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Articles indexed in May 2014

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• SSH connection falling down

  - by kappa

  I've set up a connection with autossh that creates some tunnels at system startup, but if I try to connect, after successful login (with RSA key) connection fall down, here a trace: debug1: Authentication succeeded (publickey). debug1: Remote connections from LOCALHOST:5006 forwarded to local address localhost:22 debug1: Remote connections from LOCALHOST:6006 forwarded to local address localhost:80 debug1: channel 0: new [client-session] debug1: Requesting [email protected] debug1: Entering interactive session. debug1: remote forward success for: listen 5006, connect localhost:22 debug1: remote forward success for: listen 6006, connect localhost:80 debug1: All remote forwarding requests processed debug1: Sending environment. debug1: Sending env LANG = it_IT.UTF-8 debug1: Sending env LC_CTYPE = en_US.UTF-8 debug1: client_input_channel_req: channel 0 rtype exit-status reply 0 debug1: client_input_channel_req: channel 0 rtype [email protected] reply 0 debug1: channel 0: free: client-session, nchannels 1 Transferred: sent 2400, received 2312 bytes, in 1.3 seconds Bytes per second: sent 1904.2, received 1834.4 debug1: Exit status 1 What can be the problem? All this stuff is managed by a script already running on another machine (creating reverse tunnels on the same machine but with different ports)

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• Hopping a VPN Tunnel

  - by lellouch

  My central office and remote offices are connected to each other over site to site ipsec vpn. We use Fortigate firewalls and everything is working fine. On the other hand, only central office is also connected to another company's network over ipsec vpn as well. In this situation, everything is also fine and employees at the central office is able to reach the other company's resources without problem. Now i want the employees working on our remote office can reach the other company's network over central office without creating new vpn tunnels. http://imgur.com/ozrXfGv How can i do that? Thanks for your answers in advance.

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• Centos running Apache Tomcat keep getting "java.net.SocketException: Too many open files"

  - by Gerard Moroney

  We're running Apache Tomcat 7.0.41 on CentOS 6 with java version "1.7.0_21". We were getting a lot of too many open files errors so I did some research. The consensus was that it was to to with the number of open files. So I did the following: Increased max files in /etc/security/limits.conf soft nofile 100000 hard nofile 100000 Rebooted the server Checked the limits were valid for the user which was to run the process [app_admin@xxx ~]$ ulimit -Hn 100000 [app_admin@xxx ~]$ ulimit -Sn 100000 Monitored open files on the server using the lsof command What I observed was when the total open files reached circa 13000 and tomcat had around 4500 open files the error reappeared. I am confused. I thought it would have resolved the problem but clearly I don't fully understand the root cause and also how to set the parameter correctly. To (maybe) help I have not modified the server.xml file for Tomcat (although I'm tempted). I don't want to start fiddling with that and make things worse. I'm more than happy to share any more information if someone can give me some hints on where to start looking.

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• Cannot open mailbox

  - by AdamL

  Ive got a server running Windows Server 2012 and Exchange 2013 on my local domain. Ive got the mailboxes setup and can send/receive emails fine when using OWA. If i try to open the mailbox using Outlook 2010, I get the below error If i try to expand the folder, i get the same message but am still able to use OWA. When I try to run a send/receive, I get the below error: After a bit of researching on the web, people suggest that the profile is corrupt. I have recreated the profile and tried again but still same error. Another post has suggested that the issue is DNS and client PCs should be able to resolve autodiscover.domain.local and server.domain.local to the IP of the server. This is fine on my network. Every user has got the same issue and I am struggling to identify what is causing the fault. Advise appreciated.

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• Exchange 2013 attachments too big?

