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TCP Wrappers is a host-based ACL system you can use to fine-tuned access permissions for some services in your server.

Compatible services

Some of those compatible services are: sshd, vsftpd, and nfs. You can find out which installed services are compatible with TCP Wrappers with lsof (this command lists open files, in this case it shows which current processes have opened a file. Run as root or use sudo):

lsof <libwrap library path>
  • libwrap library path may be different depending on the distribution. Some usual paths are:
    • /lib/x86_64-linux-gnu/libwrap.so.0
    • /lib64/libwrap.so.0

hosts.allow and hosts.deny

Config files for TCP Wrappers are /etc/hosts.allow (for allowed hosts) and /etc/hosts.deny (for denied hosts). The syntax for both files is similar.

# /etc/hosts.deny
<service> : <hosts> : <options>
  • You can specify a service (sshd for example) or type ALL if that rule applies to all compatible services.
  • <hosts> can be an IP address or hostname (or a comma-separated list). You can use ALL as well.
  • You can specify a command to be executed when the rule is triggered (using spawn and the command).

Rules on /etc/hosts.allow do not imply that hosts that do not appear in this file are not allowed. When a client wants to connect to the server, this happens:

  • Service will check if this host is inside hosts.allow. If it is, it won’t look further and will allow the connection.
  • If not, it will look inside hosts.deny. If the host is inside this file, server won’t allow the client to connect.
  • If there is no deny rule that affects the client host, server will allow it to connect.

Examples

Some rule examples:

sshd : 172.0.0.5 : spawn /bin/echo `date` %c %d >> /var/log/ssh-logins/log
  • Use full paths for the command and when referring to files.
  • %c prints client info, like IP.
  • %d prints service name.

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