, 2012 at 22:49

191 votes

5 answers

26k views

I am under DDoS. What can I do?

This is a Canonical Question about DoS and DDoS mitigation. I found a massive traffic spike on a website that I host today; I am getting thousands of connections a second and I see I'm using all ...

Falcon Momot

25.3k

asked Aug 19, 2013 at 9:14

155 votes

2 answers

973k 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 ?

user9517

116k

asked Sep 28, 2015 at 12:13

187 votes

73 answers

247k views

What tool do you use to monitor your servers?

For a more comprehensive list of monitoring tools and their features, check out this Wikipedia page. As the question states, what are the most commonly used tools used for this task and what are ...

Community wiki

10 revs, 6 users 44%
voretaq7

190 votes

2 answers

47k views

What are the different widely used RAID levels and when should I consider them?

This is a Canonical Question about RAID levels. What are: the RAID levels typically used (including the RAID-Z family)? deployments are they commonly found in? benefits and pitfalls of each?

MDMarra

101k

asked Dec 8, 2011 at 19:40

59 votes

2 answers

24k views

What are SPF records, and how do I configure them?

This is a canonical question about setting up SPF records. I have an office with many computers that share a single external ip (I'm unsure if the address is static or dynamic). Each computer ...

vulgarbulgar

749

asked Mar 14, 2012 at 5:45

111 votes

6 answers

66k views

Fighting Spam - What can I do as an: Email Administrator, Domain Owner, or User?

This is a Canonical Question about Fighting Spam. Also related: How to stop people from using my domain to send spam? What are SPF records, and how do I configure them? There are so ...

Chris S

78.1k

asked Aug 20, 2012 at 20:30

372 votes

6 answers

569k 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/...

Nic

13.5k

asked Feb 6, 2012 at 1:50

64 votes

12 answers

97k views

Loopback to forwarded Public IP address from local network - Hairpin NAT

This is a Canonical Question about Hairpin NAT (Loopback NAT). The generic form of this question is: We have a network with clients, a server, and a NAT Router. There is port forwarding on the ...

adopilot

1,531

asked Aug 18, 2009 at 15:16

98 votes

5 answers

119k views

Windows Active Directory naming best practices?

This is a Canonical Question about Active Directory domain naming. After experimenting with Windows domains and domain controllers in a virtual environment, I've realized that having an active ...

Anton Gogolev

1,582

asked Oct 21, 2009 at 13:10

165 votes

3 answers

109k views

Why can't a CNAME record be used at the apex (aka root) of a domain?

This is a Canonical Question about CNAMEs at the apices (or roots) of zones It's relatively common knowledge that CNAME records at the apex of a domain are a taboo practice. Example: example.com. IN ...

Andrew B

32.8k

asked Jul 19, 2014 at 10:53

57 votes

10 answers

77k views

How to use DNS/Hostnames or Other ways to resolve to a specific IP:Port

This is a Canonical Question about DNS/Hostnames resolution to IPs/Ports Example 1 I'm running a web server on port 80 and another on port 87. I would like to use DNS so that www.example.com goes to ...

Tom Smykowski

1,125

asked Oct 14, 2009 at 13:12

45 votes

3 answers

10k 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 ...

user135199

asked Nov 16, 2012 at 4:29

205 votes

5 answers

150k views

What is a glue record?

This is a Canonical Question about DNS glue records. What exactly (but briefly) is a DNS glue record? Why are they needed and how do they work?

LanceBaynes

3,107

asked Sep 9, 2011 at 3:54

110 votes

5 answers

79k views

Multiple SSL domains on the same IP address and same port?

This is a Canonical Question about Hosting multiple SSL websites on the same IP. I was under the impression that each SSL Certificate required it's own unique IP Address/Port combination. But the ...

John

7,413

asked Feb 4, 2010 at 22:36

30 votes

4 answers

11k views

Prevent mail being marked as spam

This is a canonical question about avoiding outgoing mail being classified as spam. Also related: Fighting Spam - What can I do as an: Email Administrator, Domain Owner, or User? What ...

jonepatr

405

asked Jan 26, 2011 at 17:48

31 votes

5 answers

14k views

Why not use a WAMP stack?

This is a canonical question about the use of a *AMPP's stack. I recently had a talk with some experienced people and they suggested to me not to use a WAMP stack, and instead install apache, mysql ...

matang

446

asked Nov 29, 2012 at 21:58

176 votes

18 answers

281k 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 ...

initall

2,335

asked May 30, 2011 at 12:29

10 votes

1 answer

6k views

How can I forward requests from my web server?

This is a Canonical Question about reverse proxies, how they work and how they are configured. How can I serve requests from a service on a different port or from a different server with the same ...

Gerald Schneider

24.3k

asked Sep 23, 2020 at 10:04

151 votes

2 answers

327k 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, ...

MDMarra

101k

asked Jun 27, 2012 at 3:47

103 votes

6 answers

24k 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?