  - by KPS

  I am having the toughest time sending large attachments, everywhere I have checked my file size limit for send/receive is 100mb but yet users are unable to receive files even at the size of 14mb. I'm using a spam filter (Appriver) and have worked with there support for a very long time, we see the following errors in logs 13:32:40.260 4 SMTP-000036([myserverIP]) rsp: 354 Start mail input; end with <CRLF>.<CRLF> 13:33:41.038 3 SMTP-000033([myserverIP]) write failed. Error Code=connection reset by peer 13:33:41.038 3 SMTP-000033([myserverIP]) [659500] failed to send. Error Code=connection reset by peer 13:33:41.038 4 SMTP([myserverIP]) [659500] batch reenqueued into tail Windows firewall is disabled on the exchange server, all other emails that are of smaller value come through just fine. Here is a print out of size limits: ConnectorType ConnectorName MaxReceiveMessageSize MaxSendMessageSize ------------- ------------- --------------------- ------------------ Send InternetSendConnector - 35 MB (36,700,160 bytes) Send Appriver-Smarthost - 35 MB (36,700,160 bytes) Receive Default EXCHSRVR 100 MB (104,857,600 bytes) - Receive Client Proxy EXCHSRVR 100 MB (104,857,600 bytes) - Receive Default Frontend EXCHSRVR 100 MB (104,857,600 bytes) - Receive Outbound Proxy Frontend EXCHSRVR 100 MB (104,857,600 bytes) - Receive Client Frontend EXCHSRVR 100 MB (104,857,600 bytes) - Receive ExchangeRelay 100 MB (104,857,600 bytes) - TransportConfig - 100 MB (104,857,600 bytes) 10 MB (10,485,760 bytes) ADSiteLink DEFAULTIPSITELINK Unlimited Unlimited There is a no anti-virus on the server either that could be interfering, I am out of ideas at this point :( EDIT 1 After running BPA, it gives and error: Exchange Organization: Check whether the incoming message(CN=MyDomain,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=WG,DC=local) size isn't set The maximum incoming message size isn't set in organization 'CN=MyDomain,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=WG,DC=local'. This can cause reliability problems. Here are the sizes as of now: [PS] C:\Temp>Get-TransportConfig | ft MaxSendSize, MaxReceiveSize MaxSendSize MaxReceiveSize ----------- -------------- Unlimited Unlimited [PS] C:\Temp>Get-ReceiveConnector | ft name, MaxMessageSize Name MaxMessageSize ---- -------------- Default EXCHSRVR 100 MB (104,857,600 bytes) Client Proxy EXCHSRVR 100 MB (104,857,600 bytes) Default Frontend EXCHSRVR 100 MB (104,857,600 bytes) Outbound Proxy Frontend EXCHSRVR 100 MB (104,857,600 bytes) Client Frontend EXCHSRVR 100 MB (104,857,600 bytes) ExchangeRelay 100 MB (104,857,600 bytes) Again, smaller emails come through just fine. Seems like there is a 10mb receive limit somewhere that I cannot find.

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• uWSGI and python virtual env

  - by user27512

  I'm trying to use uWSGI with a virtual env in order to use the Trac bug tracker on it. I've installed system-wide uwsgi via pip. Next, I've installed trac in a virtualenv $ virtualenv venv $ . venv/bin/activate $ pip install trac I've then written a simple uWSGI configuration script: [uwsgi] master = true processes = 1 socket = localhost:3032 home = /srv/http/trac/venv/ no-site = true gid = www-data uid = www-data env = TRAC_ENV=/srv/http/trac/projects/my_project module = trac.web.main:dispatch_request But when I try to launch it, it fails: $ uwsgi --http :8000 --ini /etc/uwsgi/vassals-available/my_project.ini --gid www-data --uid www-data ... Set PythonHome to /srv/http/trac/venv/ ... *** Operational MODE: single process *** ImportError: No module named trac.web.main unable to load app 0 (mountpoint='') (callable not found or import error) I think uWSGI isn't using the virtual env. When inside the virtual env, I can import trac.web.main without having an ImportError. How can I do that ? Thanks

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• Office 365 Powershell - Export user, license type, and company field to csv file

