ur public key and try SSH to the server, you should be able to login without a password now.
# ssh server1.myserver.com The authenticity of host 'server1.myserver.com (192.168.44.2)' can't be established. RSA key fingerprint is e3:c3:89:37:4b:94:37:d7:0c:d5:6f:9a:38:62:ce:1b. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added 'server1.myserver.com' (RSA) to the list of known hosts. Last login: Tue July 13 12:40:34 2014 from server2.myserver.com
3) Public key authentication can prevent brute force SSH attacks, but only if all password-based authentication methods are disabled. Once public key authentication has been confirmed to be working, disable regular password authentication by editing /etc/ssh/sshd_config and set the following option to “no”.
Ques 9. Explain the different Scenarios involved in TCP 3 way handshake ?
Ans – The TCP three way handshake is the process for establishing a TCP connection.We can explain 3 way handshake with a simple scenario where we assume a client computer is contacting a server to send it some information.
a) The client sends a packet with the SYN bit set and a sequence number of N.
b) The server sends a packet with an ACK number of N+1, the SYN bit set and a sequence number of X.
c) The client sends a packet with an ACK number of X+1 and the connection is established.
d) The client sends the data.
The first three steps in the above process is called the three way handshake.
Ques 10. As the disk space utilization was so high in the server, the Administrator has removed few files from the server but still the disk utilization is showing as high. What would be the reason ?
Ans – In Linux even if we remove a file from the mounted file system, that will still be in use by some application and for this application it remains available. Its because file descriptor in /proc/ filesystem is held open..So if there are such open descriptors to files already removed, space occupied by them considered as used. You find this difference by checking them using the “df” and “du” commands. While df is to show the file system usage, du is to report the file space usage. du works from files while df works at filesystem level, reporting what the kernel says it has available.
You can find all unlinked but held open files with:
# lsof | grep '(deleted)'
This will list the filename which is open witht he pid in which it is running. We can kill those Pids and which will stop these process and will recover the disk space responsible for this file.
Ques 11. What is rDNS and explain its benefits in the Linux Domain Name Systems ?
Ans – A typical DNS lookup is used to determine which IP address is associated with a hostname, and this is called Forward DNS lookup. A reverse DNS lookup is used for the opposite, to determine which hostname is associated with an IP address. Sometimes reverse DNS lookups are required for diagnostic purposes. Today, reverse DNS lookups are used mainly for security purposes to trace a hacker or spammer. Many modern mailing systems use reverse mapping to provide simple authentication using dual lookup: hostname-to-address and address-to-hostname. The rDNS ( reverse DNS ) is implemented using a specialized zone record for reverse lookups called PTR record. PTR records always resolve to names, never IP addresses.
Ques 12. What is sosreport, how do you generate it while working with your Redhat Support Team in production ?
Ans : Sosreport is a command-line utility in Redhat based linux destros (RHEL / CentOS) which collects system configuration and diagnostic information of your linux box like running kernel version, loaded modules, and system and service configuration files. This command also runs external programs to collect further information, and stores this output in the resulting archive. Sosreport is required when you have open a case with redhat for technical support. Redhat support Engineers will require sosreport of your server for troubleshooting purpose. To run sosreport, sos package should be installed. Sos package is part of default installation