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20.5. tcp_wrapper
tcp_wrapper is a library which can help you to protect service against misuse.
20.5.1. 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)
20.5.2. 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)
20.5.3. Usage
tcp_wrapper is controlled by two files name /etc/hosts.allow and /etc/hosts.deny. For more information see
$ man hosts.allow |
20.5.3.1. 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. [3ffe:ffff:100:200::]/64
daytime-stream: 1.2.3. [3ffe:ffff:100:200::]/64
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Note: there are broken implementations around, which uses following broken IPv6 network description: [3ffe:ffff:100:200::/64]. Hopefully, such versions will be fixed soon.
20.5.3.2. Example for /etc/hosts.deny
This file contains all negative filter entries and should normally deny the rest using
ALL: ALL
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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)
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20.5.4. Logging
Depending on the entry in the syslog daemon configuration file /etc/syslog.conf the tcp_wrapper logs normally into /var/log/secure.
20.5.4.1. 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=3ffe:ffff:100:200::212:34ff:fe12:3456
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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 3ffe:ffff:100:200::212:34ff:fe12:3456
¬ (3ffe:ffff:100:200::212:34ff:fe12:3456)
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20.5.4.2. 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=3ffe:ffff:100:200::212:34ff:fe12:3456
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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 3ffe:ffff:100:200::212:34ff:fe12:3456 port 33380 ssh2
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Hosting by: Hurra Communications Ltd.
Generated: 2007-01-26 17:58:19