43.5.2. TCP Wrappers Configuration Files

43.5.2. TCP Wrappers Configuration Files

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

When a TCP-wrapped service receives a client request, it performs the following steps:

  1. It 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. It 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, it grants access to the service.

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

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 character. 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

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