hosts.allow format and example on Linux

A Linux system can utilize the hosts.allow file to specify which IP addresses, hostnames, or domains are permitted to connect to it. This works specifically for TCP wrapped services. The hosts.allow file is used in conjunction with hosts.deny to determine whether a connection attempt gets accepted or denied.

The hosts.allow file is just a plain text configuration file with a rather simple syntax. In this tutorial, you will see an example of the hosts.allow file, as we show you how to format the file for different possible scenarios.

In this tutorial you will learn:

What does the hosts.allow file do?
How to edit and format the hosts.allow file

Software Requirements and Linux Command Line Conventions
Category	Requirements, Conventions or Software Version Used
System	Any Linux distro
Software	N/A
Other	Privileged access to your Linux system as root or via the `sudo` command.
Conventions	# – requires given linux commands to be executed with root privileges either directly as a root user or by use of `sudo` command $ – requires given linux commands to be executed as a regular non-privileged user

hosts.allow format and example on Linux

The hosts.allow file contains a list of rules for which hosts or networks are allowed to access the specified services inside of the file. The accepted format is to list one rule per line. Below, we will show various examples for rules you can use in the file.

NOTE
When a connection is attempted, the hosts.allow file is consulted to see whether it will be allowed or not. If it is permitted, then the hosts.deny file is consulted afterwards, to see if there is a rule that specifically denies the connection.

The usual syntax is as follows. Each value is separated by a colon :.
```
service : host/network
```
You can also supply an option, but this is not as common. We will cover some other niche choices below. More options can be added if necessary, with each one separated by another colon.
```
service : host/network [: <option>: <option>: ...]
```
The following line would allow all traffic to the sshd service. ALL is used as a wildcard.
```
sshd : ALL
```
This line would allow connections from all hosts on the 10. network. Connections from all other hosts can then be denied by the hosts.deny file. This type of configuration would work as intended since the allow line precedes our corresponding deny line in the other file, thus will be triggered first.
```
sshd : 10.
```
Accept connections from a particular IPv4 and IPv6 address:
```
sshd : 10.10.136.241
sshd : [2a02:2149:88f1:4c00:9991:9daa:b580:aee1]
```
Notice the IPv6 address must be enclosed in [ ] brackets.
Rather than using IPs, you can also specify hostnames to accept or deny connections from.
```
sshd : some.host
```
Accept connections from all hosts using the .linuxconfig.org domain name.
```
sshd : .linuxconfig.org
```
You can also use a wildcard for both the service and the host/network field. This will accept all connections to any service. This would make all other rules (including those in hosts.deny) irrelevant, as all connections will be accepted by this rule before they have a chance to be denied.
```
ALL : ALL
```
To accept only local connections, the LOCAL wildcard can be used.
```
sshd : LOCAL
```
The EXCEPT operator can be used to create an exception in an otherwise all encompassing rule. For example, this rule would allow all connections from the .linuxconfig.org domain name, except for one host.
```
sshd : .linuxconfig.org EXCEPT terminal.linuxconfig.org
```

Closing Thoughts

In this tutorial, we saw how to format the hosts.allow with various filtering rules on a Linux system. This can be an effective way to filter traffic for TCP wrapped services, although it has fallen out of common use with the rise of the powerful iptables/nftables firewall built into the Linux kernel.

hosts.allow format and example on Linux

Closing Thoughts

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