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April 2021: OpenSMTPD, plakar, ipcmsg, privsep and a small hypnosis talk

·2666 words·13 mins·
Gilles Chehade
technology plakar backups
Gilles Chehade
I’m not a cat.
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I worked on OpenSMTPD-portable, did a lot of plakar, a lot of Go and gave a technical talk on hypnosis.

Let’s start with some LoFi #


I have a youtube channel (subscribe ! now !)

This one caused me a copyright strike so I hope it was worth it :-)

OpenSMTPD-portable #

I did a bit of review and test for diffs sent to me, helped test a diff for a reproducible crash in the new libtls code on my machines, and shared some of my nooSMTPD diffs with eric@ so he could decide to reuse them or not in OpenSMTPD.

With OpenBSD 6.9 coming out, OpenSMTPD 6.9.0 was tagged and eric@ asked me if I could synchronize the OpenSMTPD portable repository so it matches OpenBSD. I spent a few hours bringing back every individual commit, fixing conflicts and ended up with a libtls-powered OpenSMTPD-portable which … didn’t build anywhere because the world still uses OpenSSL.

Since I had already dealt with this in nooSMTPD, I brought back my libtls compat layer so that on systems with OpenSSL the compat layer allows using the libtls interface on top of OpenSSL. This fixed the CI for Ubuntu and Fedora, unfortunately the CI for Alpine and ArchLinux uses LibreSSL and remain broken until the latest LibreSSL is packaged there.

Because the libtls conversion is such a major change, there will be a delay between the time OpenSMTPD is released for OpenBSD users and the time it is released for other systems: the change to the libtls interface has been heavily tested and I’ve been running it for a while now, but the portable adaptation of it has been virtually untested. I’ll send a mail this week-end to call for testing before we can tag a release.

Plakar #

I wrote about plakar last month and made a lot of progress since then.

I’m not publishing the code yet as there are many things I want to finish first before the first people start commenting and bikeshedding.

restructured the project #

I’m not too familiar with Go so I didn’t structure the project very nicely at first. I spent a few hours creating specific modules and reworking things so that they are properly split.

I’m not done but it is starting to look less shameful :-)

removed namespaces #

I initially thought namespaces within a plakar was a nice idea, but the more I played with them the more I realized it wasn’t and didn’t bring any benefit over creating multiple plakars. I decided to remove namespaces to simplify things.

plakar-level encryption #

I brought support for encryption and tried two different approaches: snapshot-level encryption and plakar-level encryption.

With the first approach, a plakar repository doesn’t care about encryption. It is the snapshots themselves that are encrypted on an individual basis and the same plakar can host both encrypted and cleartext snapshots.

With the second approach, a plakar repository is initialized as encrypted or cleartext. The snapshots are automatically encrypted if needed and the same plakar ony hosts all encrypted or all cleartext snapshots.

I played with both but decided to go with the second approach because the first one came with additional unnecessary complexity.

encryption macro-details #

A user generates a P384 keypair as well as a random master key, the bundle is protected by a pbkdf2-derived passphrase.

When pushing to an encrypted plakar, the chunks and objects are aes256-gcm encrypted using a subkey encrypted itself by the master key.

The snapshot index containing all the checksums for all objects and chunks is encrypted and signed.

Upon restore, the index signature is verified and the chunks are decrypted.

keypair and master key generation #

The P384 keypair and master key bundle is generated using the plakar keygen command:

% plakar keygen 
passphrase (confirm): 
keypair saved to local store

This results in the passphrase-encrypted bundle being saved to ~/.plakar/plakar.key.

plakar initialization #

To be able to create snapshots, an initialized plakar repository must be available. A local cleartext plakar is initialized by default in ~/.plakar so that the command will work out of the box for local snapshots.

However it can also be initialized elsewhere, defaulting to an encrypted plakar:

% plakar init /tmp/plakar
/tmp/plakar: store initialized

or can be made cleartext by passing the -cleartext option:

% plakar init -cleartext /tmp/plakar.ct
/tmp/plakar.ct: store initialized

Reworked the command line interface #

Last month, to use an alternative plakar instead of ~/.plakar, it was necessary to use the -store option which I disliked… so I reworked the command line to introduce a notion of direction.

When using the default plakar, no change is required:

% plakar push /private/etc
% plakar ls
2021-04-30T22:35:35Z 702b5b48-15dc-41cf-bfc1-1c8b94d1e985 3.1 MB (files: 242, dirs: 41)
% plakar pull 702b5b48

But when using an alternate plakar, instead of providing the -store option it is now possible to push to a plakar:

% plakar push /private/etc to /tmp/plakar.ct

.. and run commands from a plakar:

% plakar ls from /tmp/plakar.ct            
2021-04-30T22:30:51Z d499fd92-01cb-4eb3-bb60-b147417b68a1 3.1 MB (files: 242, dirs: 41)
% plakar pull d499fd92 from /tmp/plakar.ct

All commands support the direction option.

