The FTP User

After installing Ubuntu, an FTP user will be created. This user is not a normal user per se, but a name for anonymous FTP users. The FTP user entry in /etc/passwd looks like

ftp:x:14:50:FTP User:/home/ftp:/bin/false 

Note

The FTP user, as discussed here, applies to anonymous FTP configurations and server setup.

Also, note that other Linux distributions might use a different default directory, such as /usr/local/ftp, for FTP files and anonymous users.

This entry follows the standard /etc/passwd enTRy: username, password, User ID, Group ID, comment field, home directory, and shell.

To learn more about /etc/password, see the section "The Password File" in Chapter 14, "Managing Users."

Each of the items in this entry is separated by colons. In the preceding example, you can see that the Ubuntu system hosting the server uses shadowed password because an x is present in the traditional password field. The shadow password system is important because it adds an additional level of security to Ubuntu; the shadow password system is normally installed during the Ubuntu installation.

The FTP server software uses this user account to assign permissions to users connecting to the server. By using a default shell of /bin/false for anonymous FTP users versus /bin/bash or some other standard, interactive shell, an anonymous FTP user, will be unable to log in as a regular user. /bin/false is not a shell, but a program usually assigned to an account that has been locked. As root inspection of the /etc/shadow file shows (see Listing 23.2), it is not possible to log in to this account, denoted by the * as the password.

Listing 23.2. Shadow Password File ftp User Entry

# cat /etc/shadow bin:*:11899:0:99999:7::: daemon:*:11899:0:99999:7::: adm:*:11899:0:99999:7::: lp:*:11899:0:99999:7::: ... ftp:*:12276:0:99999:7::: ... 

The shadow file (only a portion of which is shown in Listing 23.2) contains additional information not found in the standard /etc/passwd file, such as account expiration, password expiration, whether the account is locked, and the encrypted password. The * in the password field indicates that the account is not a standard login account; thus, it does not have a password.

Although shadow passwords are in use on the system, passwords are not transmitted in a secure manner when using FTP. Because FTP was written before the necessity of encryption and security, it does not provide the mechanics necessary to send encrypted passwords. Account information is sent in plain text on FTP servers; anyone with enough technical knowledge and a network sniffer can find the password for the account you connect to on the server. Many sites use an anonymous-only FTP server specifically to prevent normal account passwords from being transmitted over the Internet.

Figure 23.4 shows a portion of an ethereal capture of an FTP session for an anonymous user and his password ("foobarbaz"). The ethereal client is a graphical browser used to display network traffic in real time, and it can be used to watch packet data, such as an FTP login on a LAN.

Figure 23.4. The ethereal client can filter and sniff FTP sessions to capture usernames and passwords.

Quick and Dirty FTP Service

Conscientious Linux administrators will take the time to carefully install, set up, and configure a production FTP server before offering public service or opening up for business on the Internet. However, you can set up a server very quickly on a secure LAN by following a few simple steps:

1.	Ensure that the FTP server package is installed, networking is enabled, and firewall rules on the server allow FTP access. See Chapter 18, "Network Connectivity," to learn about firewalling.
2.	If anonymous access to server files is desired, populate the /home/ftp/pub directory. Do this by mounting or copying your content, such as directories and files under this directory.
3.	Edit, and then save the appropriate configuration file (such as vsftpd.conf for vsftpd) to enable access.
4.	If you are using wu-ftpd, you must then start or restart inetd like so: /etc/init.d/inetutils-inetd restart. Make sure you have the inetutilsinetd package installed through Synaptic/APT. If you are using vsftpd, you must then start or restart the server like so: service vsftpd start.