Quick HOWTO : Ch15 : Linux FTP Server Setup
From Linux Home Networking
The File Transfer Protocol (FTP) is used as one of the most common means of copying files between servers over the Internet. Most web based download sites use the built in FTP capabilities of web browsers and therefore most server oriented operating systems usually include an FTP server application as part of the software suite. Linux is no exception.
This chapter will show you how to convert your Linux box into an FTP server using the default Very Secure FTP Daemon (VSFTPD) package included in Fedora.
FTP relies on a pair of TCP ports to get the job done. It operates in two connection channels as I'll explain:
FTP Control Channel, TCP Port 21: All commands you send and the ftp server's responses to those commands will go over the control connection, but any data sent back (such as "ls" directory lists or actual file data in either direction) will go over the data connection.
FTP Data Channel, TCP Port 20: This port is used for all subsequent data transfers between the client and server.
In addition to these channels, there are several varieties of FTP.
Types of FTP
From a networking perspective, the two main types of FTP are active and passive. In active FTP, the FTP server initiates a data transfer connection back to the client. For passive FTP, the connection is initiated from the FTP client. These are illustrated in Figure 15-1.
Figure 15-1 Active And Passive FTP Illustrated
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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.
The sequence of events for active FTP is:
FTP active mode therefore transfers data in a counter intuitive way to the TCP standard, as it selects port 20 as it's source port (not a random high port that's greater than 1024) and connects back to the client on a random high port that has been pre-negotiated on 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 works differently:
Passive FTP should be viewed as the server never making an active attempt to connect to the client for FTP data transfers. Because client always initiates the required connections, passive FTP works better for clients protected by a firewall.
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.
By default, the VSFTPD package allows regular Linux users to copy files to and from their home directories with an FTP client using their Linux usernames and passwords as their login credentials.
VSFTPD also has the option of allowing this type of access to only a group of Linux users, enabling you to restrict the addition of new files to your system to authorized personnel.
The disadvantage of regular FTP is that it isn't suitable for general download distribution of software as everyone either has to get a unique Linux user account or has to use a shared username and password. Anonymous FTP allows you to avoid this difficulty.
Anonymous FTP is the choice of Web sites that need to exchange files with numerous unknown remote users. Common uses include downloading software updates and MP3s and uploading diagnostic information for a technical support engineers' attention. Unlike regular FTP where you login with a preconfigured Linux username and password, anonymous FTP requires only a username of anonymous and your email address for the password. Once logged in to a VSFTPD server, you automatically have access to only the default anonymous FTP directory (/var/ftp in the case of VSFTPD) and all its subdirectories.
As seen in Chapter 6, "Installing Linux Software", using anonymous FTP as a remote user is fairly straight forward. VSFTPD can be configured to support user-based and or anonymous FTP in its configuration file which you'll see later.
Problems With FTP And Firewalls
FTP frequently fails when the data has to pass through a firewall, because firewalls are designed to limit data flows to predictable TCP ports and FTP uses a wide range of unpredictable TCP ports. You have a choice of methods to overcome this.
Note: The Appendix II, "Codes, Scripts, and Configurations", contains examples of how to configure the VSFTPD Linux firewall to function with both active and passive FTP.
Client Protected By A Firewall Problem
Typically firewalls don't allow any incoming connections at all, which frequently blocks active FTP from functioning. With this type of FTP failure, the active FTP connection appears to work when the client initiates an outbound connection to the server on port 21. The connection then appears to hang, however, as soon as you use the ls, dir, or get commands. The reason is that the firewall is blocking the return connection from the server to the client (from port 20 on the server to a high port on the client). If a firewall allows all outbound connections to the Internet, then passive FTP clients behind a firewall will usually work correctly as the clients initiate all the FTP connections.
Table 15-1 shows the general rules you'll need to allow FTP clients through a firewall:
Table 15-1 Client Protected by Firewall - Required Rules for FTP
Server Protected By A Firewall Problem
Typically firewalls don't let any connections come in at all. When a an incorrectly configured firewall protects an FTP server, the FTP connection from the client doesn't appear to work at all for both active and passive FTP.
Table 15-2 Rules needed to allow FTP servers through a firewall.
Most RedHat and Fedora Linux software product packages are available in the RPM format, whereas Debian and Ubuntu Linux use DEB format installation files. When searching for these packages remember that the filename usually starts with the software package name and is followed by a version number, as in vsftpd-1.2.1-5.i386.rpm. (For help on downloading and installing the required packages, see Chapter 6, Installing Linux Software).
Managing the vsftpd Server
Managing the FTP daemon is easy to do, but the procedure differs between Linux distributions. Here are some things to keep in mind.
Armed with this information you can know how to:
For more details on this, please take a look at the "Managing Daemons" section of Chapter 6 "Installing Linux Software"
Note: If you modify your daemon configuration file remember that the changes won't take effect till you restart the daemon.
Note: Remember to configure your daemon to start automatically upon your next reboot.
Testing the Status of VSFTPD
You can always test whether the VSFTPD process is running by using the netstat -a command which lists all the TCP and UDP ports on which the server is listening for traffic. This example shows the expected output.
[root@bigboy root]# netstat -a | grep ftp tcp 0 0 *:ftp *:* LISTEN [root@bigboy root]#
If VSFTPD wasn't running, there would be no output at all.
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
This file uses a number of default settings you need to know about.
The configuration file is fairly straight forward as you can see in the snippet below where we enable anonymous FTP and individual accounts simultaneously.
# 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:
Descriptions on this and more can be found in the vsftpd.conf man pages.
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.
