File Transfer Protocol (FTP) is a standard network protocol used to exchange and manipulate files over a TCP/IP based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server applications. Applications were originally interactive command-line tools with a standardized command syntax, but graphical user interfaces have been developed for all desktop operating systems in use today. FTP is also often used as an application component to automatically transfer files for program internal functions. FTP can be used with user-based password authentication or with anonymous user access.
Types of FTP
Figure : Active And Passive FTP Illustrated
From a user management perspective there are also two types of FTP: regular FTP in which files are transferred using the username and password of a regular user FTP server, and anonymous FTP in which general access is provided to the FTP server using a well known universal login method.
Take a closer look at each type.
Active FTP
The sequence of events for active FTP is: - Your 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.
- Thus the ls listing that you asked for comes back over the port 20 to high port connection, not the port 21 control connection.
Active FTP may fail in cases where the client is protected from the Internet via many to one NAT (masquerading). This is because the firewall will not know which of the many servers behind it should receive the return connection.
Passive FTP
Passive FTP works differently: - Your 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 that connection.
- Whenever the client requests data over the control connection, the client initiates the data transfer connections to the server. The source port of these data transfer connections is always a high port on the client with a destination port of a high port on the server.
As Windows defaults to active FTP, and Linux defaults to passive, you'll probably have to accommodate both forms when deciding upon a security policy for your FTP server.
Configuruing FTP Server in Fedora Core 11 :
Install rpm by yum or rpm command:
[root@satish.malanchlinux.com]#cd /satishdata/install/Packages
[root@satish.malanchlinux.com Packages]#rpm –ivh –adi vsftpd*
Or
[root@satish.malanchlinux.com Packages]#yum install *ftp*
Start/Restart vsftpd demon:
[root@satish.malanchlinux.com]#/etc/inid/vsftpd start
[root@satish.malanchlinux.com]#/etc/inid/vsftpd restart
With Redhat / Fedora you can configure VSFTPD to start at boot you can use the chkconfig command.
[root@satish.malanchlinux.com]# chkconfig vsftpd on
The vsftpd.conf File
VSFTPD only reads the contents of its vsftpd.conf configuration file only when it starts, so you'll have to restart VSFTPD each time you edit the file in order for the changes to take effect. The file may be located in either the /etc or the /etc/vsftpd directories depending on your Linux distribution. This file uses a number of default settings you need to know about.
- VSFTPD runs as an anonymous FTP server. Unless you want any remote user to log into to your default FTP directory using a username of anonymous and a password that's the same as their email address, I would suggest turning this off. The configuration file's anonymous_enable directive can be set to no to disable this feature. You'll also need to simultaneously enable local users to be able to log in by removing the comment symbol (#) before the local_enable instruction.
- If you enable anonymous FTP with VSFTPD, remember to define the root directory that visitors will visit. This is done with the anon_root directive.
anon_root=/data/directory- VSFTPD allows only anonymous FTP downloads to remote users, not uploads from them. This can be changed by modifying the anon_upload_enable directive shown later.
- VSFTPD doesn't allow anonymous users to create directories on your FTP server. You can change this by modifying the anon_mkdir_write_enable directive.
- VSFTPD logs FTP access to the /var/log/vsftpd.log log file. You can change this by modifying the xferlog_file directive.
- By default VSFTPD expects files for anonymous FTP to be placed in the /var/ftp directory. You can change this by modifying the anon_root directive. There is always the risk with anonymous FTP that users will discover a way to write files to your anonymous FTP directory. You run the risk of filling up your /var partition if you use the default setting. It is best to make the anonymous FTP directory reside in its own dedicated partition.
# Allow anonymous FTP?anonymous_enable=YES...# The directory which vsftpd will try to change# into after an anonymous login. (Default = /var/ftp)anon_root=/data/directory...# Uncomment this to allow local users to log in.local_enable=YES...# Uncomment this to enable any form of FTP write command.# (Needed even if you want local users to be able to upload files)write_enable=YES...# Uncomment to allow the anonymous FTP user to upload files. This only# has an effect if global write enable is activated. Also, you will# obviously need to create a directory writable by the FTP user.#anon_upload_enable=YES...# Uncomment this if you want the anonymous FTP user to be able to create# new directories.#anon_mkdir_write_enable=YES...# Activate logging of uploads/downloads.xferlog_enable=YES...# You may override where the log file goes if you like.# The default is shown below.xferlog_file=/var/log/vsftpd.log...To activate or deactivate a feature, remove or add the # at the beginning of the appropriate line. Other vsftpd.conf Options
There are many other options you can add to this file: - Limiting the maximum number of client connections (max_clients)
- Limiting the number of connections by source IP address (max_per_ip)
- The maximum rate of data transfer per anonymous login. (anon_max_rate)
- The maximum rate of data transfer per non-anonymous login. (local_max_rate)
FTP Security Issues
FTP has a number of security drawbacks, but you can overcome them in some cases. You can restrict an individual Linux user's access to non-anonymous FTP, and you can change the configuration to not display the FTP server's software version information, but unfortunately, though very convenient, FTP logins and data transfers are not encrypted. The /etc/vsftpd.ftpusers File
For added security, you may restrict FTP access to certain users by adding them to the list of users in the /etc/vsftpd.ftpusers file. The VSFTPD package creates this file with a number of entries for privileged users that normally shouldn't have FTP access. As FTP doesn't encrypt passwords, thereby increasing the risk of data or passwords being compromised, it is a good idea to let these entries remain and add new entries for additional security. Anonymous Upload
If you want remote users to write data to your FTP server, then you should create a write-only directory within /var/ftp/pub. This will allow your users to upload but not access other files uploaded by other users. The commands you need are: [root@satish.malanch.com tmp]# mkdir /var/ftp/pub/upload[root@satish.malanch.com tmp]# chmod 722 /var/ftp/pub/uploadFTP Greeting Banner
Change the default greeting banner in the vsftpd.conf file to make it harder for malicious users to determine the type of system you have. The directive in this file is. ftpd_banner= Welcome to Satish Malnch ftp services
