docs

Documentation

Additional documentation relevant to use of libnss_shim.

Downloading

It is also possible to get the latest release URLs using the rate-limited public GitHub API:

   curl -s https://api.github.com/repos/xenago/libnss_shim/releases/latest | grep "browser_download_url.*.$" | cut -d : -f 2,3 | tr -d \" | tr -d " "

By filtering those results further and adding a final curl download, the latest version of a specific package can be acquired in one line. For instance, this command will download the latest 64-bit x86 deb installer:

   curl -s https://api.github.com/repos/xenago/libnss_shim/releases/latest | grep "browser_download_url.*.$" | cut -d : -f 2,3 | tr -d \" | tr -d " " | grep amd64.deb | xargs -n 1 curl -sLO

Attestations

Validating downloaded packages before installation can be done with artifact attestations. With the use of artifact attestations, it is possible to check the hashes of downloaded packages to establish build provenance and confirm details like the commit SHA, pipeline trigger, etc. Validation is recommended as it can prevent issues arising from corrupt files or malicious actors sharing modified builds.

Artifact attestations are available for libnss_shim releases as of version 1.2.1. To verify artifact attestations for official packages built with GitHub Actions, the GitHub CLI can be used. Example command:

gh attestation verify /path/to/libnss_shim.deb -R xenago/libnss_shim

The GitHub CLI requires using a GitHub account, but it is also possible to view attestations online without an API key.

Configuration

Functions for 3 NSS databases are supported:

• group
  • get_all_entries()
  • get_entry_by_gid(uint32 gid)
  • get_entry_by_name(str name)
• passwd
  • get_all_entries()
  • get_entry_by_uid(uint32 uid)
  • get_entry_by_name(str name)
• shadow
  • get_all_entries()
  • get_entry_by_name(str name)

Codes can be used to insert relevant query data at runtime into the environment variables or launch arguments of commands run by libnss_shim:

• <$gid>
• <$name>
• <$uid>

Using only that information, here is the extremely basic test example of config.json - one database is defined, group, with just a single function, get_all_entries:

{
  "databases": {
    "group": {
      "functions": {
        "get_all_entries": {
          "command": "echo 'test-shim-group::1008:fake-username,another-user'"
        }
      }
    }
  }
}

The command defined for get_all_entries prints out a single line to stdout, describing a fake group called test-shim-group with gid=1008 and two members. That output is then captured by libss_shim and returned to NSS whenever a call is made requesting all the group entries (e.g. getent group).

To support command execution, the following options can be set globally and overridden for specific databases and/or functions:

• "env": Add environment variables to the set inherited from libnss_shim
• "workdir": Set the working directory before running the command

To enable debug printing to the user terminal, "debug": true can be set at the global level. This is false by default.

The following is a much more complex fake example of /etc/libnss_shim/config.json - more databases and functions are defined (but with made-up commands this time), codes are used to pass data at runtime as arguments/environment variables, debug output is enabled, and there are global defaults set for env & workdir with some database/function level overrides:

{
  "databases": {
    "group": {
      "functions": {
        "get_all_entries": {
          "command": "command.sh --all"
        },
        "get_entry_by_name": {
          "command": "another_command.sh -n <$name>",
          "workdir": "/different/folder/to/run/in"
        }
      }
    },
    "passwd": {
      "functions": {
        "get_all_entries": {
          "command": "python3 command.py"
        },
        "get_entry_by_uid": {
          "command": "python3 command.py -uid <$uid>"
        },
      },
      "env": {
        "VARIABLE": "overrides_default_for_all_passwd_database_functions",
        "PYTHONDONTWRITEBYTECODE": "1"
      }
    },
    "shadow": {
      "functions": {
        "get_entry_by_name": {
          "command": "/retrieve/shadow",
          "env": {
            "SHADOW_REQ_NAME": "<$name>",
            "VARIABLE": "overrides_default_env_variable_for_this_function"
          }
        }
      }
    }
  },
  "debug": true,
  "env": {
    "SOME_VARIABLE": "set this variable for all databases and their functions"
  },
  "workdir": "/folder/to/execute/in"
}

Commands

Commands can have input arguments passed as environment variables or as arguments using the codes defined in the Configuration section (such as <$name>). To return a response to libnss_shim, they can simply print a line to stdout in the comma-separated *nix format common to /etc/shadow, group, and passwd, or alternatively in JSON form as described below. It is important to note that the NSS compat options are not supported (e.g. +@netgroup). Information about the colon-separated format used for group, passwd, etc. on a variety of *nix systems is available online.

Although it is best to set all fields explicitly to avoid unexpected issues with default/unset values (nobody wants a password to be blank for some reason), only the following fields are required in command output:

• group
  • name
  • gid
• passwd
  • name
  • uid
  • gid
• shadow
  • name

If using standard Unix colon-split format, optional fields can be left blank. With JSON, they can be omitted entirely.

