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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

17.2. TCP Wrappers Configuration Files

To determine if a client machine is allowed to connect to a service, TCP wrappers reference the following two files, which are commonly referred to as hosts access files:

  • /etc/hosts.allow

  • /etc/hosts.deny

When a client request is received by a TCP wrapped service, it takes the following basic steps:

  1. References /etc/hosts.allow. — The TCP wrapped service sequentially parses the /etc/hosts.allow file and applies the first rule specified for that service. If it finds a matching rule, it allows the connection. If not, it moves on to the next step.

  2. References /etc/hosts.deny. — The TCP wrapped service sequentially parses the /etc/hosts.deny file. If it finds a matching rule, it denies the connection. If not, access to the service is granted.

The following are important points to consider when using TCP wrappers to protect network services:

  • Because access rules in hosts.allow are applied first, they take precedence over rules specified in hosts.deny. Therefore, if access to a service is allowed in hosts.allow, a rule denying access to that same service in hosts.deny is ignored.

  • The rules in each file are read from the top down and the first matching rule for a given service is the only one applied. The order of the rules is extremely important.

  • If no rules for the service are found in either file, or if neither file exists, access to the service is granted.

  • TCP wrapped services do not cache the rules from the hosts access files, so any changes to hosts.allow or hosts.deny take effect immediately without restarting network services.

Warning Warning
 

If the last line of a hosts access file is not a newline character (created by pressing the [Enter] key), the last rule in the file fails and an error is logged to either /var/log/messages or /var/log/secure. This is also the case for a rule that spans multiple lines without using the backslash. The following example illustrates the relevant portion of a log message for a rule failure due to either of these circumstances:

warning: /etc/hosts.allow, line 20: missing newline or line too long

17.2.1. Formatting Access Rules

The format for both /etc/hosts.allow and /etc/hosts.deny are identical. Any blank lines or lines that start with a hash mark (#) are ignored, and each rule must be on its own line.

Each rule uses the following basic format to control access to network services:

<daemon list>: <client list> [: <option>: <option>: ...]

The following is a basic sample hosts access rule:

vsftpd : .example.com 

This rule instructs TCP wrappers to watch for connections to the FTP daemon (vsftpd) from any host in the example.com domain. If this rule appears in hosts.allow, the connection is accepted. If this rule appears in hosts.deny, the connection is rejected.

The next sample hosts access rule is more complex and uses two option fields:

sshd : .example.com  \
: spawn /bin/echo `/bin/date` access denied>>/var/log/sshd.log \
: deny

Note that each option field is preceded by the backslash (\). Use of the backslash prevents failure of the rule due to length.

This sample rule states that if a connection to the SSH daemon (sshd) is attempted from a host in the example.com domain, execute the echo command (which logs the attempt to a special file), and deny the connection. Because the optional deny directive is used, this line denies access even if it appears in the hosts.allow file. For a more detailed look at available options, refer to Section 17.2.2 Option Fields.

17.2.1.1. Wildcards

Wildcards allow TCP wrappers to more easily match groups of daemons or hosts. They are used most frequently in the client list field of access rules.

The following wildcards may be used:

  • ALL — Matches everything. It can be used for both the daemon list and the client list.

  • LOCAL — Matches any host that does not contain a period (.), such as localhost.

  • KNOWN — Matches any host where the hostname and host address are known or where the user is known.

  • UNKNOWN — Matches any host where the hostname or host address are unknown or where the user is unknown.

  • PARANOID — Matches any host where the hostname does not match the host address.

Caution Caution
 

The KNOWN, UNKNOWN, and PARANOID wildcards should be used with care as a disruption in name resolution may prevent legitimate users from gaining access to a service.

17.2.1.2. Patterns

Patterns can be used in the client list field of access rules to more precisely specify groups of client hosts.

The following is a list of the most common accepted patterns for a client list entry:

  • Hostname beginning with a period (.) — Placing a period at the beginning of a hostname matches all hosts sharing the listed components of the name. The following example applies to any host within the example.com domain:

    ALL : .example.com
    
  • IP address ending with a period (.) — Placing a period at the end of an IP address matches all hosts sharing the initial numeric groups of an IP address. The following example applies to any host within the 192.168.x.x network:

    ALL : 192.168.
    
  • IP address/netmask pair — Netmask expressions can also be used as a pattern to control access to a particular group of IP addresses. The following example applies to any host with an address range of 192.168.0.0 through 192.168.1.255:

    ALL : 192.168.0.0/255.255.254.0
     
    
    Important Important
     

    When working in the IPv4 address space, the address/prefix length (prefixlen) pair declarations are not supported. Only IPv6 rules can use this format.

  • [IPv6 address]/prefixlen pair — [net]/prefixlen pairs can also be used as a pattern to control access to a particular group of IPv6 addresses. The following example would apply to any host with an address range of 3ffe:505:2:1:: through 3ffe:505:2:1:ffff:ffff:ffff:ffff:

    ALL : [3ffe:505:2:1::]/64
     
    
  • The asterisk (*) — Asterisks can be used to match entire groups of hostnames or IP addresses, as long as they are not mixed in a client list containing other types of patterns. The following example would apply to any host within the example.com domain:

    ALL : *.example.com
    
  • The slash (/) — If a client list begins with a slash, it is treated as a file name. This is useful if rules specifying large numbers of hosts are necessary. The following example refers TCP wrappers to the /etc/telnet.hosts file for all Telnet connections:

    in.telnetd : /etc/telnet.hosts
    

Other, lesser used, patterns are also accepted by TCP wrappers. Refer to the hosts_access man 5 page for more information.

Warning Warning
 

Be very careful when using hostnames and domain names. Attackers can use a variety of tricks to circumvent accurate name resolution. In addition, disruption in DNS service prevents even authorized users from using network services.

