How do I find out running processes were associated with each open port on Linux? How do I find out what process has open TCP port # 111 or UDP port 7000 under Linux using the CLI?
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I‘ve configured my Apache in chrooted jail at /jail/apache directory. However, my syslogd is not working and nothing gets logged using /dev/log and /jail/apache/dev/log. How do I fix this problem under CentOS 5.x AMD64 with SELinux?
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While going though my logs I found that BIND9 on Debian version 5.x is giving a warning which read as follows:

max open files (1024) is smaller than max sockets (4096)

How do I fix this problem?
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Q. How do I find out which service is listening on a specific port? How do I find out what program is listening on a specific TCP Port?
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Q. I’ve custom compile kernel and I’d like to upgrade my system to latest stable release. How do I upgrade FreeBSD to new release without using freebsd-update binary tool?
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Q. I’m new to Linux and how do I examine filesystem and identify regular files or directories?

A. Both Linux and UNIX comes with ls command for examining the filesystem. You can use ls command to look at the filesystem. ls command can display:
=> Character devices
=> Regular files
=> Sym links (symbolic links)
=> Directories
=> Pipes
=> Sockets
=> Block devices

ls command examples

Display /etc directory files, enter:
$ ls /etc
When invoked without any arguments, ls lists the files in the current working directory:
$ ls
Use -l (long option) to lists filenames, sizes, permissions, type and all other information:
$ ls /etc/passwd
Output:

-rw-r--r-- 1 root root 2453 Jul 17 16:25 /etc/passwd

See Linux / UNIX file permissions for more information

Directory

A directory is marked with a d as the first letter of the permissions field:
ls -ld /etc
Output:

drwxr-xr-x 88 root root 12288 Aug  5 23:46 /etc

Symbolic link

A symbolic link is marked with an l (lower case L) as the first letter of the permissions string:
ls -l /bin/nisdomainname
Output:

lrwxrwxrwx 1 root root 8 Jul 10 08:50 /bin/nisdomainname -> hostname

Similarly,

• A named pipe is marked with a p as the first letter of the permissions string.
• A socket is marked with a s as the first letter of the permissions string.
• A character device is marked with a c as the first letter of the permissions strings. (ls -l /dev/console)
• A block device is marked with a b (ls -l /dev/sdb1).