Wednesday, 23 October 2013

Interview Questions FTP Server

                         ############  Interview Questions FTP Server  ############

Q. What is ftp?
Ans: ftp is a file transfer protocol, used to share files ,directories.
Q: How to deny specific users access to the FTP server ?
To deny specific users access to the FTP server, add their usernames to the /etc/vsftpd/
ftpusers file. By default, system users such as root and nobody are included in this list.

Q: Can we create logs for ftp authenticated sessions ?
Ans: Yes, If the xferlog_enable directive in vsftpd.conf is set to YES, file transfers using the FTP protocol are logged to /var/log/xferlog. Information such as a time stamp, IP address of the client, the file being transferred, and the username of the person who authenticated the connection is included in the log entry.

Q: What is meaning of max_clients parameter ?
Ans: Maximum number of clients that can connect at one time.
If set to 0, the number of clients is unlimited.

Q: On which port VSFTP server works ?
Ans: FTP uses two ports, 20 and 21. By default, the FTP server listens for requests on port 21.
After a connection is established, the client sends commands to the server on port 21.
However, port 20 is used when the server sends data back to the client.
Q: How to restart VSFTP server ?
service vsftpd restart

Q: How to allow Anonymous FTP ?
Ans: Anonymous FTP is enabled by default by setting the anonymous_enable directive in
/etc/vsftpd/vsftpd.conf to YES.

Q: - For Redhat Linux or Fedora which package is required for FTP service ?
Ans: Red Hat Enterprise Linux 6 or FEDORA includes the vsftpd FTP service.
Rpm -qa | grep vsftpd to chceck before installing
yum install vsftpd* -y to install vsftpd package

Q: - Important Configuration file for vsftp server ?
The FTP server uses the /etc/vsftpd/vsftpd.conf configuration file. Using this file, you
can set options for displaying a custom banner message after users log in, setting the
default file permissions for uploaded files, and setting the port on which to listen for
incoming connections.

Q: - What is FTP ?
Ans: FTP client connects to the FTP server by establishing an FTP control connection to port 21 of the server. Your commands such as 'ls' and 'get' are sent over this connection. Whenever the client requests data over the control connection, the server initiates data transfer connections back to the client. The source port of these data transfer connections is always port 20 on the server, and the destination port is a high port (greater than 1024) on the client.

Q: - What is Passive mode?
Ans: Passive mode, like active mode, is initiated by the FTP client application. When requesting data from the server, the FTP client indicates it wants to access the data in passive mode and the server provides the IP address and a random, unprivileged port (greater than 1024) on the server. The client then connects to that port on the server to download the requested information.

Q: - Explain directive "session_support?
When enabled, vsftpd attempts to maintain login sessions for each user through Pluggable Authentication Modules (PAM).

Q: - Is there any way to monitor clients connected to vsftpd?
Ans : Yes. We actually have two slightly different methods to monitor vsftpd clients. First, make sure you have enabled the config option, "setproctitle_enable=YES" like in our example above and restart your vsftpd server. Then run the command "watch ps -Cvsftpd -o user, pid, stime, cmd" to watch the processes including ip, username and actions like idle or data retrieval.
Q: - I want to copy multiple files with out prompting for any info, how can I do that one?
ftp -i ftpserver

Q: - Local users cannot log in. How to resolve this issue?
Ans :Check "local_enable=YES" in your /etc/vsftpd/vsftpd. conf to allow local users to log in.

Q: - How to change vsftpd default port?
Ans :Set "listen_port" option in "vsftpd.conf"

Q: - How to restrict some IP's not use my FTP server?
go to /etc/hosts.deny
vsftpd: (or) ip address
save it and do the following
service vsftpd restart
chkconfig vsftpd on

Q: - Does vsftpd support IPv6?
Ans :Yes

Q. How to change ftp default port? 
Ans :By setting listen_port option in vsftpd.conf

Q. How much data can be send in a Single FTP session
Ans :The amount of data which can be send is infinite. FTP uses TCP. The TCP channel can be used until the complete data is transferred. Multiple TCP segments can be used within the same TCP channel.

Q. Can an IP packet carry FTP data
Ans :An IP packet is a carrier protocol used for carrying application layer data. The FTP data is packaged in TCP, which is sent to the intended recepient using the IP protocol in an IP based network

Q. How does Active FTP communication Work
Ans :When a FTP client initiates a connection with FTP server in Active FTP mode, initially the FTP client initiates tcp 3 way handshake with the FTP server. This is initiated to port 21 on the server and is referred to as the control connection. Once the control connection is successful, the client responds with the port command to the server, which contains the port number to which the server needs to initiate the second TCP connection. The server initiates the connection ( 2ND tcp 3 way handshake) from port 20 on it to the port number provided by the client. This is known as data connection.Once the data connection connection is succesfull, the data transmission can commence.

Q. How many TCP connections are used by Active and Passive FTP respectively 
Ans :Both Active and Passive FTP uses two TCP connections

Q. Why does Active FTP not work when a firewall is involved between the client and the server. 
Ans :In Active FTP , once the FTP client establishes the connectivity with the server, the server initiates the data connection to the client. This is a TCP connection which is initiated from a system (FTP Server) which resides outside the network. All traffic initiated from outside is considered as untrusted, and the firewall would drop it by default.

Q. How does Passive FTP resolve the issue with firewalls
Ans :In Passive FTP, the FTP client initiates the connection with the server and not the FTP server with the client. Since the client is residing within the network , the connection is considered as trusted by the firewall.

Q. In Active FTP, which TCP port is used for initiating the control channel 
Ans :Port 21

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