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Open Management Infrastructure - PowerShell Edition

Test workflow PowerShell Gallery

This is a fork of Microsoft OMI repository. Read more about this fork in my blog post Wacky WSMan on Linux.

Changes from Upstream OMI

The main goal of this fork is to produce an alternative build of libmi that is used by PowerShell for WinRM based PSRemoting. This alternative build is designed to fix the current problems with the shipped version of libmi. Unfortunately there is no interest in upstream to accept any patches for this use case so I've decided to store them in this fork and produce my own builds. The following changes have been made:

  • Create a compiled build against a newer version of OpenSSL (1.1.x) for distros that include this version
  • Fixed the compiler to work on newer distributions that now have stricter defaults
    • Upstream failed on macOS due to the code using system defined macros
    • Upstream failed on newer gcc versions that had stricter default set
  • Fixed up the GSSAPI implementation to work properly
    • macOS works with Kerberos out of the box as the correct symbols and workflow is being used
    • Technically NTLM auth on macOS can work but only does when running over HTTPS. This is due to a fundamental problem with macOS and NTLM when it's wrapped in SPNEGO
    • Ensure the mech set favours Kerberos and only fallsback to NTLM if Kerberos fails
    • Added the GSS_C_DELEG_POLICY_FLAG flag to support credential delegation with Kerberos
    • Use GSS_C_NT_HOSTBASED_SERVICE with the service http@<hostname> principal which works in more situations than before
    • Don't try and resolve the hostname when building the SPN, this goes against the guidelines of GSSAPI, if that behaviour is desired then set it in your krb5.conf
    • Commented out unused GSSAPI symbols to improve compatibility on versions that don't have those symbols available
  • Added support for using implicit Kerberos credentials retrieved with kinit
    • Will work when both -Authentication Negotiate or -Authentication Kerberos but the former will only work with Kerberos with no NTLM fallback
    • Should technically require -Authentication NegotiateWithImplicitCredential but PowerShell does not pass in the required flags to know this
  • Rewrote the message decryptor used when HTTP message encryption was used over Negotiate or Kerberos auth
    • This now works in more situations whereas before it would fail with an unhelpful MI_RESULT_FAILED error
    • Also works against an on-prem Exchange remoting endpoint due to the unique nature of how that encrypted the message
  • Added additional log entries to make debugging slightly easier than before
  • Increased the password length limit to 8KiB to support modern authentication required by O365 WSMan connections
    • The original limit was 1KiB and I've seen JWT tokens that modern auth in O365 exceed 1.5KiB
    • I've set it to 8KiB as that seems to be a common default of HTTP header sizes, if it does exceed that then the server would return a 413 anyway
  • Added support for sending the channel binding tokens when using GSSAPI on a HTTPS connection
    • This will allow the client to authenticate when the WSMan service has set Auth/CbtHardeningLevel = Strict
    • If the client fails to derive the CBT token, further information can be found in the logs
  • Turned on HTTPS certificate verification by default
    • Any HTTPS connections will have OpenSSL check the server's certificate like a proper HTTPS connection
    • For PowerShell versions older than 7.2, you still need to tell PowerShell to skip the checks but those skip options are ignored in OMI
    • See https_validation for more details on this topic
  • Also create a slightly customised libpsrpclient
    • This enables WSMan on distributions that Microsoft does not include libpsrpclient for
    • Also allows this fork to fix things that are outside of the OMI codebase

I am not looking at fixing any underlying problems in this library or work on the server side part of OMI. This is purely focusing on improving the experience when using WinRM as a client on non-Windows based hosts within PowerShell. There are no guarantees of support, you are free to change whatever you wish on your own builds but use the code here at your own risk.

Build

See build for more information on how to manually build these libraries.

Installing

Since the 2.0.0 release there is now a PowerShell module that can be used to install this library on known distributions. You can see this package at PSGallery PSWSMan. To install the WSMan libs through this module you can run the following in PowerShell:

Install-Module -Name PSWSMan

# Requires root access to install, Install-WSMan can be run directly if already running as root
sudo pwsh -Command 'Install-WSMan'

If you wish to build your own changes you can manually build the module. Make sure to run this step after you've manually built OMI into the build/lib directory.