  - by ASGJim

  I need to be able to export user name or email address (doesn't matter which), company (from the company field under the organization tab in a user account of the exchange admin console), and license type (e.g. exchange online e1, exchange online kiosk etc...) I am able to export both values in two statements into two separate files but that doesn't do me much good. I can export the username and license type with the following: Get-MSOLUser | % { $user=$_; $_.Licenses | Select {$user.displayname},AccountSKuid } | Export-CSV "sample.csv" -NoTypeInformation And, I can get the company values with the following: Get-User | select company | Export-CSV sample.csv Someone on another forum suggested this - $index = @{} Get-User | foreach-object {$index.Add($_.userprincipalname,$_.company)} Get-MsolUser | ForEach-Object { write-host $_.userprincipalname, $index[$_.userprincipalname], $_.licenses.AccountSku.Skupartnumber} That seems like it should work but it doesn't display any license info in my powershell, it's just blank. Also I wouldn't know how to export that to a csv file. Any help would be appreciated. Thanks.

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• Creating a fallback error page for nginx when root directory does not exist

  - by Ruirize

  I have set up an any-domain config on my nginx server - to reduce the amount of work needed when I open a new site/domain. This config allows me to simply create a folder in /usr/share/nginx/sites/ with the name of the domain/subdomain and then it just works.™ server { # Catch all domains starting with only "www." and boot them to non "www." domain. listen 80; server_name ~^www\.(.*)$; return 301 $scheme://$1$request_uri; } server { # Catch all domains that do not start with "www." listen 80; server_name ~^(?!www\.).+; client_max_body_size 20M; # Send all requests to the appropriate host root /usr/share/nginx/sites/$host; index index.html index.htm index.php; location / { try_files $uri $uri/ =404; } recursive_error_pages on; error_page 400 /errorpages/error.php?e=400&u=$uri&h=$host&s=$scheme; error_page 401 /errorpages/error.php?e=401&u=$uri&h=$host&s=$scheme; error_page 403 /errorpages/error.php?e=403&u=$uri&h=$host&s=$scheme; error_page 404 /errorpages/error.php?e=404&u=$uri&h=$host&s=$scheme; error_page 418 /errorpages/error.php?e=418&u=$uri&h=$host&s=$scheme; error_page 500 /errorpages/error.php?e=500&u=$uri&h=$host&s=$scheme; error_page 501 /errorpages/error.php?e=501&u=$uri&h=$host&s=$scheme; error_page 503 /errorpages/error.php?e=503&u=$uri&h=$host&s=$scheme; error_page 504 /errorpages/error.php?e=504&u=$uri&h=$host&s=$scheme; location ~ \.(php|html) { include /etc/nginx/fastcgi_params; fastcgi_pass 127.0.0.1:9000; fastcgi_intercept_errors on; } } However there is one issue that I'd like to resolve, and that is when a domain that doesn't have a folder in the sites directory, nginx throws an internal 500 error page because it cannot redirect to /errorpages/error.php as it doesn't exist. How can I create a fallback error page that will catch these failed requests?

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• cant remove index.php from url in codeigniter

  - by Ashiq

  iam new in codeigniter frame work,i want to remove index.php from url and tried many times bt its not working..... here is my .htaccess file RewriteEngine on RewriteBase /test/ RewriteCond $1 !^(index\.php|resources|robots\.txt) RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule ^(.*)$ test/index.php/$1 [L,QSA] iam also change $config['index_page'] = ''; bt when running this i got an error message... Internal Server Error The server encountered an internal error or misconfiguration and was unable to complete your request. Please contact the server administrator at [email protected] to inform them of the time this error occurred, and the actions you performed just before this error. More information about this error may be available in the server error log. here is my appache error log [Sat Jan 05 16:59:53.265625 2013] [core:error] [pid 3976:tid 1152] [client ] Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace. pls help to solve this........ Thanks

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• What is the ip range of EC2

