Start Mosh with AWS SSM Session Manager

start_mosh.sh

#!/bin/bash
#
# Mush mosh and SSM sessions together.
#
# Run mosh-server on an EC2 instance as part of an AWS Session Manager SSH
# session, and feed the output into mosh-client locally. This is helpful
# if you have an EC2 instance which:
#
# * Is publicly addressable
# * Has a predictable Name tag but a shifting IP
# * Allows inbound Mosh traffic (UDP 60000-61000) from your location
# * Does not allow inbound SSH, except via SSM
#
# Dependencies:
#
# * AWS CLI
# * jq
# * mosh (on both the client and server)

set -euo pipefail

usage() {
	echo "Usage: $0 -p <AWS CLI profile> -n <instance name filter> -r <region> -u <remote user>"
}

help() {
	echo -e "Start a mosh session on an EC2 instance via AWS SSM Session Manager.\n"
	usage
}

while getopts ":hp:n:r:u:" options; do
	case "${options}" in
		h)
			help; exit 0;;
		p)
			PROFILE=${OPTARG};;
		n)
			NAME_FILTER=${OPTARG};;
		r)
			REGION=${OPTARG};;
		u)
			REMOTE_USER=${OPTARG};;
		:)
			echo -e "Error: -${OPTARG} requires an argument.\n"
			usage; exit 1;;
		*)
			echo -e "Unknown option -${OPTARG}.\n"
			usage; exit 1;;
	esac
done

if [[ -z "$PROFILE" || -z "$NAME_FILTER" || -z "$REMOTE_USER" || -z "$REGION" ]]; then
	help
	exit 1
fi

shift $((OPTIND - 1))

target_instance=$(
	aws ec2 describe-instances \
		--profile "${PROFILE}" \
		--region "${REGION}" \
		--filter "Name=tag:Name,Values=[${NAME_FILTER}]"
)
target_instance_ip=$(jq -nr "$target_instance | .Reservations[].Instances[].PublicIpAddress")
target_instance_id=$(jq -nr "$target_instance | .Reservations[].Instances[].InstanceId")

echo "Target instance: $target_instance_id ($target_instance_ip)"

echo "Starting mosh-server..."
output=$(
	ssh -o ProxyCommand="aws ssm start-session \
		--profile ${PROFILE} \
		--region ${REGION} \
		--target $target_instance_id \
		--document-name AWS-StartSSHSession \
		--parameters 'portNumber=22'" \
		${REMOTE_USER}@${target_instance_id} /usr/local/bin/mosh-server \
	| grep "MOSH CONNECT" | tr -d '\r\n'
)

echo "Getting key/port output..."
echo $output
mosh_port=$(echo $output | cut -d' ' -f3)
mosh_key=$(echo $output | cut -d' ' -f4)

echo "Starting mosh-client on port $mosh_port with key $mosh_key..."
MOSH_KEY="$mosh_key" mosh-client "$target_instance_ip" "$mosh_port"