43.5.2. TCP Wrappers Configuration Files

43.5.2.1. Formatting Access Rules

The format for both /etc/hosts.allow and /etc/hosts.deny is identical. Each rule must be on its own line. Blank lines or lines that start with a hash (#) are ignored.

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

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

<daemon list> — A comma-separated list of process names (not service names) or the ALL wildcard. The daemon list also accepts operators (refer to Section 43.5.2.1.4, “Operators”) to allow greater flexibility.
<client list> — A comma-separated list of hostnames, host IP addresses, special patterns, or wildcards which identify the hosts affected by the rule. The client list also accepts operators listed in Section 43.5.2.1.4, “Operators” to allow greater flexibility.
<option> — An optional action or colon-separated list of actions performed when the rule is triggered. Option fields support expansions, launch shell commands, allow or deny access, and alter logging behavior.

Note

More information on the specialist terms above can be found elsewhere in this Guide:

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 to append the attempt to a special log 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. Refer to Section 43.5.2.2, “Option Fields” for a more detailed look at available options.

43.5.2.1.1. 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 are available:

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

The KNOWN, UNKNOWN, and PARANOID wildcards should be used with care, because they rely on functioning DNS server for correct operation. Any disruption to name resolution may prevent legitimate users from gaining access to a service.

43.5.2.1.2. Patterns

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

The following is a list of common patterns for entries in the client field:

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
When working in the IPv4 address space, the address/prefix length (prefixlen) pair declarations (CIDR notation) 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

Be very careful when using hostnames and domain names. Attackers can use a variety of tricks to circumvent accurate name resolution. In addition, disruption to DNS service prevents even authorized users from using network services. It is, therefore, best to use IP addresses whenever possible.

43.5.2.1.3. Portmap and TCP Wrappers

Portmap's implementation of TCP Wrappers does not support host look-ups, which means portmap can not use hostnames to identify hosts. Consequently, access control rules for portmap in hosts.allow or hosts.deny must use IP addresses, or the keyword ALL, for specifying hosts.

Changes to portmap access control rules may not take effect immediately. You may need to restart the portmap service.

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

43.5.2.1.4. 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 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

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.