update-gateway-ssh-config

#/bin/bash

# given three machines, A, B and C
# < LAN > | < Public >
#    A    B     C
# A is a machine inside the network with a dynamically assigned IP address
# B is a machine reachable from inside and out with a hostname that resolves to an IP
# C is a machine on the public internet
# this script updates the SSH config on B such that a user at B (or C logged into to B)
# could SSH into A without having to know the IP address of A

# we achieve this by regularly updating the SSH config on our gateway machine
# such that from the gateway, `ssh <hostname>` will get us to this machine

# N.B. we do not have permission to edit the /etc/hosts file on B

set -ex

if [[ -z $1  ]]; then
    echo "No gateway username supplied"
    exit 1
fi

if [[ -z $2  ]]; then
    echo "No gateway hostname supplied"
    exit 1
fi

GATEWAY_USERNAME=$1
GATEWAY_HOSTNAME=$2
TMP_FILE=$(mktemp -q /tmp/tmp-XXXXXXXX)

# read our network config for our local network IP
LOCAL_NETWORK_IP=$(ifconfig | grep "inet 192.168" | awk '{printf $2}')

cat << EOF > $TMP_FILE
# this file generated at $(date) on $(hostname) and copied to ${GATEWAY_HOSTNAME}
# any manual edits to this file may be overridden before long...
Host $(hostname)
    HostName ${LOCAL_NETWORK_IP}
    User $(whoami)
    ForwardAgent yes
EOF

scp $TMP_FILE $GATEWAY_USERNAME@$GATEWAY_HOSTNAME:~/.ssh/config

rm $TMP_FILE