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623 votes
13 answers
156k 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 ...
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  • 6,330
45 votes
5 answers
26k 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 ...
user avatar
142 votes
3 answers
39k 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 ...
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  • 79.1k
279 votes
5 answers
111k 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 ...
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  • 81.6k
353 votes
7 answers
772k 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 ...
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470 votes
8 answers
177k 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 ...
95 votes
8 answers
186k 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 ...
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  • 1,785
124 votes
5 answers
74k 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 ...
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  • 11.9k
185 votes
5 answers
24k 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 ...
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  • 24.9k
142 votes
2 answers
881k 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 ?
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  • 114k
187 votes
73 answers
246k 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 ...
188 votes
2 answers
45k 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?
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  • 100k
57 votes
2 answers
22k 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 ...
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110 votes
6 answers
64k 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 ...
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  • 77.2k
356 votes
6 answers
525k 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/...
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  • 13k
97 votes
5 answers
113k 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 ...
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59 votes
12 answers
86k 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 ...
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  • 1,481
55 votes
10 answers
68k 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 ...
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149 votes
3 answers
91k 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 ...
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  • 31.6k
45 votes
3 answers
9k 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 ...
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185 votes
4 answers
132k 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?
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  • 2,897
110 votes
5 answers
78k 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 ...
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  • 7,073
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 ...
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  • 405
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 ...
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  • 446
164 votes
18 answers
253k 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 ...
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  • 2,195
151 votes
2 answers
325k 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, ...
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  • 100k
8 votes
1 answer
2k 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 ...
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103 votes
6 answers
23k 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?
user avatar
13 votes
6 answers
43k 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 ...
user avatar
  • 194k
268 votes
2 answers
81k 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)...
user avatar
  • 2,383
96 votes
21 answers
51k 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. ...
user avatar
  • 1,925
67 votes
9 answers
85k 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, ...
user avatar
  • 905
205 votes
7 answers
317k 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 ...
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  • 23.9k
36 votes
2 answers
10k 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 ...
user avatar
  • 11.9k
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 ...
user avatar
32 votes
2 answers
3k 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 ...
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  • 31.6k
60 votes
5 answers
42k 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 ...
user avatar
  • 1,447
26 votes
7 answers
63k 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. ...
user avatar
  • 361
230 votes
4 answers
179k 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 ...
user avatar
  • 8,353
107 votes
8 answers
22k 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 ...
user avatar
  • 3,339
30 votes
5 answers
13k 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,...
user avatar
  • 403
47 votes
6 answers
30k 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 ...
user avatar
  • 1,674
39 votes
7 answers
53k 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%...
user avatar
546 votes
11 answers
554k 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 ...
user avatar
  • 8,222
163 votes
5 answers
32k 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 ...
user avatar
  • 14.1k
12 votes
4 answers
36k 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 ...
user avatar
  • 4,330
197 votes
3 answers
261k 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:...
user avatar
31 votes
4 answers
50k 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 ...
user avatar
  • 194k
17 votes
2 answers
12k views

Adding a whole IPv6 /64 block to an network interface on debian

I tried adding a whole IPv6 (/64) block to an interface using ip route add local 2001:41d0:2:ad64::/64 dev lo like described here on my Debian server, but I seem to be missing something. If I ping ...
user avatar
203 votes
9 answers
32k views

Heartbleed: What is it and what are options to mitigate it?

This is a Canonical Question about understanding and remediating the Heartbleed security issue. What exactly is CVE-2014-0160 AKA "Heartbleed"? What is the cause, what OSs and versions of OpenSSL are ...
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  • 9,084

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