A ssh/gpg-agent wrapper leveraging cgroups and systemd/socket activation
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Latest commit 50c069f Mar 13, 2016 @vodik libsystemd-daemon doesn't exit anymore
Its been renamed since v229. Thanks to @holomorph and @oschwede. Closes
issue #70.

README.md

NOTICE

I've had a lot of fun developing and supporting this tool and learned temendously from developing it. However, I won't be dedicating much more effort into it going foward. Recent changes in gpg-agent have weakened the rational for using envoyd around gpg-agent.

Its simpler and better to just wrap gpg-agent in a service now. That leaves envoy-exec, for this configuration, the only useful component. I've seperated it into a seperate project gpg-tools under the name gpg-exec.

Those using ssh-agent can continue to use this project, but since I primarily use gpg-agent, I can't speak for the quality of it. I will continue to try to support this project and fix bugs.

envoy

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Envoy helps you to manage SSH keys in a similar fashion to keychain, but is implemented in C and takes advantage of cgroups and systemd.

The daemon, envoyd, starts the agent of choice in a sanitized environment and caches the associated environmental variables in memory. The agent is started on demand and its lifetime is tracked through cgroups for accuracy. envoyd is typically started as root and can thus serve all the users on the system at once. It checks the credentials of the incoming connection and starts the agent under that uid/guid. If it is started as a user it will only be able to serve that particular user's requests.

The envoy command connects to the daemon and gets all the information associated with the current running agent. It can then do things like add new keys to the agent or output shell code to inject these variables into a shell.

This effectively allows a user to share a single long-running authentication agent between all shells and sessions in a clean and managed fashion that doesn't clutter user login sessions.

Setup

To setup envoy, first enable the socket:

# systemctl enable envoy@ssh-agent.socket    # to make ssh-agent the default agent
# systemctl enable envoy@gpg-agent.socket    # or to make it gpg-agent

Then add the following to your shell's rc file.

envoy [key ...]
source <(envoy -p)

The -t flag lets you override the default agent. So envoy -t gpg-agent will launch gpg-agent even if ssh-agent is the system default.

The envoyd daemon will also run just fine under a user session, just note that it won't be able to serve multiple users at once in this configuration.

Usage

usage: envoy [options] [key ...]
Options:
 -h, --help            display this help
 -v, --version         display version
 -d, --defer           defer adding keys until the next envoy invocation
 -a, --add             add private key identities
 -x, --expunge         remove private key identities
 -k, --kill            kill the running agent
 -r, --reload          reload the agent (gpg-agent only)
 -l, --list            list fingerprints of all loaded identities
 -u, --unlock=[PASS]   unlock the agent's keyring (gpg-agent only)
 -p, --print           print out environmental arguments
 -s, --sh              print sh style commands
 -c, --csh             print csh style commands
 -f, --fish            print fish style commands
 -t, --agent=AGENT     set the preferred agent to start

Note that when passing in keys, if they reside in ~/.ssh/, then just providing the filename is sufficient.

Envoy with ssh-agent

When invoking envoy causes ssh-agent to start, on that first run any keys passed to envoy will be added to the agent. The default behavior is to check for the presence of the files .ssh/id_rsa, .ssh/id_dsa, .ssh/id_ecdsa and .ssh/id_ed25519 and load those files if present.

Envoy with gpg-agent

Keys are never implicitly added with gpg-agent. Instead, keys have to be explicitly added through either envoy -a or ssh-add. The agent will then continue track those identities automatically without having to be specified in the future.

The agent will also still respect ~/.gnupg/gpg-agent.conf. For example, to disable scdaemon, put disable-scdaemon in that file.

Note that invoking envoy also updates gpg-agent with the current status, if available, of the tty and X. It is the same effect of running echo UPDATESTARTUPTTY | gpg-connect-agent. This may cause some odd behaviour with the pinentry. The pinentry may appear in an inappropriate place if this data becomes stale. This is a limitation of gpg-agent itself.

Envoy's pam integration

Envoy provides a pam module to load the agent into the environment at login instead of relying on envoy -p. To use it, edit /etc/pam.d/login and add:

session   optional    pam_envoy.so

Its also possible provide an optional argument to choose which agent type to start:

session   optional    pam_envoy.so    gpg-agent

Envoy can also optionally unlock gpg-agent's keyring automatically with your password, but in order to do so it needs an auth token. To enable this, add:

auth      optional    pam_envoy.so
session   optional    pam_envoy.so

Note that this relies on gpg-agent's passphrase presetting support. To enable this, ensure allow-preset-passphrase is also in ~/.gnupg/gpg-agent.conf.

Wrappers with envoy

Envoy has support for wrapping commands through envoy-exec. The utility will connect to the daemon, setup the environment, and launch the provided command. For example:

envoy-exec ssh git@github.com

It is also possible to write an envoy-exec "script" to provide a terser wrapper.

#!/usr/bin/envoy-exec
/usr/bin/ssh

This script will behave as if its been invoked as envoy-exec ssh.

Cgroups support

Having been unable to find a simple cgroups library targeted at embedding, I wrote my own. cgroups.c has been borrowed from my own project here.

Any bugs with the cgroups support or confusions with terminology (I'm pretty sure my terminology is way off) should be reported there.