./build.py module

Once built you can import the module and install the WSMan components.

# Import PSWSMan from the repo source, that will source the libs from PSWSMan/lib/{distribution} of the repo
Import-Module -Name ./build/PSWSMan
Install-WSMan

You can also manually install the libraries by copying the files build/lib/{distribution}/lib* into the PowerShell directory. The location of the PowerShell directory differs based on each distribution or how it was installed. An easy way to determine this directory is by running dirname "$( readlink "$( which pwsh )" )"

To enable Kerberos authentication you will need to ensure you install the Kerberos system packages that can vary between distros. See the .json files in distribution_meta to see the test_deps that are required to test PowerShell with Kerberos auth. NTLM auth also requires the gss-ntlmssp package which is another separate package that can be installed. This is also documented in the .json files for each distribution.

A few thing to note when using the WSMan transport in PowerShell

  • When wanting to use Kerberos auth you need to specify the user in the UPN format, e.g. username@DOMAIN.COM. Do not use the Netlogon form DOMAIN\username
  • When using Basic auth you MUST connect over HTTPS and skip cert verification by adding -SessionOption (New-PSSession -SkipCACheck -SkipCNCheck)
    • While this tells PowerShell to skip the certificate checks, this library will still continue to do so
    • See https_validation for more details on this topic

Testing

See testing for more information on how to test the changes here.

Troubleshooting

There are a few steps you can follow to troubleshoot any problems when using this library in PowerShell. Most problems are split into 3 different categories:

The libmi library also has a builtin logging mechanism that you can enable to help with debugging issues at runtime. To enable logging for OMI you first need to create a file at /opt/omi/etc/omicli.conf with the following contents:

# Can be ERROR, WARNING, INFO, DEBUG, VERBOSE (requires a debug build) with the default being WARNING
# Any previous values will also be set, i.e. setting INFO will enable ERROR and WARNING
loglevel = DEBUG

# The directory (must end with /) to place the log file in
logpath = /tmp/

# The name of the logfile to write to
logfile = omi-pwsh.log

If you have specified a custom --prefix path when you built libmi, then /opt/omi should be subsituted with the prefix you specified. You may also want to create the dir /opt/omi/var/log as this directory is used to place the HTTP trace files.

Library Errors

If PowerShell fails to load the libpsrpclient or libmi library it can fail with the error

This parameter set requires WSMan, and no supported WSMan client library was found. WSMan is either not installed or unavailable for this system.

The main thing you can do to test this out is to very the linked libraries are present on the system. Run the following to get information on all the linked libraries

PWSHDIR="$( dirname "$( readlink "$( which pwsh )" )" )"

# On Linux
ldd "${PWSHDIR}/libpsrpclient.so"
ldd "${PWSHDIR}/libmi.so"

# On macOS
otool -L "${PWSHDIR}/libpsrpclient.dylib"
otool -L "${PWSHDIR}/libmi.dylib"

Read through this list and make sure each of the libraries it reference actually resolve to a path on the system. There may be a chance that you never replaced the original libmi library with one from this fork.

Authentication Failures

Authentication can be a tricky thing to debug due to the complexities of GSSAPI on Linux. If you would like to use Kerberos authentication (you should) then the first time you should verify is that you are able to get a Kerberos ticket for your user. You can test this by running kinit username@DOMAIN.COM and entering your password. If this fails then you have a system setup issue with your Kerberos config and should fix that first. Once you have verified the system can talk to the domain controller then you can start testing using it in PowerShell. A few things you should be aware of when it comes to Kerberos authentication:

  • Make sure you are connecting to the host with its fully qualified domain name
    • Kerberos is highly dependent on DNS working and uses the FQDN of the host to verify its info in the domain controller
  • Make sure the time on the PowerShell host and the remote host are in sync
  • If passing in an explicit credential, make sure you use the UPN form username@DOMAIN.COM

A helpful way to troubleshoot Kerberos issues is to set the env var KRB5_TRACE=/dev/stdout before opening PowerShell. This will output any any GSSAPI events to the console allowing you to see the steps it follows and potentially trace down the underlying problem in your scenario.