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@bigboy tmp]# mkdir /var/ftp/pub/upload [root@bigboy tmp]# chmod 722 /var/ftp/pub/upload
FTP 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= New Banner Here
Using SCP As Secure Alternative To FTP
One of the disadvantages of FTP is that it does not encrypt your username and password. This could make your user account vulnerable to an unauthorized attack from a person eavesdropping on the network connection. Secure Copy (SCP) and Secure FTP (SFTP) provide encryption and could be considered as an alternative to FTP for trusted users. SCP does not support anonymous services, however, a feature that FTP does support.
You should always test your FTP installation by attempting to use an FTP client to log in to your FTP server to transfer sample files.
The most common sources of day-to-day failures are incorrect usernames and passwords.
Initial setup failures could be caused by firewalls along the path between the client and server blocking some or all types of FTP traffic. Typical symptoms of this are either connection timeouts or the ability to use the ls command to view the contents of a directory without the ability to either upload or download files. Follow the firewall rule guidelines to help overcome this problem. Connection problems could also be the result of typical network issues outlined in Chapter 4, "Simple Network Troubleshooting".
FTP has many uses, one of which is allowing numerous unknown users to download files. You have to be careful, because you run the risk of accidentally allowing unknown persons to upload files to your server. This sort of unintended activity can quickly fill up your hard drive with illegal software, images, and music for the world to download, which in turn can clog your server's Internet access and drive up your bandwidth charges.
In this example, anonymous FTP is not desired, but a group of trusted users need to have read only access to a directory for downloading files. Here are the steps:
1) Disable anonymous FTP. Comment out the anonymous_enable line in the vsftpd.conf file like this:
# Allow anonymous FTP? anonymous_enable=NO
2) Enable individual logins by making sure you have the local_enable line uncommented in the vsftpd.conf file like this:
# Uncomment this to allow local users to log in. local_enable=YES
3) Start VSFTP.
[root@bigboy tmp]# systemctl start vsftpd.service
4) Create a user group and shared directory. In this case, use /home/ftp-users and a user group name of ftp-users for the remote users
[root@bigboy tmp]# groupadd ftp-users [root@bigboy tmp]# mkdir /home/ftp-docs
5) Make the directory accessible to the ftp-users group.
[root@bigboy tmp]# chmod 750 /home/ftp-docs [root@bigboy tmp]# chown root:ftp-users /home/ftp-docs
6) Add users, and make their default directory /home/ftp-docs
[root@bigboy tmp]# useradd -g ftp-users -d /home/ftp-docs user1 [root@bigboy tmp]# useradd -g ftp-users -d /home/ftp-docs user2 [root@bigboy tmp]# useradd -g ftp-users -d /home/ftp-docs user3 [root@bigboy tmp]# useradd -g ftp-users -d /home/ftp-docs user4 [root@bigboy tmp]# passwd user1 [root@bigboy tmp]# passwd user2 [root@bigboy tmp]# passwd user3 [root@bigboy tmp]# passwd user4
7) Copy files to be downloaded by your users into the /home/ftp-docs directory
8) Change the permissions of the files in the /home/ftp-docs directory for read only access by the group
[root@bigboy tmp]# chown root:ftp-users /home/ftp-docs/* [root@bigboy tmp]# chmod 740 /home/ftp-docs/*
write_enable = NO
Remember, you must restart VSFTPD for the configuration file changes to take effect.
Sample Login Session To Test Functionality
Here is a simple test procedure you can use to make sure everything is working correctly:
1) Check for the presence of a test file on the ftp client server.
[root@smallfry tmp]# ll total 1 -rw-r--r-- 1 root root 0 Jan 4 09:08 testfile [root@smallfry tmp]#
2) Connect to bigboy via FTP
[root@smallfry tmp]# ftp 192.168.1.100 Connected to 192.168.1.100 (192.168.1.100) 220 ready, dude (vsFTPd 1.1.0: beat me, break me) Name (192.168.1.100:root): user1 331 Please specify the password. Password: 230 Login successful. Have fun. Remote system type is UNIX. Using binary mode to transfer files. ftp>
ftp> put testfile local: testfile remote: testfile 227 Entering Passive Mode (192,168,1,100,181,210) 553 Could not create file. ftp>
ftp> ls 227 Entering Passive Mode (192,168,1,100,35,173) 150 Here comes the directory listing. -rwxr----- 1 0 502 76288 Jan 04 17:06 vsftpd-1.1.0-1.i386.rpm 226 Directory send OK. ftp> get vsftpd-1.1.0-1.i386.rpm vsftpd-1.1.0-1.i386.rpm.tmp local: vsftpd-1.1.0-1.i386.rpm.tmp remote: vsftpd-1.1.0-1.i386.rpm 227 Entering Passive Mode (192,168,1,100,44,156) 150 Opening BINARY mode data connection for vsftpd-1.1.0-1.i386.rpm (76288 bytes). 226 File send OK. 76288 bytes received in 0.499 secs (1.5e+02 Kbytes/sec) ftp> exit 221 Goodbye. [root@smallfry tmp]#
[root@smallfry tmp]# ftp 192.168.1.100 Connected to 192.168.1.100 (192.168.1.100) 220 ready, dude (vsFTPd 1.1.0: beat me, break me) Name (192.168.1.100:root): anonymous 331 Please specify the password. Password: 530 Login incorrect. Login failed. ftp> quit 221 Goodbye. [root@smallfry tmp]#
Now that testing is complete, you can make this a regular part of your FTP server's operation.
FTP is a very useful software application that can have enormous benefit to a Web site or to collaborative computing in which files need to be shared between business partners. Although insecure, it is universally accessible, because FTP clients are a part of all operating systems and Web browsers. If data encryption security is of great importance to you, then you should probably consider SCP as a possible alternative. You can find more information on it in Chapter 17, "Secure Remote Logins and File Copying".