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Programming language: Haskell
License: MIT License
Tags: System    

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README

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sshd-lint

The ssh daemon is the front door to your server. It's unfortunate that the parser for sshd_config file is unhelpful in detecting incorrectly-entered configuration options or settings that may leave your system open to attackers. sshd-lint checks a sshd_config file for adherence to security best practices. It warns about repeated configuration lines, contradictory settings in the same file, and settings that don't adhere to best practices.

It can be run from the command line to show suggestions for your sshd_config file, or an option can be provided to emit nagios-compatible output so that any changes deviating from best practices will be detected by a monitoring system.

Wat? Weird things happen in ssh config file parsing.

Like in JavaScript, trying to understand how sshd reads your configuration file leads to several "Wat?" moments.

Wat #1 - Earliest option wins for some settings

sshd has two checking modes, -t and -T. Neither one alerts if config directives are repeated, regardless of whether the value being assigned is the same or different.

If you have two different values for some config file value, the first one takes effect (wat?). So the following allows cleartext passwords, but putting the directives in the other order turns off cleartext passwords. Again, there is no warning nor error from sshd abouth the duplicate lines.

PasswordAuthentication yes
PasswordAuthentication no

sshd-lint takes the weird sshd parsing into account and makes sure that settings adhere to best practices. It also lets you know if there are duplicated lines which may lead to unexpected behavior from the SSH daemon.

Wat #2 - No warning for contradictory settings

sshd allows contradictory settings. Fortunately, for certain options like AllowUser, AllowGroup, DenyUser and DenyGroup, the most restrictive option takes effect. Still, it's a bit unexpected that if you add user 'joe' to both AllowUser and DenyUser, joe is Denied without any warning that you told sshd to use contradictory settings.

sshd-lint considers contradictory settings to be an error.

Installation

If you don't have the Haskell platform installed, you'll need to install it. On Ubuntu, apt-get install haskell-platform. On Mac OS, you can install haskell-platform with homebrew or from the pkg file.

Then, install sshd-lint from Hackage.

To install sshd-lint in your ~/.cabal/bin directory:

cabal update && cabal install sshd-lint

This will install sshd-lint in your ~/.cabal/bin, which you should add to your PATH. Alternatively you can install in /usr/local/bin by using the --global option on cabal install.

Usage

Invoking sshd-lint without any arguments defaults to checking /etc/ssh/sshd_config, which should be present on most systems running OpenSSH. You can run sshd_lint on any configuration file by specifying it after the -f argument, eg:

sshd-lint -f /tmp/new_sshd_config

sshd_lint supports a Nagios output mode, which can be enabled with the -n flag.

Other options can be seen by invoking sshd_lint with the -h flag.

sshd-lint's Best Practices

There should be no duplicate lines present in the ssh config file (with the exception of the HostKey configuration option, which we ignore). Currently the following settings are considered best practices by sshd-lint.

PermitEmptyPasswords no
PasswordAuthentication no
HostbasedAuthentication no
PermitRootLogin no
IgnoreRhosts yes
Protocol 2
StrictModes yes
UsePrivilegeSeparation yes

Output

If sshd-lint has any suggestions they will be printed to standard output. The return value of the program will be 1 if there are suggestions.

The output of sshd-lint will be "No suggestions" if there are no suggestions, and the return value will be 0.

The format of sshd-lint may change while the project is still young, so it is suggested that scripts using sshd-lint depend on the return value, but not necessarily the exact formatting of output until the project matures.

Other Reading

Many of the sshd best practices come from the article, Top 20 OpenSSH Server Best Security Practices.

Author

Justin Leitgeb

License

MIT

Copyright

(C) 2015 Stack Builders Inc.


*Note that all licence references and agreements mentioned in the sshd-lint README section above are relevant to that project's source code only.