  - by Nicolas Kassis

  I'd like to setup a rule to block ssh request from EC2 since I've been seeing a large amount of ssh based attack from there and was wondering if anyone knew what their IP ranges are. EDIT: Thank you for the answer, I went ahead and implemented the iptables rules as follow. I ignore all traffic for the moment. Logging it just to see if the rules are working and for stats on how much crap EC2 is sending out ;) #EC2 Blacklist $IPTBLS -A INPUT -s 67.202.0.0/18 -j LOG --log-prefix "<firewall> EC2 traffic " $IPTBLS -A INPUT -s 67.202.0.0/18 -j DROP $IPTBLS -A INPUT -s 72.44.32.0/19 -j LOG --log-prefix "<firewall> EC2 traffic " $IPTBLS -A INPUT -s 72.44.32.0/19 -j DROP $IPTBLS -A INPUT -s 75.101.128.0/17 -j LOG --log-prefix "<firewall> EC2 traffic " $IPTBLS -A INPUT -s 75.101.128.0/17 -j DROP $IPTBLS -A INPUT -s 174.129.0.0/16 -j LOG --log-prefix "<firewall> EC2 traffic " $IPTBLS -A INPUT -s 174.129.0.0/16 -j DROP $IPTBLS -A INPUT -s 204.236.192.0/18 -j LOG --log-prefix "<firewall> EC2 traffic " $IPTBLS -A INPUT -s 204.236.192.0/18 -j DROP $IPTBLS -A INPUT -s 204.236.224.0/19 -j LOG --log-prefix "<firewall> EC2 traffic " $IPTBLS -A INPUT -s 204.236.224.0/19 -j DROP $IPTBLS -A INPUT -s 79.125.0.0/17 -j LOG --log-prefix "<firewall> EC2 traffic " $IPTBLS -A INPUT -s 79.125.0.0/17 -j DROP

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• Installing Linux on a Windows 8.1 laptop

  - by nicoX

  I would like to clean install a linux distribution as Ubuntu etc. My laptop that runs Windows 8.1. I have two options in mind. Clean install or dual boot. My technical question is: my laptop have a 8GB SSD drive, which it uses to boot Windows with and a 500GB for storage. I wonder what that 8GB SSD stores? It can't store the whole Windows install as that would be much more than 8GB. Also if I would do a clean install of Ubuntu could I use the 8GB SSD to have Ubuntu boot up quicker. How would I install it. Option two, if I would like to dual boot, how would I proceed having the SSD to boot both systems? I also wish to ask about the Legacy and UEFI differences. Windows runs with UEFI. So when I'm installing Linux, should I run Legacy, and if I dual boot, what option to I choose?

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• Windows 7 Not Booting After Moving Partition

  - by Guillermo Phillips

  I have a Sony Vaio laptop. After using GParted to move the primary Windows partion, the laptop no longer boots, saying 'Operating system not found'. I don't have a recovery disc and the only other machine I have access to is a Mac Mini. I have tried creating a bootable USB using the recovery ISO from Microsoft. I can see all the files on the USB stick from my Mac. I followed the instructions here: http://borgstrom.ca/2010/10/14/os-x-bootable-usb.html I have set the laptop BIOS boot order to be 'External' first, but the laptop refuses to boot from the USB stick. I have previously been able to boot from a linux installation on the USB stick. Any help or ideas would be appreciated.

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• Windows 7 Enterprise, Service Pack 1. Software MS Office Excel 2010

  - by user327560

  In Excel I understand there is no mechanism to customise & re-label the Rows & Columns (i.e. Renaming Col. A to some text like "Item Number" and so on. My question is regarding if it's possible to start Row Numbering at zero, or to determine a pre-allocated number of rows which contain my Headers, and then the first Row with the detail is infact seen as Row 1? Reason for question is I work multiple INternational Projects and we use Excel to trsack alot of activities & issues. Oddly, many people will refer to, for example "Point 7"... Some people mean the ID 7 (which I have the first Column dedicated to ID Number), some mean Excel Row 7, which infact could be really ID 3, or 4 from Col. A.... Any easy way or workaround to just use the Excel Row Numbers but select from when Row 1 is counted?

