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I'm trying to login using ssh keys but server is only allows to login me using a password. The following message is logged into server /var/log/secure file:

Feb 25 06:36:50 ns5 sshd[26681]: Authentication refused: bad ownership or modes for file /home/user/.ssh/authorized_keys2

How do I fix this problem?
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Q. I've couple of remote servers and I'd like to access few admin only application running on port 10000 and 3001. My firewall only allows port 80, 443, 25, 22 and 110 for public access. Do I need to open port 10000 and 3001 for everyone using firewall? How do I access my admin only apps without opening port 10000 and 3001?
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How do I restart dovecot IMAP4 / IMAP (Internet Message Access Protocol) under RHEL / CentOS / Fedora / Debian / Ubutnu Linux?
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How do I restart NFS server when network link goes down between two servers or NFS client / server under CentOS 4.x / RHEL / CentOS Linux 5.x?
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Linux: Tune NFS Performance

What values can be tuned to improve NFS performance? How do I improve my client performance under Linux operating systems?
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Edit DNS Configuration Files

Q. How do I edit dns configuration file to add my ISPs nameservers?
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How do I find out my DHCP server address?

Q. In Windows XP / 2000 / Vista ipconfig /all command will show me DNS server and DHCP server ip address. But, how to know DHCP server address in Linux?

A. The DHCP protocol allows a host to contact a central server which maintains a list of IP addresses which may be assigned on one or more subnets. A DHCP client may request an address from this pool, and then use it on a temporary basis for communication on network.

dhclient.leases file

Under Linux you use command dhclient to obtain and managing dhcp tasks. In order to keep track of leases across system reboots and server restarts, dhclient keeps a list of leases it has been assigned in the dhclient.leases file. On startup, after reading the dhclient.conf file, dhclient reads the dhclient.leases file to refresh its memory about what leases it has been assigned.

When a new lease is acquired, it is appended to the end of the dhclient.leases file. In order to prevent the file from becoming arbitrarily large, from time to time dhclient creates a new dhclient.leases file from its in-core lease database. The old version of the dhclient.leases file is retained under the name dhclient.leases~ until the next time dhclient rewrites the database.

Find out DHCP server address

Usually dhclient.leases file is located at /var/lib/dhcp3/dhclient.leases, type the following command:
less /var/lib/dhcp3/dhclient.leases
OR
cat /var/lib/dhcp3/dhclient.leases
Alternatively, you can just use grep command to get DHCP server address, enter:
grep dhcp-server-identifier /var/lib/dhcp3/dhclient.leases
Output:

lease {
  interface "ra0";
  fixed-address 192.168.1.106;
  option subnet-mask 255.255.255.0;
  option dhcp-lease-time 86400;
  option routers 192.168.1.1;
  option dhcp-message-type 5;
  option dhcp-server-identifier 192.168.1.1;
  option domain-name-servers 208.67.222.222,208.67.220.220;
  option dhcp-renewal-time 43200;
  option dhcp-rebinding-time 75600;
  option host-name "vivek-desktop";
  renew 0 2007/12/9 05:17:36;
  rebind 0 2007/12/9 15:06:37;
  expire 0 2007/12/9 18:06:37;
}
lease {
  interface "ra0";
  fixed-address 192.168.1.106;
  option subnet-mask 255.255.255.0;
  option routers 192.168.1.1;
  option dhcp-lease-time 86400;
  option dhcp-message-type 5;
  option domain-name-servers 208.67.222.222,208.67.220.220;
  option dhcp-server-identifier 192.168.1.1;
  option dhcp-renewal-time 43200;
  option dhcp-rebinding-time 75600;
  option host-name "vivek-desktop";
  renew 0 2007/12/9 06:11:22;
  rebind 0 2007/12/9 16:13:50;
  expire 0 2007/12/9 19:13:50;
}

A note about RHEL / CentOS / Fedora Linux user

Redhat and friends uses /var/lib/dhcp/dhclient.leases file:
less /var/lib/dhcp/dhclient.leases