If the defined command ran correctly but no results are found for that query, it is expected to exit normally and print either empty JSON {} or nothing at all (other than whitespace/newlines) to stdout.

Commands and arguments are split according to POSIX shell syntax, but are not run through a shell, so bash-specific syntax will not function. For example, a command such as program1 && program2 will be interpreted as running program1 with arguments && and program2. Although it is not recommended (see the Security section), it remains possible to run a shell directly, e.g. sh -c 'program1 && program2'.

Here is the expected JSON format from running each database's supported commands, with types indicated. All numbers are expected in base-10 integer form and must fit within the ranges of the indicated numeric int types (isize and usize are platform-dependent and can be 32 or 64-bits):

• group

  • get_all_entries()

      {
        "group-name-here": {
          "passwd": (str),
          "gid": (uint32),
          "members": [
            (str),
            (str),
            ...
          ]
        },
        "another-groupname": {
          ...
        }
      }
    

  • get_entry_by_gid(uint32 gid) - Response should be the same format as get_all_entries(), but only a single record

  • get_entry_by_name(str name) - Response should be the same format as get_entry_by_gid(uint32 gid)

• passwd

  • get_all_entries()

      {
        "username-here": {
          "passwd": (str),
          "uid": (uint32),
          "gid": (uint32),
          "gecos": (str),
          "dir": (str),
          "shell": (str)
        },
        "second-username-here": {
          ...
        }
      }
    

  • get_entry_by_uid(uint32 uid) - Response should be the same format as get_all_entries(), but only a single record

  • get_entry_by_name(str name) - Response should be the same format as get_entry_by_uid(uint32 uid)

• shadow

  • get_all_entries()

      {
        "first-username-here": {
          "passwd": (str),
          "last_change": (isize),
          "change_min_days": (isize),
          "change_max_days": (isize),
          "change_warn_days": (isize),
          "change_inactive_days": (isize),
          "expire_date": (isize),
          "reserved": (usize)
        },
        "username-two": {
          ...
        }
      }
    

  • get_entry_by_name(str name) - Response should be the same format as get_all_entries(), but only a single record

  • Note: The final field, reserved, is seemingly unused in practice and is typically omitted

Interaction with /etc/nsswitch.conf

Originally from issue #5.

When implementing get_entry_by_gid do we need to return a value for preexisting gid in /etc/group file? Also, same question regarding get_entry_by_name for group and passwd.

No need to return entries if that same content is already present on disk in e.g. /etc/group/etc/passwd. That is taken care of by the existing mappings as set up by the default installation process.

From README.md:

libnss_shim is mapped as shim in /etc/nsswitch.conf as the last source for all supported databases

What this looks like in /etc/nsswitch.conf by default after installing libnss_shim on a clean AlmaLinux system:

(...)
passwd:     sss files systemd shim
shadow:     files shim
group:      sss files systemd shim
(...)

This means that by default the shim will be called only after lookups to the other databases listed before it, and only if the others failed. So if there is already an entry on disk (or in any preceding database) for that particular query then libnss_shim will never be called.

You can demonstrate this effect by configuring libnss_shim to respond with different information than is on disk, and running these queries:

getent group root
getent -s files group root
getent -s shim group root

The first one makes a normal request which follows the priority order forgroup set in /etc/nsswitch.conf: sss, files, systemd, shim. Because sss responds with nothing, NSS moves on and requests it from files, which succeeds so systemd/shim are never queried.

The second command specifically requests it from files (i.e. /etc/group/), so the same information as the first command is returned.

The third command requests it from shim, so libnss_shim is used and the custom entry value is returned.

If the order of group in /etc/nsswitch.conf is changed so that shim is first, then getent group root will return the custom value right away and never check the other databases:

Development

Builds can be run inside temporary containers, like in CI:

1. Ensure Docker is installed and the docker command is available to the running user

2. Ensure the libnss_shim repository has been cloned:

   git clone https://github.com/xenago/libnss_shim.git
   

3. Run the build script inside a container using the docker run command

   • Replace /path/to/cloned/libnss_shim with the actual location of the cloned repository

   • Edit --version_number=0.0.0 if a specific version number is desired (SemVer format)

     docker run -v /path/to/cloned/libnss_shim:/libnss_shim --rm -it quay.io/pypa/manylinux2014_x86_64:latest bash -c 'cd /libnss_shim && bash build.sh --version_number=0.0.0'
     

   Note: it may be possible to build libnss_shim for additional architectures and operating systems by using different manylinux versions or other containers in combination with QEMU. cross might also help in this regard.

4. The build script will output packages in the following subdirectories of the cloned repo:

   • target/debian/*.deb
   • target/generate-rpm/*.rpm

Useful resources

See ACKNOWLEDGMENTS.md