Generate a tarball #

I thought it would be nice if I could restore a snapshot or part of it into a tarball, as I often want to extract a bit of a snapshot on a machine to send elsewhere.

I introduced a tarball command which allows generating a tarball:

% plakar tarball d499fd92 > /tmp/d499df92.tar.gz

It also supports partial restore:

% plakar tarball d499fd92:/etc > /tmp/d499df92_etc.tar.gz

plakar server and client #

I implemented a proof of concept for a plakar server and client protocol, allowing the use of plakar over the network.

On one end, I initialize a plakar repository and run a server from it:

nas% plakar init /tmp/plakar
/tmp/plakar: store initialized
nas% plakar server from /tmp/plakar

On the other end, I simply push to a remote plakar:

laptop% plakar push /etc to plakar:// 

There is absolutely no difference or limitation from the client point of view, any command that works locally will work remotely just as well.

It is even possible to chain servers in order to proxify a plakar server:

nas% plakar server from /tmp/plakar

gate% plakar server from plakar://

laptop% plakar push /etc to plakar:// 

Does it serve a purpose ? nope, it just works by accident.

plakar ui #

Finally, I implemented a basic web UI so that I could browse the snapshots easily.

% plakar ui
Launched UI on port 40717

This opens a web browser which lets me browse the snapshots as a filesystem:

allows me to inspect individual files:

get a preview:

including of images, pdf, videos or sound:

and even search for files matching a pattern in every snapshot:

Because the web UI is a plakar client that doesn’t do anything but plakar commands, it can be launched from any plakar store, cleartext or encrypted, local or remote.

Some work in Go #

I first wrote Go code with filter-rspamd and filter-senderscore two years ago, but never really dived into the language more than these two small projects because I still have some love-hate issues with some aspects of it.

I decided this month to get more familiar with it and started looking at what it would take to write a tiny daemon, not just a program that runs an endless loop and does all work in the same process, but one that does things the OpenBSD style with privileges separation, message passing and fd passing.

I was not disappointed: there’s not much out there to do that.

go-ipcmsg #

The first thing that is missing is a package that provides something like the imsg(3) framework.

For those not familiar with it, the imsg(3) framework provides functions that allow two processes to exchange messages, including file descriptors, while guaranteeing that messages are always received whole.

Typically, you will create a socketpair(2) before fork(2)-ing a process. The parent and the child will both use one end of the pair to communicate with each other, using the imsg(3) framework that will take care of buffering I/O and exposing full messages to receiving end.

There are a lot of gory details on how it achieves this and requires understanding iovec, the semantic of sendmsg(2) and recvmsg(2), how control messages and SCM_RIGHTS works, as well as some of the side effets of cmsgbuf alignments. I did a bit of work related to resources exhaustion there a few years ago and, while the interface is lovely, I can’t say I really missed diving in that code.

When I figured that there was nothing similar in Go, I started reproducing the imsg(3) API but realized that while the API was fine in C, it could be made much simpler in Go using the language-provided channels and goroutines.

So I wrote a Channel struct which creates two channels, one for reads and one for writes, and associates them to a socketpair(2) end:

// parent process main routine, forks a child then sets up an ipcmsg
// Channel on the socketpair, returning read and write channels. The
// channels can be used to emit messages to the child process.
func parent() {
    pid, fd := fork_child()
    child_r, child_w := ipcmsg.Channel(pid, fd)

    // send a message to child
    child_w <- ipcmsg.Message(42, []byte("foobar"))

    // read a message from child
    msg := <- child_r
    log.Printf("Received message: %s\n", string(msg.Data))

// child process main routine, sets up an ipcmsg Channel on fd 3,
// the socketpair end inherited from parent,
// read and write channels can be used to communicate with parent
// process.
func child() {
    parent_r, parent_w := ipcmsg.Channel(os.Getppid(), 3)

    // receive a message from parent
    msg := <- parent_r
    log.Printf("Received message: %s\n", string(msg.Data))

    // write a message back
    parent_w <- ipcmsg.Message(42, []byte("barbaz"))

In practice, you wouldn’t just inline the calls but the parent and the child would call a dispatch function to handle messages and act upon them, something similar to below:

const (

func dispatcher(r chan ipcmsg.IPCMessage, w chan ipcmsg.IPCMessage) {
    for msg := range r {
        switch msg.Hdr.Type {
        case IPCMSG_PING:
            log.Printf("data: %s\n", string(msg.Data))
            w <- ipcmsg.Message(IPCMSG_PONG, []byte("barbaz"))

func parent() {
    pid, fd := fork_child()
    child_r, child_w := ipcmsg.Channel(pid, fd)

    dispatcher(child_r, child_w)