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• Can I remove the original file while running "sort"?

  - by Spaceman

  I'm sorting a huge file, around 400 gigabytes. I'm running out of the disk space and I must do something quickly. Let's assume the original file is called original_file. So I execute (simplified) it as "sort original_file | gzip -c output_file" I use /home/tmp as a temporary dir. From what I see, there are a lot of intermediate files, like so: tmpA465 tmpB154 ... and so on. The smallest ones have size 12 megabytes. The largest have ~182 megabytes. So, it seems that the "sort" command have already split the original file into small pieces, and have sorted them, and now it is merging them into bigger parts (which will be, eventually, sorted as well). Please correct me if I'm wrong. Can I remove the original file right now without terminating the sort process? I've been waiting for a few days for that and it's important that the "sort" command will not fail and I will get the result file, finally. The OS is Ubuntu server 13.04, x64. Thanks!

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• Why is piping dd through gzip so much faster than a direct copy?

  - by Foo Bar

  I wanted to backup a path from a computer in my network to another computer in the same network over a 100MBit/s line. For this I did dd if=/local/path of=/remote/path/in/local/network/backup.img which gave me a very low network transfer speed of something about 50 to 100 kB/s, which would have taken forever. So I stopped it and decided to try gzipping it on the fly to make it much smaller so that the amount to transfer is less. So I did dd if=/local/folder | gzip > /remote/path/in/local/network/backup.img.gz But now I get something like 1 MB/s network transfer speed, so a factor of 10 to 20 faster. After noticing this, I tested this on several paths and files and it was always the same. Why does piping dd through gzip also increase the transfer rates by a large factor instead of only reducing the bytelength of the stream by a large factor? I'd expected even a small decrease in transfer rates instead, due to the higher CPU consumption while compressing, but now I get a double plus. Not that I'm not happy, but just wondering. ;)

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• two shops network

  - by edward

  okay so, I just opened up two shops in my hometown. The two stores is about 6 blocks apart, connecting them by wire is not really feasible in cost wise. What kind of network topology should I use for my small shops, there will be 5 computers, one is the sales computer ,another 4 as mentioned is the guest computer. I want the sales and guest computers network to be seperated. Both shops have same computers. The guest computers serves up simple website that has my shop catalog on it, I'm thinking of using a web server. So, how am i suppose to setup these networks, im planning to add in more computers in the future. Is it I need to station a single server at a shop, and all the computer connected to it? or is there any more effective methods? I'm no networking expert, would love to hear some advice.

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• Laptop white screen on power-up. Still displays via HDMI output

  - by Inno

  my wife's laptop recently started displaying a white screen. It doesn't show post or anything, just a white screen when it's powered on. However, it works normally with HDMI output to our television. I took it apart and fiddled with both ends of the display cable, but I either didn't fiddle correctly or that's just not the problem. I also noticed that the screen won't turn off anymore when the laptop is closed. Is there a name for the mechanism that controls this function, so I can try and locate it? My guesses are that the problem lies with the screen itself or the display cable, but I'm curious if there's anything else I might be overlooking. Also of note is that the left hinge is partially broken. The corner of the plastic computer case broke off, so the hinge is exposed and doesn't stay in place. I've tried holding it in place, wiggling it around, tapping various parts of the computer, but the white screen remains.

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• INACCESSIBLE_BOOT_DEVICE on Windows 8.1 Bootcamp

  - by Ruby MewMew

  I'm on my MacBook Pro and I want to use my Bootcamp partition to work on one screen. I've installed Parallels 9 and after I installed it and configured it to look at my Windows partition through the Bootcamp Wizard it gives me an "INACCESSIBLE_BOOT_DEVICE" error when I try and boot. I figured it must be an issue with parallels so I've rebooted and now I'm getting it when I try to boot into Windows 8.1. Any ideas? Thanks

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• Finding out with process binds a hotkey [duplicate]

  - by Christian

  This question already has an answer here: How can I determine which process owns a hotkey in Windows? 2 answers Somehow I have set a hotkey that start DoubleCommander whenever I press AltGr+s. Unfortunately I forgot which program is responsible for that behavior and therefore how to change it back. How do I find out which program is responsible to change it back?

