consrv

Command consrv is a SSH to serial console bridge server, originally designed for deployment on gokrazy.org devices. Apache 2.0 Licensed.

Overview

SSH can be used to conveniently access remote machines over the network, but only if the machine has functional networking.

Serial consoles can be used to remotely access a machine with broken or no networking, but often require running a cable from another machine to remotely rescue a machine.

consrv combines the best of both worlds: an SSH interface running on a Raspberry Pi which can provide serial console access to one or more remote machines, all secured by an SSH channel. I (Matt Layher) run consrv on two Raspberry Pi 4s using gokrazy to act as remote serial console servers for my headless machines.

-- Ethernet --> [Raspberry Pi + consrv]
                  |-- USB to serial --> [desktop]
                  |-- USB to serial --> [router]
                  |-- USB to serial --> [server]

I use the following hardware, but any serial equipment supported by Linux should just work:

• Syba dual port DB9 COM RS232 PCIe x1 card
• StarTech USB to Serial RS232 null-modem adapter

Setup (gokrazy)

After formatting and mounting /perm on a gokrazy device, create the following files:

• /perm/consrv/host_key: an OpenSSH format private key for the host (generate using ssh-keygen, I recommend ssh-keygen -t ed25519)
• /perm/consrv/consrv.toml: the configuration file for consrv

Setup (Linux/other OS)

When consrv is built for a non-gokrazy Linux or other operating system (without build tag gokrazy), flags are available to specify the location of the configuration and SSH host key files:

$ ./consrv -h
Usage of ./consrv:
  -c string
        path to consrv.toml configuration file (default "consrv.toml")
  -k string
        path to OpenSSH format host key file (default "host_key")

Configuration

The TOML configuration file should have device entries for each serial device, and SSH public key identities which can be used to access the devices. Password authentication is not supported. For example:

# Configure the SSH server listener. If no configuration is specified, consrv
# binds the SSH server to ":2222" by default.
[server]
address = ":2222"

# Configure one or more USB to serial devices with friendly names which are used
# as the SSH username to access a device's serial console. You must specify either
# "device" as the path to the device or "serial" to look up the device's path
# by the adapter's serial number (useful for machines with many connections).
#
# Optionally a list of identities which are allowed to access a device may be
# provided on a per-device basis. If no identities key is configured, all
# identities are allowed to access the device.
[[devices]]
name = "server"
serial = "A64NMAJS"
baud = 115200
identities = ["mdlayher"]

[[devices]]
name = "desktop"
device = "/dev/ttyUSB1"
baud = 115200

# Configure one or more SSH public key identities which can authenticate against
# consrv to access the devices.
[[identities]]
name = "mdlayher"
public_key = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIN5i5d0mRKAf02m+ju+I1KrAYw3Ny2IHXy88mgyragBN Matt Layher (mdlayher@gmail.com)"

# Enable or disable the debug HTTP server for facilities such as Prometheus
# metrics and pprof support.
#
# Warning: do not expose pprof on an untrusted network!
[debug]
address = "localhost:9288"
prometheus = true
pprof = false

Now you can log in to either device's serial console over SSH using port 2222 on the consrv host. When you're ready to end your session, use the SSH escape ENTER ~ . to break the connection:

$ ssh -i ~/.ssh/mdlayher_ed25519 -p 2222 server@monitnerr-1
consrv> opened serial connection "server": path: "/dev/ttyUSB0", serial: "A64NMAJS", baud: 115200

servnerr-3 login: matt
Password:

[matt@servnerr-3:~]$ w
 19:49:16 up 8 days,  1:01,  1 user,  load average: 0.12, 0.06, 0.02
USER     TTY        LOGIN@   IDLE   JCPU   PCPU WHAT
matt     ttyS0     19:49    4.00s  0.03s  0.00s w

[matt@servnerr-3:~]$ Shared connection to monitnerr-1 closed.