The file descriptors passing is handled with the function MessageWithFd() which takes an additional parameter:

    fd, err := syscall.Open(os.Args[0], 0700, 0)
    if err != nil {
    w <- ipcmsg.MessageWithFd(IPCMSG_PING, []byte("barbaz"), fd)

The sending end will have the descriptor closed upon sending, the receiving end will be receive a message with a HasFd option set in its header and an open descriptor:

    msg := <- r
    if msg.Hdr.HasFd == 1 {
        if msg.Fd == -1 {
            // expected a descriptor but got none, handle this
        } else {

The package is already commited on Github, however it hasn’t been heavily tested and I would not recommend using it for anything serious before it has matured a bit. I would LOVE to receive testing feedbacks though !

go-privsep #

The second thing missing is the ability to easily do privileges separation, that is creating multiple processes that are inherited from a parent process with different privileges, and have the ability to communicate one with another.

On OpenBSD, daemons follow the fork+reexec pattern which boils down to the following:

The daemon is started, it forks all of its child processes after setting up the plumbing for communicating with them, and each child process re-executes itself so that it benefits from ASLR and doesn’t retain the memory layout of the parent process.

This is an improvement over the more widely used fork pattern, where each child inherits from parent, because the reexec causes all inherited data to be lost. It prevents inheriting sensitive information by accident at the cost of forcing the parent to send back the necessary information to each process. And again, it makes all child processes benefit from ASLR too which is nice.

This pattern was popularized in OpenBSD a few years ago, long after many of the daemons were already in place, so each one did what it could to make it happen. There was no attempt at unifying this through a framework like was done for IPC and imsg(3).

Since I had a good understanding of how to do that and no framework to inspire myself, I worked on a privsep package that would make it easier to write such daemons. The idea is that a daemon will describe the different processes that compose it, as well as the communication channels between them, and the privsep package will then handle all the plumbing so that processes are created according to expectations.

Here is a simple example of how it works:

package main

import (


const (

func parent_main() {
	<-make(chan bool) // sleep forever

func foobar_main() {
	parent := privsep.GetParent()
	parent.Write(ipcmsg.Message(IPCMSG_PING, []byte("abcdef")))
	<-make(chan bool)

func parent_dispatcher(r chan ipcmsg.IPCMessage, w chan ipcmsg.IPCMessage) {
	for msg := range r {
		if msg.Hdr.Type == IPCMSG_PING {
			log.Printf("[parent] received PING, sending PONG\n")
			w <- ipcmsg.Message(IPCMSG_PONG, []byte("abcdef"))

func foobar_dispatcher(r chan ipcmsg.IPCMessage, w chan ipcmsg.IPCMessage) {
	for msg := range r {
		if msg.Hdr.Type == IPCMSG_PONG {
			log.Printf("[foobar] received PONG, sending PING\n")
			w <- ipcmsg.Message(IPCMSG_PING, []byte("abcdef"))

func main() {

	parent := privsep.Parent(parent_main)
	foobar := privsep.Child("foobar", foobar_main)

	parent.Channel(foobar, parent_dispatcher)
	foobar.Channel(parent, foobar_dispatcher)


As you can see, it uses the ipcmsg package to handle IPC between the two processes, allowing my pingpong daemon to play ping pong indefinitely. Each process can have more specific settings set so that it runs under a specific user, a specific group or from a chroot(2) jail.

I still have a lot of work to do on this, but I commited the current state on Github anyways.

small talk on hypnosis #

This is not tech related but since my other main activity is hypnosis and hypnotherapy, I spent a bit of time this month working on a 3-hours talk I gave to a community of street hypnotists and hypnotherapists in Nantes.

I’ve been working these last three months on a model to describe the organization of the psychic apparatus, based on what I learnt, experienced and observed these last six years from hundreds of hypnosis sessions, tons of psychology readings and many experiments in lucid dreaming.

The model describes how the psychic apparatus organization changes with the altered state of consciousness of a subject and what it means in terms of access to the subconscious and repressed psychic content.

This is not limited to hypnosis as it can be used to evaluate if a particular kind of therapy even makes sense, but applied to hypnosis it is particularly useful to understand resistance, what is happening in a subject when a technique is used, what is happening when a technique isn’t working, and how to bring the subject to the proper state of consciousness.

I don’t think this post is the right place to dive into details and, well, the talk lasted 3 hours and it was very superficial so I will stop there. If people are interested in this topic, let me know and I’ll think of a way to present this as the slides I presented are not really informative without me talking over them.

What’s next ? #

Not much.

May will be a calm month for me here as the sponsoring has decreased and I need to do some freelance to buy myself spare time in June/July. Furthermore, I have had to deal with some personal issues in April and I’m not in the mood to do a lot of stuff at the moment.

I’ll probably do a few things, as usual, but I’m not sure what at this point as it depends on how much time I’ll have available and how my mood evolves.

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