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Running a container with "--add-host blah:host-gateway" adds an /etc/hosts entry for host "blah" and an address on the docker host - to give the container a convenient way of reaching the host. If no --host-gateway-ip option is supplied, the IPv4 address of the default bridge is used - and that's been fine until now, it's a host address we know will exist. But, in a container that's only connected to IPv6-only networks, that doesn't work. So: - if the default bridge has an IPv6 address, create an additional /etc/hosts entry with that adddress - allow two --host-gateway-ip options - at most one IPv4 and one IPv6 address - in daemon.json, allow a JSON array value in --host-gateway-ips (plural) - for a single address, a JSON string is also allowed For example: --host-gateway-ip 192.0.2.1 --host-gateway-ip 2001:db8::1111 And the daemon.json version would be: "host-gateway-ips": ["192.0.2.1", "2001:db8::1111"] But, this is also still valid: "host-gateway-ip": "192.0.2.1" Note that the /etc/hosts entries follow the usual rules. If IPv6 is disabled in a container (by sysctl, or lack of kernel support), IPv6 addresses are not included in the file. In other cases, IPv4 and IPv6 addresses will both be included, whether or not the container currently has network endpoints that support IPv4 or IPv6. buildx has its own code to interpret the host-gateway-ip option. When it's updated to understand two addresses, moby will need to pass it both. For now, it passes an IPv4 address if there is one, else IPv6. Signed-off-by: Rob Murray <rob.murray@docker.com>