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• Windows 8 screen goes black after a minute

  - by Deni

  My laptop screen goes black after about a minute of inactivity. I've already tried setting the Screen Saver to anything other than Blank and increasing the Wait but it still keeps happening. Also checked my Power Options, the plan I currently have is set to turn the screen off only after an hour. If I move the mouse or press any key on the keyboard the screen wakes up. Any other ideas why this is happening and how to fix it?

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• Windows 8 Automatic Logon Tick Box Missing

  - by Luke Kenny

  Recently (in the past few days,) perhaps following the latest Windows Update, it appears the tick box to allow automatic logon in "control userpasswords2" or "netplwiz" has disappeared. I have two machines running Windows 8 and the option is no longer available for either. Both machines user a Microsoft account, rather than a local account, for the primary user to logon. The only other recent change I can think of, and I am confident this change was made well before this issue arose, was enabling HomeGroup. How can I re-enable automatic logon for the affected user?

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• domain MX record not found while installing Zimbra

  - by user1347219

  I am getting this error: DNS ERROR resolving MX for localhost.localdomain It is suggested that the domain name have an MX record configured in DNS Re-Enter domain name? [Yes] named file: $ttl 38400 localhost.localdomain. IN SOA centoslpt.localhost.localdomain. test.localhost.localdomain. ( 1357549995 10800 3600 604800 38400 ) localhost.localdomain. IN NS centoslpt.localhost.localdomain. centoslpt.localhost.localdomain. IN A 192.168.1.15 mail.localhost.localdomain. IN MX 10 192.168.1.15 why is MX record not detected pls, I am using BIND and webmin.

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• Passing two arguments to a command using pipes

  - by firebat

  Usually, we only need to pass one argument: echo abc | cat echo abc | cat some_file - echo abc | cat - some_file Is there a way to pass two arguments? Something like {echo abc , echo xyz} | cat cat `echo abc` `echo xyz` I could just store both results in a file first echo abc > file1 echo xyz > file2 cat file1 file2 But then I might accidentally overwrite a file, which is not ok. This is going into a non-interactive script. Basically, I need a way to pass the results of two arbitrary commands to cat without writing to a file. UPDATE: Sorry, the example masks the problem. While { echo abc ; echo xyz ; } | cat does seem to work, the output is due to the echos, not the cat. A better example would be { cut -f2 -d, file1; cut -f1 -d, file2; } | paste -d, which does not work as expected. With file1: a,b c,d file2: 1,2 3,4 Expected output is: b,1 d,3 RESOLVED: Use process substitution: cat <(command1) <(command2) Alternatively, make named pipes using mkfifo: mkfifo temp1 mkfifo temp2 command1 > temp1 & command2 > temp2 & cat temp1 temp2 Less elegant and more verbose, but works fine, as long as you make sure temp1 and temp2 don't exist before hand.

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• How to Add a Taskbar to the Desktop in Ubuntu 14.04

