Controlling FTP Server Access

You can use the following configuration files in the /etc/ftpd directory to control access to the FTP server.

• ftpusers is used to list users who are denied access to the FTP server.

• ftphosts is used to allow or deny login from various hosts to various accounts on the FTP server.

• ftpaccess is the main FTP configuration file. The FTP server only reads the /etc/ftpd/ftpaccess file if called with the -a option. When the ftpaccess file is used, all users must be members of a class to be allowed access to the FTP server. You can specify many ftpaccess directives that apply only to a particular class.

  For further information, see ftpusers(4), ftphosts(4), and ftpaccess(4)

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  Note -

  In all FTP server configuration files, lines beginning with # signs are treated as comments.

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How to Define FTP Server Classes

To log in to the FTP server, users must be members of a class when the ftpaccess file is used. To add the class directive to the ftpaccess file, you specify the class name, typelist of users who are permitted access from a particular host.

1. Become superuser.

2. Add entries for anonymous, guest, and real users in the ftpaccess file.

   class class typelist addrglob[addrglob...]

   class	Keyword that is used to define FTP users.
   class	A name that is defined by the class keyword. Each login is compared against a list of defined classes. The logged in user is considered a member of the first class matched.
   typelist	A comma-separated list of the keywords that match the three types of users: anonymous, guest, and real.
   addrglob	A globbed domain name or a globbed numeric address. The addrglob can also be the name of a file, starting with a slash (`/`), which contains additional address globs: address:netmask or address/cidr. Here are some examples of globbed addresses: Numeric IPv4 address: 10.1.2.3 Globbed domain name *.provider.com Globbed numeric IPv4 address 10.1.2.* Numeric IPv4 address:netmask 10.1.2.0:255.255.255.0 Numeric IPv4 address/CIDR 10.1.2.0/24 Numeric IPv6 address: 2000::56:789:21ff:fe8f:ba98 Numeric IPv6 address/CIDR: 2000::56:789:21ff:fe8f:ba98/120

Example-Defining FTP Server Classes

class  local  real,guest,anonymous *.provider.com
class  remote real,guest,anonymous *

The previous example defines the local class as any user of the type real, guest, or anonymous who logs in from *.provider.com. The last line defines remote as any user who logs in from anywhere other than *.provider.com.

How to Set User Login Limits

You can limit the number of simultaneous logins by users of a certain class with directives that are set in the ftpaccess file. Each login limit contains the name of a class, a UUCP-style days-of-week list, and a message file to display if the limit is exceeded.

To set user login limits, follow the steps in the next procedure.

1. Become superuser.

2. Add the following entries to the ftpaccess file:

   limit class n times [message_file]

   limit	Keyword that is used to restrict simultaneous logins by the specified number of users of a defined class at certain connection times.
   class	A name that is defined by the class keyword. Each login is compared against a list of defined classes. The logged-in user is considered a member of the first class matched.
   n	Number of users.
   times	Day-of-week and time-of-day when the class can connect. Use Any for any day.
   message_file	Message file that is displayed if a user is denied access.

Example-Setting User Login Limits

limit   anon     50  Wk0800-1800		/etc/ftpd/ftpmsg.deny
limit   anon    100  Any				/etc/ftpd/ftpmsg.deny
limit   guest   100  Any				/etc/ftpd/ftpmsg.deny

The first line of the preceding example shows a limit of 50 simultaneous logins that are allowed to users of class anon during weekly work hours. The second line limits anon users to 100 simultaneous logins outside of working hours. The last line shows a limit of 100 guest logins that are allowed at any time. For information on how to specify day and time parameters, see ftpaccess(4).

The example further indicates that the content of the file /etc/ftpd/ftpmsg.deny is returned when a specified login limit is reached, assuming ftpmsg.deny exists. For information on using the /usr/sbin/ftpcount command to view the number and login limit for each class of user who is logged in at a particular time, see ftpcount(1).

Users are allowed login to the FTP server unless a specified limit is reached. Anonymous users are logged in as the user ftp. Real users are logged in as themselves, and guests are logged in as real users with a chroot environment to limit access privileges.

For information on using the /usr/sbin/ftpwho command to check the identities of the users logged into the FTP server, see ftpwho(1).

How to Control the Number of Invalid Login Attempts

If a login to the FTP server fails because of a problem such as misspelling required information, login is usually repeated. The user is allowed a specific number of consecutive login attempts before a message is logged to the syslog file. At that point, the user is disconnected. You can set a failure limit on the number of login attempts by following steps in the next procedure.

1. Become superuser.

2. Add the following entries to the ftpaccess file.

   loginfails n

   loginfails	Keyword that is used to assign the number of login failures that are permitted before the FTP connection is terminated
   n	Number of times a login can fail

Example-Controlling the Number of Invalid Login Attempts

loginfails 10

The preceding example states that the user is disconnected from the FTP server after 10 failed login attempts.

How to Disallow FTP Server Access to Particular Users

The /etc/ftpd/ftpusers file lists names of users who are not allowed to log in to the FTP server. When login is attempted, the FTP server checks the /etc/ftpd/ftpusers file to determine whether the user should be denied access. If the user's name is not found in that file, the server then searches the /etc/ftpusers file.

If the user's name is matched in /etc/ftpusers, a syslogd message is written with a statement that the match was found in a deprecated file. The message also recommends the use of /etc/ftpd/ftpusers instead of /etc/ftpusers.

Support for the /etc/ftpusers file has been deprecated in this release. If the /etc/ftpusers file exists when the FTP server is installed, the file is moved to /etc/ftpd/ftpusers.

For additional information, see syslogd(1M), in.ftpd(1M), and ftpusers(4)

1. Become superuser.

2. Add entries to the /etc/ftpd/ftpusers file for users who are not allowed to log in to the FTP server.

Example-How to Disallow FTP Server Access

root
daemon
bin
sys
adm
lp
uccp
nuucp
listen
nobody
noaccess
nobody4

The previous example lists the typical entries in the ftpusers file. User names match entries in the /etc/passwd. The list generally includes the superuser root and other administrative and system application identities.

The root entry is included in the ftpusers file as a security measure. The default security policy is to disallow remote logins for root. The policy is also followed for the default value that is set as the CONSOLE entry in the /etc/default/loginfile. See login(1).

How to Restrict Access to the Default FTP Server

In addition to the controls mentioned previously, you can add explicit statements to the ftpaccess file to restrict access to the FTP server.

1. Become superuser.

2. Add the following entries to the ftpaccess file.

   1. By default, all users are allowed access to the default (non-virtual) FTP server. To deny access for specific users (other than anonymous), add the following entry:

      defaultserver deny username [username...]

      defaultserver	Keyword that is used to identify the non-virtual server to which access can be denied or allowed
      username	Login name of a user with restricted access to the defaultserver

   2. To allow access for users who are not listed on the deny line, add the following line:

      defaultserver allow username [username...]

   3. To prevent access by anonymous users, add the entry:

      defaultserver private

Example-Restricting Access to the Default FTP Server

defaultserver deny *
defaultserver allow username

The previous example states that the FTP server denies access to all users except anon users and those users who are listed on the allow line.

You can also use the ftphosts file to deny access to particular login accounts from various hosts. See ftphosts(4) for additional information.