vsftpd - An Introduction to the Very Secure FTP Daemon

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System Administration

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TranslationInfo:[Author + translation history. mailto: or http://homepage

original in de Mario M. Knopf

de to en Jürgen Pohl

AboutTheAuthor:

Mario enjoys to keep busy with Linux, networks and other topics concerning security related issues. In his spare time he takes care of two webpages: neo5k.org and linuxwallpapers.de.

Abstract:

This article gives a basic introduction to the "Very Secure FTP Daemon". I am beginning with a general description of FTP and vsftpd. After that we will have a look at the installation, configuration and start options of the vsftp daemon. We are closing with a short functions test.

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vsftp

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Introduction

The File Transfer Protocol's purpose is the platform independent data transfer of the internet, it is based on a server/client architecture. RFC 959[1] determins FTP to be split in two different channels, one serves for the data (TCP-port 20) and the other for the control (TCP-port 21). Over the control channel the two sides (server and client) exchange commands for the initiation of the data transfer
A FTP connection involves four steps:

The FTP is using is the connection controlling TCP (Transmission Control Protocol) as transmission protocol which assures the arrival of the data at the recipient. Therefore there is no need for FTP to be concerned about paket loss or error checking during the data transfer. Simply expressed TCP makes sure that each data paket is arriving only once - without errors and in the correct sequence.

Data transmission differentiates between three different types of transfer whereas the completion of the stream mode is marked by an end-of-file (EOF) and in the two other transfer modii with an end-of-record (EOR) marker.

In addition there are two different transfer modes:

The ASCII-mode is being used for the transfer of text files, the binary mode is being used to transfer programs and similar data. The user does not need to select the transfer mode specifically since by now all FTP clients switch to the recognized type of file to be transfered.

Since the user recognicion and the password of the authentification are not encrypted it is very important to point out this potential security risk. This is the reason for some thoughts about the security of the FTP. In October 1997 RFC 2228[2] was finally published, which defined security specific addendums to the File Transfer Protocol.

vsftpd

vsftpd represents a server for unix like operating systems, it runs on platforms like Linux, ?BSD, Solaris, HP-UX and IRIX. It supports many features which are very much missed on other FTP-servers. Some of them are:

The name vsftpd stands for "very secure FTP daemon", which is one of the primary concerns of developer Chris Evans. From the very beginning of the development and the design of the FTP server high security was one of the concerns.

One example is the fact that vsftpd is operated in chroot mode, which means a program (in this case vsftpd) is assigned a new root directory (l), it can no longer access programs or files outside of that directory - it is so to speak 'locked up'. Should a FTP-server be compromised the potential attacker will be isolated from the rest of the system and extensive damage will be prevented. More information on chroot can be found in the the artikel under [3]. Article[4] is recommended for those interested in the specific security mechanism of vsftpd

With the many features - of which the requirements for the security of the FTP-service should be the highest priority - vsftpd elevates itself significantly above the other FTP-servers. WU-FTPD[5] may be mentioned here as a negative example due to a variety of numerous security gaps in the past couple of years.

Installation

The installation of the vsftpd daemon is fairly easy since complete RPM packages of vsftpd can be found in each main distribution, in many cases it is already installed. Otherwise the source can be found at [6] and manually installed.

After getting the source unpack the tarball, go to the directory created and execute make. Here is a demonstration of the necessary commands:

neo5k@phobos> tar xzvf vsftpd-x.x.x.tar.gz
neo5k@phobos> cd vsftpd-x.x.x
neo5k@phobos> make

Prior to this we should check if the user "nobody" and the directory "/usr/share/empty" exists and if necessary we need to create them. If we are planning access for anonymous users a user "ftp" with the home directory "/var/ftp" needs to be generated. This will be accomplished by entering the following two commands:

neo5k@phobos> mkdir /var/ftp
neo5k@phobos> useradd -d /var/ftp ftp

Because of security reasons the directory "/var/ftp" should not belong to the user "ftp", nor should he have writing privileges in it. With the next two commands we can change the owner and take writing privileges from other users if the user is already existing:

neo5k@phobos> chown root.root /var/ftp
neo5k@phobos> chmod og-w /var/ftp

After all pre-conditions are met we can now install the vsftp-daemon:

neo5k@phobos> make install

The manpages and the program should now be copied to the correct location in the data system. In case of unexpected complications manual copying of the files should do the job.

neo5k@phobos> cp vsftpd /usr/sbin/vsftpd
neo5k@phobos> cp vsftpd.conf.5 /usr/share/man/man5
neo5k@phobos> cp vsftpd.8 /usr/share/man/man8

Since the sample of our configuration file at this point has not been copied - it would make the introduction easier - we need another manual entry:

neo5k@phobos> cp vsftpd.conf /etc

Configuration

The configuration file may be found under "/etc/vsftpd.conf". Like with most configuration files comments are being marked with an initial hash key.