  - by Lori Kaufman

  If you’ve switched to Ubuntu from Windows, it may take some time to get used to the new and different interface. However, you can easily incorporate a familiar Windows feature, the Taskbar, into Ubuntu to make the transition easier. A tool called Tint2 provides a bar at the bottom of the Ubuntu Desktop that resembles the Windows Taskbar. We will show you how to install it and make it start every time you log into Ubuntu. NOTE: When we say to type something in this article and there are quotes around the text, DO NOT type the quotes, unless we specify otherwise. Press Ctrl + Alt + T to open a Terminal window. To install Tint2, type the following line at the prompt and press Enter. sudo apt-get install tint2 Type your password at the prompt and press Enter. The progress of the installation displays and then a message displays saying how much disk space will be used. When asked if you want to continue, type a “y” and press Enter. When the installation has finished, close the Terminal window by typing “exit” at the prompt and pressing Enter. Click the Search button at the top of the Unity bar. Start typing “startup applications” in the Search box. Items that match what you type start displaying below the Search box. When the Startup Applications tool displays, click the icon to open it. On the Startup Applications Preferences window, click Add. On the Add Startup Program dialog box, enter a name for the startup application. This name displays in the list on the Startup Applications Preferences window. Type “tint2” in the Command edit box, enter a description in the Comment edit box, if desired, and click Add. Tint2 is added as a startup program and will start every time you log into Ubuntu. Click Close to close the Startup Applications Preferences window. Log out and log back in to make the Taskbar available on the desktop. You do not need to reboot the computer for this change to take effect. Now, when you minimize a program, an icon for it displays on the Taskbar at the bottom of the screen, just like the Taskbar in Windows. If you decide that you don’t want the Taskbar to display every time you log into Ubuntu, you can uncheck the Tint2 startup program on the Startup Applications Preferences window. You don’t need to delete it from the list.

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• How do NTP Servers Manage to Stay so Accurate?

  - by Akemi Iwaya

  Many of us have had the occasional problem with our computers and other devices retaining accurate time settings, but a quick sync with an NTP server makes all well again. But if our own devices can lose accuracy, how do NTP servers manage to stay so accurate? Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites. Photo courtesy of LEOL30 (Flickr). The Question SuperUser reader Frank Thornton wants to know how NTP servers are able to remain so accurate: I have noticed that on my servers and other machines, the clocks always drift so that they have to sync up to remain accurate. How do the NTP server clocks keep from drifting and always remain so accurate? How do the NTP servers manage to remain so accurate? The Answer SuperUser contributor Michael Kjorling has the answer for us: NTP servers rely on highly accurate clocks for precision timekeeping. A common time source for central NTP servers are atomic clocks, or GPS receivers (remember that GPS satellites have atomic clocks onboard). These clocks are defined as accurate since they provide a highly exact time reference. There is nothing magical about GPS or atomic clocks that make them tell you exactly what time it is. Because of how atomic clocks work, they are simply very good at, having once been told what time it is, keeping accurate time (since the second is defined in terms of atomic effects). In fact, it is worth noting that GPS time is distinct from the UTC that we are more used to seeing. These atomic clocks are in turn synchronized against International Atomic Time or TAI in order to not only accurately tell the passage of time, but also the time. Once you have an exact time on one system connected to a network like the Internet, it is a matter of protocol engineering enabling transfer of precise times between hosts over an unreliable network. In this regard a Stratum 2 (or farther from the actual time source) NTP server is no different from your desktop system syncing against a set of NTP servers. By the time you have a few accurate times (as obtained from NTP servers or elsewhere) and know the rate of advancement of your local clock (which is easy to determine), you can calculate your local clock’s drift rate relative to the “believed accurate” passage of time. Once locked in, this value can then be used to continuously adjust the local clock to make it report values very close to the accurate passage of time, even if the local real-time clock itself is highly inaccurate. As long as your local clock is not highly erratic, this should allow keeping accurate time for some time even if your upstream time source becomes unavailable for any reason. Some NTP client implementations (probably most ntpd daemon or system service implementations) do this, and others (like ntpd’s companion ntpdate which simply sets the clock once) do not. This is commonly referred to as a drift file because it persistently stores a measure of clock drift, but strictly speaking it does not have to be stored as a specific file on disk. In NTP, Stratum 0 is by definition an accurate time source. Stratum 1 is a system that uses a Stratum 0 time source as its time source (and is thus slightly less accurate than the Stratum 0 time source). Stratum 2 again is slightly less accurate than Stratum 1 because it is syncing its time against the Stratum 1 source and so on. In practice, this loss of accuracy is so small that it is completely negligible in all but the most extreme of cases. Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

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