= README

Nsscash (a pun on cache) is a simple file-based cache for NSS similar to
nsscache [1]. The goal is to distribute users/groups/etc. to multiple systems
without having to rely on a (single) stable server. Traditional systems like
LDAP or NIS require a stable server or users/groups cannot be resolved. By
distributing the data to all systems, temporary outages of the server cause no
issues on the clients. In addition the local storage is much faster than
remote network access. To update the local caches polling via HTTP/HTTPS is
performed, for example every minute, only downloading new data if anything has
changed.

Nsscash consists of two parts: `nsscash`, written in Go, which downloads files
via HTTP or HTTPS, parses them, creates indices and writes the result to a
local file. The second part is the NSS module (`libnss_cash.so.2`), written in
C, which provides integration via `/etc/nsswitch.conf`. It's specifically
designed to be very simple and uses the data prepared by `nsscash` for
lookups. To support quick lookups, in O(log n), the files utilize indices.

Nsscash can also be used separately from the NSS module to deploy arbitrary
files to many hosts and keep them up-to-date.

Nsscash is very careful when deploying the changes:

- All files are updated using the standard "write to temporary file", "sync",
  "rename" steps which is atomic on UNIX file systems. The indices are stored
  in the same file preventing stale data during the update.
- All errors cause an immediate abort ("fail fast") with a proper error
  message and a non-zero exit status. This prevents hiding possibly important
  errors. In addition all files are fetched first and then deployed to try to
  prevent inconsistent state if only one file can be downloaded. The state
  file (containing last file modification and content hash) is only updated
  when all operations were successful.
- To prevent unexpected permissions, `nsscash` does not create new files. The
  user must create them first and `nsscash` will then re-use the permissions
  (without the write bits to discourage manual modifications) and owner/group
  when updating the file (see examples below).
- To prevent misconfigurations, empty files (no users/groups) are not
  permitted and will not be written to disk. This is designed to prevent the
  accidental loss of all users/groups on a system.
- To detect file corruption a hash of all deployed files is stored separately
  and verified on each `nsscash` run.

The passwd/group files have the following size restrictions:

- maximum number of entries: '2^64-1' (uint64_t)
- maximum passwd entry size: 65543 bytes (including newline)
- maximum group entry size: 65535 bytes (including newline, only one member)
- maximum members per group: depends on the user name length,
                             with 9 bytes per user: 5460 users
- `nsscash` checks for these restrictions and aborts with an error if they are
  violated

Nsscash has an extensive test suite for both the Go and C part testing general
requirements and various corner cases. See TODO.adoc for a list of known
issues and possible improvements.

Nsscash was created as replacement for nsscache [1] because nsscache has very
complex code, especially the parts written in C, uses multiple files for data
and indices and has had severe bugs (including temporary loss of all users and
race conditions when updating data and index files).

Nsscash is licensed under AGPL version 3 or later.

[1] https://github.com/google/nsscache


== REQUIREMENTS

- Go, for `nsscash`
  * github.com/BurntSushi/toml
  * github.com/google/renameio
  * github.com/pkg/errors
- C compiler, for `libnss_cash.so.2`

- HTTP(S) server to provide the passwd/group/etc. files

- NSS module is only supported on Little-endian systems (for now)

Tested on Debian Buster, but should work on any GNU/Linux system. With
adaptations to the NSS module it should work on any UNIX-like system which
uses NSS.


== USAGE

Install `libnss_cash.so.2` somewhere in your library search path (see
`/etc/ld.so.conf`), e.g. `/usr/lib/x86_64-linux-gnu/`.

Update `/etc/nsswitch.conf` to include the cash module; `passwd` and `group`
are currently supported. For example:

    passwd:         files cash
    group:          files cash
    [...]

Create the cache files with the proper permissions (`nsscash fetch` won't
create new files to prevent using incorrect permissions):

    touch /etc/passwd.nsscash
    touch /etc/group.nsscash
    chmod 0644 /etc/passwd.nsscash
    chmod 0644 /etc/group.nsscash

Configure the `nsscash` configuration file `nsscash.toml`, see below.

Then start `nsscash`:

    nsscash fetch /path/to/config/nsscash.toml

This will fetch the configured files and update the local caches. The files
are atomically overwritten (via temporary file, sync, and rename).

Verify the users/groups are available, e.g. with `getent`. If everything
works, remember to reboot the host as changes to `nsswitch.conf` don't affect
running processes!

Now configure `nsscash` to run regularly, for example via cron or a systemd
timer.

To monitor `nsscash` for errors one can use the last modification time of the
state file (see below). It's written on each successful run and not modified
if an error occurs.

=== CONFIGURATION

Nsscash is configured through a simple configuration file written in TOML. A
typical configuration looks like this:

    statepath = "/var/lib/nsscash/state.json"

    [[file]]
    type = "passwd"
    url = "https://example.org/passwd"
    path = "/etc/passwd.nsscash"

    [[file]]
    type = "group"
    url = "https://example.org/group"
    path = "/etc/group.nsscash"

    # Optional, but useful to deploy files which are not supported by the
    # nsscash NSS module, but by libc's "files" NSS module. nsscash takes care
    # of the atomic replacement and updates; an "netgroup: files" entry in
    # "/etc/nsswitch.conf" makes the netgroups available.
    [[file]]
    type = "plain"
    url = "https://example.org/netgroup"
    path = "/etc/netgroup"

The following global keys are available:

- `statepath`: Path to a JSON file which stores the last modification time and
  hash of each file; automatically updated by `nsscash`. Used to fetch data
  only when something has changed to reduce the required traffic, via
  `If-Modified-Since`. When the hash of a file has changed the download is
  forced.

Each `file` block describes a single file to download/write. The following
keys are available (all keys are required unless marked as optional):

- `type`: Type of this file; can be either `passwd` (for files in
  `/etc/passwd` format), `group` (for files in `/etc/group` format), or
  `plain` (arbitrary format). Only `passwd` and `group` files are supported by
  the nsscash NSS module. But, as explained above, `plain` can be used to
  distribute arbitrary files. The type is required as the `.nsscash` files are
  preprocessed for faster lookups and simpler C code which requires a known
  format.

- `url`: URL to fetch the file from; HTTP and HTTPS are supported

- `ca`: Path to a custom CA in PEM format. Restricts HTTPS requests to accept
  only certificates signed by this CA. Defaults to the system's certificate
  store when omitted. (optional)

- `username`/`password`: Username and password sent via HTTP Basic-Auth to the
  webserver. The configuration file must not be readable by other users when
  this key is used. (optional)

- `path`: Path to store the retrieved file


== AUTHORS

Written by Simon Ruderich <simon@ruderich.org>.


== LICENSE

This program is licensed under AGPL version 3 or later.

Copyright (C) 2019-2021  Simon Ruderich

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.
