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1500 questions
107
votes
6 answers

"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 on the standard port and acknowledging port…
Mike B
  • 12,016
106
votes
5 answers

Best way to gracefully restart CentOS?

I always used the command: shutdown -r now However, sometimes that causes MySQL issues. What's the most graceful way to restart CentOS? I've seen: reboot and halt How can I gently reboot the machine?
User
  • 1,405
106
votes
15 answers

ssh tunnel refusing connections with "channel 2: open failed"

All of a sudden (read: without changing any parameters) my netbsd virtualmachine started acting oddly. The symptoms concern ssh tunneling. From my laptop I launch: $ ssh -L 7000:localhost:7000 user@host -N -v Then, in another shell: $ irssi -c…
lorenzog
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106
votes
5 answers

Should a wildcard SSL certificate secure both the root domain as well as the sub-domains?

I ask this question, because Comodo are telling me that a wildcard certificate for *.example.com will also secure the root domain example.com. So with a single certificate, both my.example.com and example.com are secured without warning from a…
josswinn
  • 1,175
106
votes
2 answers

escaping double quotes and percent signs (%) in cron

The following command works from prompt but not from crontab. grep abc /var/log/messages | grep "`date '+%B %d'`" | mail -s"abc log of `hostname`" s.o+`hostname`@gmail.com I need to add it to daily cron.
shantanuo
  • 3,619
106
votes
6 answers

list all packages from a repository in ubuntu / debian

is there a command to see what packages are available from a certain ppa repository?
opensas
  • 1,191
106
votes
3 answers

What is .crt and .key files and how to generate them?

I've the following configuration: SSLEngine on SSLCertificateFile /etc/httpd/conf/login.domain.com.crt SSLCertificateKeyFile /etc/httpd/conf/login.domain.com.key SSLCipherSuite ALL:-ADH:+HIGH:+MEDIUM:-LOW:-SSLv2:-EXP but I don't know how to…
Mohammad Ali Akbari
  • 1,793
106
votes
12 answers

nmap find all alive hostnames and IPs in LAN

How can I issue a nmap command that shows me all the alive machines' IP addresses and corresponding hostname s in the LAN that I am connected? (if this can be done in another way/tool you surely are welcome to answer)
şaloma
  • 1,217
105
votes
3 answers

What is the difference between service and systemctl?

Maybe this is a trivial question, but it is not totally clear to me. On one of our servers we have some background processes running which were started with service and some others which were started with systemctl, like this: $ service nginx…
Aidas Bendoraitis
  • 1,515
105
votes
8 answers

How to determine JAVA_HOME on Debian/Ubuntu?

On Ubuntu it is possible to have multiple JVMs at the same time. The default one is selected with update-alternatives. But this does not set the JAVA_HOME environment variable, due to a debian policy. I am writing a launcher script (bash), which…
Witek
  • 1,422
105
votes
2 answers

How to make PuTTY settings persistent?

Some PuTTY settings are valid only for the current session, and when I start it again, they are at the default value again. How can I change the default values?
Jader Dias
  • 4,745
105
votes
8 answers

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 an Apache server with ab that it's pretty easy to…
Ben Williams
  • 2,396
104
votes
11 answers

How can I get diff to show only added and deleted lines? If diff can't do it, what tool can?

How can I get diff to show only added and deleted lines? If diff can't do it, what tool can?
C. Ross
  • 3,105
104
votes
6 answers

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, why is this required?
dayuloli
  • 1,293
104
votes
17 answers

CentOS vs. Ubuntu

I had a web server that ran Ubuntu, but the hard drive failed recently and everything was erased. I decided to try CentOS on the machine instead of Ubuntu, since it's based on Red Hat. That association meant a lot to me because Red Hat is a…
DLH
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