ssh config to virtualbox with unknown ip - find out from guest properies

config

# can not use HostName because ip is dynamic
# target machine is vm in virtual box and has bridged interface to the network
# this is useful when you run vm that shall be accessible the same way as host
# machine is and you can not have statically assigned ip from dhcp

# edit: following comment is outdated
# prerequisity is to have `socat` and `arp-scan` commands available (installed)
# also need of providing password for sudo may be avoided by setting sudoers
# with proper NOPASSWD directive

Host zs
#    HostName 10.4.1.2
#    ProxyCommand bash -c 'socat stdio tcp:$(sudo arp-scan --interface em1 -l | grep 08:00:23:78:ac:23 | cut -f 1):22'
# edit: more convenient way without sudo and nasty arp-scan
#    ProxyCommand bash -c 'socat stdio tcp:$(arp -a | grep 08:00:23:78:ac:23 | cut -d " " -f 2 | tail -c +2 | head -c -2):22'
# edit: previous doesn't work every time - most convinient way is using vbox guest additions
    ProxyCommand bash -c 'socat stdio tcp:$(VBoxManage guestproperty get <VBOX_NAME> "/VirtualBox/GuestInfo/Net/0/V4/IP" | cut -d " " -f 2):22'
    User root

Please use VBoxManage guestproperty get <VM_NAME|VM_UUID> "/VirtualBox/GuestInfo/Net/0/V4/IP" if it works for you. (for vbox version < 5.1 you need to have installed guest additions extra)
You can list all available properties by VBoxManage guestproperty enumerate <VM_NAME|VM_UUID>.

Also using arp -a instead of arp-scan is more legit way to do it. arp-scan may get detected by monitoring tools as malicious behaviour.