NTLM authentication is less picky about the environment it is run in but it comes at the downside of it being less secure. NTLM auth works out of the box on macOS but on other Linux hosts you need to ensure you have installed the gss-ntlmssp package. Have a look through the distribution_meta .json files to see what name this package comes under for your distribution.

WSMan errors

These are the hardest problems to debug as it's usually a sign of a logic issue in the libmi code. The best advice I can give you here is to create a debug build of the library using ./build.py --debug and make sure you have enable the VERBOSE level logs in the omicli.conf file. Hopefully the logs can at least narrow down where the problem lies.

OpenSSL errors

OMI does not use the HTTPS stack provided by .NET which means it links directly against OpenSSL on the system. On Linux hosts this is usually nothing major and will automatically select the same OpenSSL installed by the system's package manager. This is a different story on macOS as OpenSSL isn't shipped in a form that can be used by OMI. This means macOS needs to install it's own copy of OpenSSL which is typically done with either brew or port:

# brew
brew install openssl

# port
port install openssl curl-ca-bundle

Note: For port the curl-ca-bundle package must also be installed so it's provided with a sane default of trusted root certificates.

Common errors that are encountered when using OpenSSL are:

  • No OpenSSL version found when running Install-WSMan
    • For Linux make sure the openssl package is installed using the distribution specific package manager
    • For macOS make sure openssl is installed through brew, port`, or compile it yourself
    • For the ARM macOS hosts, make sure openssl has been installed with the same architecture as what pwsh is running as - archs cannot mix and match in the same process
  • Certificate verification errors
    • This can be in the form of certificate verify failed or error:16000069:STORE routines::unregistered scheme
    • See the docs on HTTPS validation for more information on this topic

Known Issues

There are a few known issues so far that are split into the can and can't fix categories. Can fix issues are ones that are problems in this codebase that can be fixed and a new library recompiled. Can't fix issues are either issues that would take a lot of effort to implement and/or require changes in other libraries which is out of scope here.

Can Fix

  • HTTP trace files containing the HTTP payloads sent in an exchange are placed in {prefix}/var/log if that folder exists
    • These trace files should only be created if the loglevel in the omicli.conf file is set to DEBUG or higher but currently that does not happen
  • No CredSSP authentication
    • Implementing CredSSP authentication is quite complex but is theoretically possible

Can't Fix

  • Cannot do basic auth over HTTP
    • PowerShell hardcodes a check that stops you from doing this for security reasons
    • Really why would you want to do this anyway
  • When using MIT krb5 as the GSSAPI backend, Kerberos delegation will only work when /etc/krb5.conf contains [libdefaults]\nforwardable = true
    • This is a problem in that library where gss_acquire_cred_with_pass will only acquire a forwardable ticket (required for delegation) if the krb5.conf contains the forwardable = true setting
    • Recent versions of Heimdal are not affected
    • You can run kinit -f username@DOMAIN.COM to get a forwardable ticket regardless of the krb5.conf value and use the implicit credential instead

Contributing

I'm happy to look at any PRs or help with any issues but bear in mind that this is something I work on in my spare time. There is no guarantee that I will be able to solve your problems or look at a PR. If you are making any changes to the code in Unix/ then I recommend you add a comment # JBOREAN CHANGE: reason for change. This allows me to easily merge any upstream changes and compare what has been edited here and why compared to any incoming changes.

Backlog

See Can Fix for known bugs that can be fixed. Otherwise other features/changes that are in the backlog are:

  • Add a way to specify the omicli.conf file through an env var instead of the hardcoded location
  • Try and find a better way to enable NTLM auth for macOS, current implementation is a bit of hack
  • Add a force NTLM auth to be used in conjunction with -Authentication Negotiate

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