Aditya Shukla

1,031

asked Dec 14, 2010 at 1:52

13 votes

6 answers

45k views

ZFS SAS/SATA controller recommendations

I've been working with OpenSolaris and ZFS for 6 months, primarily on a Sun Fire x4540 and standard Dell and HP hardware. One downside to standard Perc and HP Smart Array controllers is that they do ...

ewwhite

198k

asked Nov 12, 2009 at 14:32

271 votes

2 answers

86k 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)...

samwise

2,413

asked Feb 28, 2012 at 21:25

217 votes

8 answers

348k 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 ...

Cakemox

25.4k

asked Nov 17, 2011 at 13:07

100 votes

21 answers

53k views

Should we host our own nameservers?

This is a Canonical Question about whether to outsource DNS resolution for ones own domains I currently have my ISP providing DNS for my domain, but they impose limitations on adding records. ...

Saif Khan

1,965

asked Jun 10, 2009 at 22:46

36 votes

2 answers

11k views

How do you do load testing and capacity planning for databases?

This is a canonical question about capacity planning for databases. Related: Can you help me with my capacity planning? How do you do load testing and capacity planning for web sites? I'm looking to ...

gWaldo

12k

asked Jan 16, 2012 at 22:55

67 votes

9 answers

87k views

What a beginner should know/learn for sysadmin job?

This is a Canonical Question about System Administration Careers When I start my job as System Administrator, what basics skills should I know/learn? Are there any key differences for Network, ...

Dhanapal

905

asked May 19, 2009 at 6:25

6 votes

1 answer

3k views

Excessive number of sleeping processes in CentOS - howto diagnose? [closed]

I have a a large number of sleeping processes, about 600, the majority of which are Apache processes. Should I kill all these sleeping processes, or will that make Apache fail completely? Why are ...

mr.soroush

171

asked Nov 25, 2014 at 7:13

33 votes

2 answers

4k views

Why is geo-redundant DNS necessary for small sites?

This is a Canonical Question about DNS geo-redundancy. It's extremely common knowledge that geo-redundant DNS servers located at separate physical locations are highly desirable when providing ...

Andrew B

32.8k

asked Aug 1, 2015 at 6:22

554 votes

11 answers

593k 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 ...

MikeN

8,482

asked Sep 21, 2009 at 14:04

60 votes

5 answers

43k 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 ...

Tim

1,497

asked Sep 22, 2009 at 16:59

26 votes

7 answers

66k views

Third-party SSD solutions in ProLiant Gen8 servers

I was wondering if anyone had any specific experience using Intel DC3700 SSDs (or similar) in the HP (DL380p) Gen8 servers? I'm upgrading a set of database servers that use direct-attached storage. ...

gwf

361

asked Aug 5, 2013 at 22:00

109 votes

8 answers

23k views

How can a single disk in a hardware SATA RAID-10 array bring the entire array to a screeching halt?

Prelude: I'm a code-monkey that's increasingly taken on SysAdmin duties for my small company. My code is our product, and increasingly we provide the same app as SaaS. About 18 months ago I moved our ...

Stu Thompson

3,359

asked Nov 16, 2011 at 11:14

30 votes

5 answers

14k 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,...

p-static

403

asked Dec 5, 2010 at 0:27

234 votes

4 answers

182k 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 ...

Gareth

8,573

asked May 11, 2009 at 5:13

77 votes

3 answers

200k 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....

Hard worker

879

asked Dec 17, 2013 at 12:05

51 votes

7 answers

34k views

Is there a name based virtual host SSH reverse proxy?

I've grown quite fond of HTTP reverse proxies in our development environment and found the DNS based virtual host reverse proxy quite useful. Having only one port (and the standard one) open on the ...

ahanson

1,714

asked Jul 1, 2009 at 16:01

42 votes

7 answers

57k 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%...

Andreas Kuntzagk

791

asked Aug 21, 2009 at 11:23

309 votes

5 answers

700k 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; ...

jeffreyveon

3,255

asked Apr 15, 2012 at 17:31

163 votes

5 answers

33k views

Are SSD drives as reliable as mechanical drives (2013)?

SSD drives have been around for several years now. But the issue of reliability still comes up. I guess this is a follow up from this question posted 4 years ago, and last updated in 2011. It's now ...

hookenz

14.5k

asked May 14, 2013 at 4:11

12 votes

4 answers

37k views

Incredibly low disk performance on HP ProLiant DL385 G7

As a test of the Opteron processor family, I bought a HP DL385 G7 6128 with HP Smart Array P410i Controller - no memory. The machine has 20GB ram 2x146GB 15k rpm SAS + 2x250GB SATA2, both in Raid 1 ...

3molo

4,330

asked Jan 12, 2011 at 12:35

202 votes

4 answers

280k 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:...

Muahmmad Zeeshan

2,123

asked Nov 17, 2009 at 10:37

32 votes

4 answers

52k views

What happens when the USB key or SD card I've installed VMware ESXi on fails?

An SD (SDHC) card installed in an HP ProLiant DL380p Gen8 server running VMware ESXi just failed :( I encountered some ominous looking messages on the vCenter console and in the HP ProLiant ILO event ...

ewwhite

198k

asked Oct 28, 2013 at 21:30

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