68
votes
15answers
12k 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 ...
68
votes
5answers
61k views

“POSSIBLE BREAK-IN ATTEMPT!” in /var/log/secure — what does this mean?

I've got a CentOS 5.x box running on a VPS platform. My VPS host misinterpreted a support inquiry I had about connectivity and effectively flushed some iptables rules. This resulted in ssh listening ...
68
votes
4answers
97k views

How can I see Time-To-Live (TTL) for a DNS record?

I would like to see the Time-To-Live (TTL) value for a CNAME record. I have access to dig (on Apple Mac OS X), which gives me an answer like this: % dig host.example.gov <*SNIP*> ;; ANSWER ...
67
votes
13answers
44k views

How Often Do Windows Servers Need to be Restarted?

A little background: We have several Windows servers (2003, 2008) for our department. We're a division of IT so we manage our own servers. Of the four of us here I'm the only one with a slight amount ...
67
votes
9answers
50k 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, ...
67
votes
10answers
91k views

How to tell if a Linux system is big endian or little endian?

I know that certain processors are Big Endian and others are Little Endian. But is there a command, bash script, python script or series of commands that can be used at the command line to determine ...
67
votes
11answers
71k views

apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1 for ServerName

I keep getting this warning when I (re)start Apache. * Restarting web server apache2 apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1 for ...
67
votes
6answers
125k views

How do I create user accounts from the Terminal in Mac OS X 10.5?

I would like to be able to create new users in Mac OS X 10.5 remotely after ssh'ing into the machine. How do I do this?
67
votes
3answers
28k views

How to re-order windows, change the scroll shortcut, and modify the status bar contents in GNU Screen?

I learned to use gnu-screen and have been using it for the past several weeks. I got a grip of the basics now, and would be very helpful if I get these queries clarified: How do I rename / reorder / ...
67
votes
13answers
28k views

How does a team of Systems Administrators share passwords securely?

What are best practices for sharing hundreds of passwords among a few people? These passwords protect mission critical data, and cannot ever be visible beyond a small team.
67
votes
4answers
59k views

Disable caching when serving static files with Nginx (for development)

We are using Nginx to serve static files on a development platform. As it is a development platform, we'd like to disable caching so that each change is propagated to the server. The configuration of ...
66
votes
37answers
22k views

What is the single most influential book every sysadmin should read?

For software developers, there are some books you must absolutely read. What is the single most influential book every programmer should read? How about for sysadmins? Is there a similar list of ...
66
votes
10answers
55k views

memcache vs memcached?

I want to use memcached http://www.danga.com/memcached/ I have installed it through yum install memcached But now I need to connect to PHP, and there is an extension named memcache and one named ...
66
votes
10answers
214k views

Run Oracle SQL script and exit from sqlplus.exe via command prompt

I would like to run an Oracle script through SQL Plus via a Windows command prompt. The script does not contain an "exit" command, but I would still like SQL Plus to exit, returning control to the ...
66
votes
8answers
45k views

What steps do you take to secure a Debian server? [closed]

I am installing a Debian server which is connected directly to the Internet. Obviously I want to make it as secure as possible. I would like you guys/gals to add your ideas to secure it and what ...
66
votes
8answers
143k 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 ...
65
votes
10answers
86k views

How can I export the privileges from MySQL and then import to a new server?

I know how to export/import the databases using mysqldump & that's fine but how do I get the privileges into the new server. For extra points, there are a couple of existing databases on the new ...
65
votes
13answers
174k views

Using wget to recursively download whole FTP directories

I want to copy all of the files and folders from one host to another. The files on the old host sit at /var/www/html and I only have FTP access to that server, and I can't TAR all the files. Regular ...
65
votes
19answers
41k views

Does SpinRite do what it claims to do? [closed]

I don't have any real (i.e. professional) experience with Steve Gibson's SpinRite so I'd like to put this to the SF community. Does SpinRite actually do what it claims? Is it a good product to use? ...
65
votes
3answers
8k views

Is it possible to reboot a Linux OS without rebooting the hardware?

Is there a way to reboot a Linux system (Debian in particular) without rebooting the hardware? I have a RAID controller that takes a bit to get itself running before the OS starts up, and I would ...
65
votes
11answers
19k views

Multiple data centers and HTTP traffic: DNS Round Robin is the ONLY way to assure instant fail-over?

Multiple A records pointing to the same domain seem to be used almost exclusively to implement DNS Round Robin as a cheap load balancing technique. The usual warning against DNS RR is that it is not ...
64
votes
11answers
20k views

Why drop caches in Linux?

In our servers we have a habit of dropping caches at midnight. sync; echo 3 > /proc/sys/vm/drop_caches When I run the code it seems to free up lots of RAM, but do I really need to do that. Isn'...
64
votes
4answers
84k views

Running Cron every 2 hours [duplicate]

I have the cron job as shown below, and wanted it to run every 2 hours, but it keeps running every 2 minutes. Can someone tell me where I'm going wrong? * */2 * * * /path-to-script
64
votes
6answers
498k views

How to determine the hostname from an IP address in a Windows network?

My LAN has 50 Windows hosts. At the Windows command line I try ping to get the IP address of a running Windows machine. The question is how to get hostname of a specific IP address in the same ...
64
votes
11answers
268k views

How can I choose between using my ISP's DNS, or Google's 8.8.8.8? [closed]

It seems like a good idea to use Google's public DNS 8.8.8.8 and 8.8.4.4 because it's really fast -- much faster than my own ISP's DNS! -- and probably more reliable, too. That seems like a ...
64
votes
14answers
28k views

GIT as a backup tool

On a server, install git cd / git init git add . git commit -a -m "Yes, this is server" Then get /.git/ to point to a network drive (SAN, NFS, Samba whatever) or different disk. Use a cron job ...
64
votes
5answers
86k views

Clean way to write complex multi-line string to a variable

I need to write some complex xml to a variable inside a bash script. The xml needs to be readable inside the bash script as this is where the xml fragment will live, it's not being read from another ...
64
votes
2answers
89k views

SFTP logging: is there a way?

