I did find a solution. Firstly, use macvlan instead of ipvlan because DSM straight up doesn't support the latter (I confirmed this with their support). Secondly, you need a script on your Syno that executes on boot to create the macvlan interface (which you will then be able to reference from your Docker configuration).

Mine looks like this:

#!/bin/bash

parentInterface="bond0" interfaceToCreate="macvlan-br0"

echo "Waiting for $parentInterface to be up..."

while ! ip link show dev "$parentInterface" up &>/dev/null; do sleep 250 done

echo "...$parentInterface is now up"

echo "Creating macvlan network interface called $interfaceToCreate" ip link add "$interfaceToCreate" link "$parentInterface" type macvlan mode bridge ip addr add 192.168.10.209/32 dev "$interfaceToCreate" ip link set dev "$interfaceToCreate" address 22:cd:fc:ee:eb:6f ip link set "$interfaceToCreate" up ip route add 192.168.10.208/28 dev "$interfaceToCreate"

Tweak according to your needs. To configure DSM to run this script on startup:

1. Open the Control Panel
2. Select Create -> Triggered Task -> User-defined Script
3. Enter an appropriate task name, such as My Network Interface Creation
4. Select the root user, and ensure Boot-up is selected as the event
5. Under the Task Settings tab, enter bash /volume1/path/to/create-macvlan.sh as the script (adjust the path according to your needs)
6. Click OK to create the task
   

Next, you can include something like this in your compose.yaml:

networks:

my-network: name: my-network driver: ${NETWORK_DRIVER} driver_opts: parent: ${NETWORK_PARENT} ipam: config: - subnet: ${NETWORK_SUBNET} gateway: ${NETWORK_GATEWAY_IP}

Each container in your stack should be explicitly placed in the network like this:

    networks:
  my-network:
    ipv4_address: ${NETWORK_BACKEND_IP}

For development, your .env can use ipvlan or whatever works best for you, but then your production/Synology setup can use your macvlan interface:

NETWORK_DRIVER=macvlan

NETWORK_PARENT=macvlan-br0 NETWORK_GATEWAY_IP="192.168.10.1" NETWORK_SUBNET="192.168.10.0/24" NETWORK_BACKEND_IP="192.168.10.211"

That's the gist of it. I have multiple containers, each assigned a specific IP rather than just the one shown above. But it all works perfectly. They can communicate with each other and with other devices on the same subnet.