It is, therefore, best to use IP addresses whenever possible.

17.2.1.3. Portmap and TCP Wrappers

When creating access control rules for portmap, do not use hostnames as portmap's implementation of TCP wrappers does not support host look ups. For this reason, only use IP addresses or the keyword ALL when specifying hosts in hosts.allow or hosts.deny.

In addition, changes to portmap access control rules may not take affect immediately without restarting the portmap service.

Widely used services, such as NIS and NFS, depend on portmap to operate, so be aware of these limitations.

17.2.1.4. Operators

At present, access control rules accept one operator, EXCEPT. It can be used in both the daemon list and the client list of a rule.

The EXCEPT operator allows specific exceptions to broader matches within the same rule.

In the following example from a hosts.allow file, all example.com hosts are allowed to connect to all services except cracker.example.com:

ALL: .example.com EXCEPT cracker.example.com

In the another example from a hosts.allow file, clients from the 192.168.0.x network can use all services except for FTP:

ALL EXCEPT vsftpd: 192.168.0.
Note Note
 

Organizationally, it is often easier to avoid using EXCEPT operators. This allows other administrators to quickly scan the appropriate files to see what hosts are allowed or denied access to services, without having to sort through EXCEPT operators.

17.2.2. Option Fields

In addition to basic rules allowing and denying access, the Red Hat Enterprise Linux implementation of TCP wrappers supports extensions to the access control language through option fields. By using option fields within hosts access rules, administrators can accomplish a variety of tasks such as altering log behavior, consolidating access control, and launching shell commands.

17.2.2.1. Logging

Option fields let administrators easily change the log facility and priority level for a rule by using the severity directive.

In the following example, connections to the SSH daemon from any host in the example.com domain are logged to the default authpriv syslog facility (because no facility value is specified) with a priority of emerg:

sshd : .example.com : severity emerg

It is also possible to specify a facility using the severity option. The following example logs any SSH connection attempts by hosts from the example.com domain to the local0 facility with a priority of alert:

sshd : .example.com : severity local0.alert
Note Note
 

In practice, this example does not work until the syslog daemon (syslogd) is configured to log to the local0 facility. Refer to the syslog.conf man page for information about configuring custom log facilities.

17.2.2.2. Access Control

Option fields also allow administrators to explicitly allow or deny hosts in a single rule by adding the allow or deny directive as the final option.

For instance, the following two rules allow SSH connections from client-1.example.com, but deny connections from client-2.example.com:

sshd : client-1.example.com : allow
sshd : client-2.example.com : deny

By allowing access control on a per-rule basis, the option field allows administrators to consolidate all access rules into a single file: either hosts.allow or hosts.deny. Some consider this an easier way of organizing access rules.

17.2.2.3. Shell Commands

Option fields allow access rules to launch shell commands through the following two directives:

  • spawn — Launches a shell command as a child process. This option directive can perform tasks like using /usr/sbin/safe_finger to get more information about the requesting client or create special log files using the echo command.

    In the following example, clients attempting to access Telnet services from the example.com domain are quietly logged to a special file:

    in.telnetd : .example.com \
      : spawn /bin/echo `/bin/date` from %h>>/var/log/telnet.log \
      : allow
    
  • twist — Replaces the requested service with the specified command. This directive is often used to set up traps for intruders (also called "honey pots"). It can also be used to send messages to connecting clients. The twist directive must occur at the end of the rule line.

    In the following example, clients attempting to access FTP services from the example.com domain are sent a message via the echo command:

    vsftpd : .example.com \
    : twist /bin/echo "421 Bad hacker, go away!"
    

For more information about shell command options, refer to the hosts_options man page.

17.2.2.4. Expansions

Expansions, when used in conjunction with the spawn and twist directives, provide information about the client, server, and processes involved.

Below is a list of supported expansions:

  • %a — Supplies the client's IP address.

  • %A — Supplies the server's IP address.

  • %c — Supplies a variety of client information, such as the username and hostname, or the username and IP address.

  • %d — Supplies the daemon process name.

  • %h — Supplies the client's hostname (or IP address, if the hostname is unavailable).

  • %H — Supplies the server's hostname (or IP address, if the hostname is unavailable).

  • %n — Supplies the client's hostname. If unavailable, unknown is printed. If the client's hostname and host address do not match, paranoid is printed.

  • %N — Supplies the server's hostname. If unavailable, unknown is printed. If the server's hostname and host address do not match, paranoid is printed.

  • %p — Supplies the daemon process ID.

  • %s —Supplies various types of server information, such as the daemon process and the host or IP address of the server.

  • %u — Supplies the client's username. If unavailable, unknown is printed.

The following sample rule uses an expansion in conjunction with the spawn command to identify the client host in a customized log file.

When connections to the SSH daemon (sshd) are attempted from a host in the example.com domain, execute the echo command to log the attempt, including the client hostname (by using the %h expansion), to a special file:

sshd : .example.com  \
: spawn /bin/echo `/bin/date` access denied to %h>>/var/log/sshd.log \
: deny

Similarly, expansions can be used to personalize messages back to the client. In the following example, clients attempting to access FTP services from the example.com domain are informed that they have been banned from the server:

vsftpd : .example.com \
: twist /bin/echo "421 %h has been banned from this server!"

For a full explanation of available expansions, as well as additional access control options, refer to section 5 of the man pages for hosts_access (man 5 hosts_access) and the man page for hosts_options.

For additional information about TCP wrappers, refer to Section 17.5 Additional Resources. For more information about how to secure TCP wrappers, refer to the chapter titled Server Security in the Red Hat Enterprise Linux Security Guide.

 
 
  Published under the terms of the GNU General Public License Design by Interspire