# Comment line

An examplary configuration could look like this:

# Anonymus FTP-access permitted? YES/NO
anonymous_enable=NO
 
# Permit anonymus upload? YES/NO
anon_upload_enable=NO
 
# Permission for anonymus users to make new directories? YES/NO
anon_mkdir_write_enable=NO
 
# Permission for anonymus users to do other write operations - like renaming or deleting? YES/NO
anon_other_write_enable=NO
 
# Log on by local users permitted? YES/NO
local_enable=YES
 
# Shall local users be locked into their home directory? YES/NO
chroot_local_user=YES
 
# Highest permitted data transfer rate in bytes per second for local logged on users. Default = 0 (unlimited)
local_max_rate=7200
 
# General write permission? YES/NO
write_enable=YES
 
# Enable messages when changing directories? YES/NO
dirmessage_enable=YES
 
# Welcome banner at users logon.
ftpd_banner="Welcome to neo5k's FTP service."
 
# Activate logging? YES/NO
xferlog_enable=YES
 
# Logging of all FTP activities? YES/NO
# Careful! This can generate large quantities of data.
log_ftp_protocol=NO
 
# Confirm connections are established on port 20 (ftp data) only. YES/NO
connect_from_port_20=YES
 
# Timeout during idle sessions
idle_session_timeout=600
 
# Data connection timeout
data_connection_timeout=120
 
# Access through Pluggable Authentication Modules (PAM)
pam_service_name=vsftpd
 
# Standalone operation? YES/NO - depending on operation mode (inetd, xinetd, Standalone)
# The author's FTP service is being startet with xinetd, therefore the value here is NO.
listen=NO

Starting the FTP-Service

vsftpd may operate in three different ways. One is through inetd or xinetd, the third is standalone operation.

inetd

If the FTP service shall be operated with inetd we open the configuration file "/etc/inetd.conf" with an editor:

neo5k@phobos> vi /etc/inetd.conf

We search for the lines pertaining to the FTP services and remove the comment mark in front of the vsftpd entry. If there is no such entry we may enter it. After that we have to restart inetd. The entry should look like this: 
# ftp   stream   tcp   nowait   root   /usr/sbin/tcpd   in.ftpd
ftp   stream   tcp   nowait   root   /usr/sbin/tcpd   vsftpd

xinetd

It is recommended to start the vsftp daemon with xinetd which is more up to date than inetd. Some updates are e.g. logging of requests, access control, binding of the service to a specific network interface and so on. A very good introduction to xinetd can be found under [7]. After the modification restart of xinetd is necessary. The configuration of xinetd could look like this: 
# vsftp daemon.
service ftp
{
     disable = no
     socket_type = stream
     wait = no
     user = root
     server = /usr/sbin/vsftpd
     per_source = 5
     instances = 200
     no_access = 192.168.1.3
     banner_fail = /etc/vsftpd.busy_banner
     log_on_success += PID HOST DURATION
     log_on_failure += HOST
     nice = 10
}

Standalone Operation

There is also the possibility to operate the vsftp daemon in standalone mode. For this we open again the file "/etc/vsftpd.conf" and make the following changes:

# Shall the vsftp daemon run in standalone operation? YES/NO
listen=YES

After that entry the daemon can be startet with following entry

neo5k@phobos> /usr/sbin/vsftpd &

If the search path has been entered correctly this entry will do the start

neo5k@phobos> vsftpd &

With the next entry we can check if the search path was entered correctly:

neo5k@phobos> echo $PATH
/usr/sbin:/bin:/usr/bin:/sbin:/usr/X11R6/bin

In standalone mode we have, of course, to watch that the vsftp daemon is not startet with inetd or xinetd.

Operation Test

After successful installation and configuration we are able to access our FTP server for the first time. 
neo5k@phobos> ftp phobos
Connected to phobos
220 "Welcome to neo5k's FTP service."
Name (phobos:neo5k): testuser
331 Please specify the password.
Password:
230 Login successful
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls -l
229 Entering Extended Passive Mode
150 Here comes the directory listing
drwxr-xr-x      11  500      100        400  May 07 16:22  docs
drwxr-xr-x       9  500      100        464  Feb 01 23:05  hlds
drwxr-xr-x      39  500      100       4168  May 10 09:15  projects
226 Directory send OK.
ftp>

Conclusion

As we noticed the vsftp daemon is not hard to install nor to configure. It offers many features and a high degree of security.

Of course, this introduction offers only a small glimpse into the environment offered by vsftpd, since the FTP server is offering numerous possibilities of configuration. Those of you who would like to research vsftpd more in depth should visit the project page[6] and review the extensive documentation.

Links

[1] ftp://ftp.rfc-editor.org/in-notes/rfc959.txt [RFC 959 - File Transfer Protocol]
[2] ftp://ftp.rfc-editor.org/in-notes/rfc2228.txt [RFC 2228 - FTP Security Extensions]
[3] linuxfocus.org: article225, January2002 [chroot]
[4] http://vsftpd.beasts.org/DESIGN [Security vsftpd]
[5] http://www.wu-ftpd.org/ [WU-FTPD]
[6] http://www.vsftpd.beasts.org/ [Home of vsftpd]
[7] linuxfocus.org: article 175, November2000 [xinetd]