I'm wondering if there is a way to log commands received by the server. It can be all SSH commands, as long as it includes information on commands related to file transfer. I'm having issues with an ...
64
votes
5answers
15k views

How can I implement ansible with per-host passwords, securely?

I would like to use ansible to manage a group of existing servers. I have created an ansible_hosts file, and tested successfully (with the -K option) with commands that only target a single host ...
64
votes
3answers
34k views

What is the difference between Load Balancer and Reverse Proxy?

I'm not clear about the difference between Load Balancer and Reverse Proxy. They both seems having same behavior: distributing incoming requests to backend servers.
64
votes
3answers
21k views

Can I create a top level TLD? (.michael for instance)

It appears that ICANN is allowing the creation of top level domains. Instead of 'registering' a domain, you would essentially be signing up to be a registrar (you'd be giving out registrations on ...
63
votes
54answers
8k views

Hidden Features of Linux

Following in the spirit of Hidden Features of PowerShell and various others on Stack Overflow, what Linux commands or combinations of commands do you find essential in your work? Also See: Hidden ...
63
votes
23answers
102k views

How to copy a large number of files quickly between two servers

I need to transfer a huge amount of mp3s between two serves (Ubuntu). By huge I mean about a million files which are on average 300K. I tried with scp but it would have taken about a week. (about 500 ...
63
votes
19answers
19k views

The perfect server room?

What do I have to consider when I'm planning a new server room for a small company (30 PCs, 5 servers, a couple of switches, routers, UPS...)? What are the most important aspects in order to protect ...
63
votes
4answers
34k views

Where does email sent to *@example.com go? [closed]

So I've wondered this for a long time. Where does email sent to *@example.com go? If I accidentally sent sensitive information to *@example.com would some evil person (potentially at the IANA) be ...
63
votes
18answers
71k views

Why hasn't rsync caught on in the Windows world? [closed]

It seems that rsync is the de-facto standard for efficient file backup and sync in Unix/Linux. Does anyone have any thoughts on why it wouldn't have caught on in the Windows world? Why hasn't it ...
63
votes
13answers
91k views

What's the best way to check if a volume is mounted in a Bash script?

What's the best way to check if a volume is mounted in a Bash script? What I'd really like is a method that I can use like this: if <something is mounted at /mnt/foo> then <Do some ...
63
votes
7answers
106k views

Apache 2 startup warning: NameVirtualHost *:80 has no VirtualHosts

When my Ubuntu Apache server (Apache 2) starts up I get a warning message that reads: [warn] NameVirtualHost *:80 has no VirtualHosts However, the web server is working fine. What might I have ...
63
votes
11answers
161k views

How do I find if there is a rogue DHCP server on my Network?

What's the best approach towards determining if I have a rogue DHCP server inside my network? I'm wondering how most admins approach these kinds of problems. I found DHCP Probe through searching, ...
63
votes
6answers
72k views

When does /tmp get cleared?

I'm taking to putting various files in /tmp, and I wondered what the rules on deleting them are? I'm imagining it's different for different distributions, and I'm particularly interested in Ubuntu ...
62
votes
3answers
4k views

Rsync triggered Linux OOM killer on a single 50 GB file

I have a single 50 GB file on server_A, and I'm copying it to server_B. I run server_A$ rsync --partial --progress --inplace --append-verify 50GB_file root@server_B:50GB_file Server_B has 32 ...
62
votes
8answers
115k views

How to Unban an IP properly with Fail2Ban

I'm using Fail2Ban on a server and I'm wondering how to unban an IP properly. I know I can work with IPTables directly: iptables -D fail2ban-ssh <number> But is there not a way to do it with ...
62
votes
11answers
110k views

IIS 7.5 (Windows 7) - HTTP Error 401.3 - Unauthorized

I'm trying to test my ASP.Net website on localhost and I'm getting this error: HTTP Error 401.3 - Unauthorized You do not have permission to view this directory or page because of the access control ...
62
votes
6answers
52k views

Is X.Y.Z.0 a valid IP address?

Are IP addresses with a 0 in the last octet valid? 10.6.43.0 In my case, I have the the following netmask 255.255.252.0 What about a 0 for the other octets?
62
votes
7answers
6k views

DNS failing to propagate worldwide

I haven't changed anything related to the DNS entry for serverfault.com, but some users were reporting today that the serverfault.com DNS fails to resolve for them. I ran a justping query and I can ...
62
votes
6answers
23k views

Why use Chef/Puppet over shell scripts?

New to Puppet and Chef tools. Seems like the job that they are doing can be done with shell scripting. Maybe it was done in shell scripts until these came along. I would agree they are more readable. ...
62
votes
5answers
19k views

Active Directory explained

If you had to explain Active Directory to someone how would you explain it?
62
votes
11answers
161k views

“Add correct host key in known_hosts” / multiple ssh host keys per hostname?

Trying to ssh into a computer I control, I'm getting the familiar message: @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ @@@@@...
62
votes
2answers
19k views

Colors in bash after piping through less?

When I have dircolors defined life is full of... color. When I pipe ls through less to scroll around I lose the colors. Any suggestions?
62
votes
6answers
15k views

Why can't MX records point to an IP address?

I understand you should not point a MX record at an IP address directly, but should instead point it to an A record, which, in turns, points to the IP address of your mail server. But, in principle, ...

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