tcp_wrapper

Linux IPv6 HOWTO (en)
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# 22.8. tcp_wrapper

tcp_wrapper is a library which can help you to protect service against misuse.

## 22.8.1. Filtering capabilities

You can use tcp_wrapper for

- Filtering against source addresses (IPv4 or IPv6)

- Filtering against users (requires a running ident daemon on the client)

## 22.8.2. Which program uses tcp_wrapper

Following are known:

- Each service which is called by xinetd (if xinetd is compiled using tcp_wrapper library)

- sshd (if compiled using tcp_wrapper)

## 22.8.3. Usage

tcp_wrapper is controlled by two files name /etc/hosts.allow and /etc/hosts.deny. For more information see

    $ man hosts.allow

### 22.8.3.1. Example for /etc/hosts.allow

In this file, each service which should be positive filtered (means connects are accepted) need a line.

    sshd:           1.2.3. [2001:0db8:100:200::]/64
    daytime-stream: 1.2.3. [2001:0db8:100:200::]/64

Note: there are broken implementations around, which uses following broken IPv6 network description: [2001:0db8:100:200::/64]. Hopefully, such versions will be fixed soon.

### 22.8.3.2. Example for /etc/hosts.deny

This file contains all negative filter entries and should normally deny the rest using

    ALL: ALL

If this node is a more sensible one you can replace the standard line above with this one, but this can cause a DoS attack (load of mailer and spool directory), if too many connects were made in short time. Perhaps a logwatch is better for such issues.

    ALL: ALL: spawn (echo "Attempt from %h %a to %d at `date`"
     | tee -a /var/log/tcp.deny.log | mail root@localhost)

## 22.8.4. Logging

Depending on the entry in the syslog daemon configuration file /etc/syslog.conf the tcp_wrapper logs normally into /var/log/secure.

### 22.8.4.1. Refused connection

A refused connection via IPv4 to an xinetd covered daytime service produces a line like following example

    Jan 2 20:40:44 gate xinetd-ipv6[12346]: FAIL: daytime-stream libwrap
    ¬ from=::ffff:1.2.3.4
    Jan 2 20:32:06 gate xinetd-ipv6[12346]: FAIL: daytime-stream libwrap
     from=2001:0db8:100:200::212:34ff:fe12:3456

A refused connection via IPv4 to an dual-listen sshd produces a line like following example

    Jan 2 20:24:17 gate sshd[12345]: refused connect from ::ffff:1.2.3.4
    ¬ (::ffff:1.2.3.4)
    Jan 2 20:39:33 gate sshd[12345]: refused connect
     from 2001:0db8:100:200::212:34ff:fe12:3456
    ¬ (2001:0db8:100:200::212:34ff:fe12:3456)

### 22.8.4.2. Permitted connection

A permitted connection via IPv4 to an xinetd covered daytime service produces a line like following example

    Jan 2 20:37:50 gate xinetd-ipv6[12346]: START: daytime-stream pid=0
    ¬ from=::ffff:1.2.3.4
    Jan 2 20:37:56 gate xinetd-ipv6[12346]: START: daytime-stream pid=0
     from=2001:0db8:100:200::212:34ff:fe12:3456

A permitted connection via IPv4 to an dual-listen sshd produces a line like following example

    Jan 2 20:43:10 gate sshd[21975]: Accepted password for user from ::ffff:1.2.3.4
    ¬ port 33381 ssh2
    Jan 2 20:42:19 gate sshd[12345]: Accepted password for user
     from 2001:0db8:100:200::212:34ff:fe12:3456 port 33380 ssh2

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