dco doesn't load, missing "topology subnet"

[thomasschaeferm]

I use openvpn with IPv6-only (transport and payload).
I want to use dco, but it complains about "topology subnet".
The whole error message is:

May 25 09:19:17 raspberrypi kernel: OpenVPN data channel offload (ovpn-dco) 0.2.20230426 -- (C) 2020-2023 OpenVPN, Inc.
May 25 09:19:31 raspberrypi openvpn[13706]: Note: --cipher is not set. OpenVPN versions before 2.5 defaulted to BF-CBC as fallback when cipher negotiation failed in this case. If you need this fallback please add '--data-ciphers-fallback BF-CBC' to your configuration and/or add BF-CBC to --data-ciphers.
May 25 09:19:31 raspberrypi openvpn[13706]: Note: NOT using '--topology subnet' disables data channel offload.
May 25 09:19:31 raspberrypi openvpn[13706]: OpenVPN 2.6.4 armv7l-unknown-linux-gnueabihf [SSL (OpenSSL)] [LZO] [LZ4] [EPOLL] [MH/PKTINFO] [AEAD] [DCO]
May 25 09:19:31 raspberrypi openvpn[13706]: library versions: OpenSSL 3.0.8 7 Feb 2023, LZO 2.10
May 25 09:19:31 raspberrypi openvpn[13706]: DCO version: 0.2.20230426
May 25 09:19:31 raspberrypi openvpn[13707]: net_route_v4_best_gw query: dst 0.0.0.0
May 25 09:19:31 raspberrypi openvpn[13707]: net_route_v4_best_gw result: via 0.0.0.0 dev
May 25 09:19:31 raspberrypi openvpn[13707]: Diffie-Hellman initialized with 2048 bit key
May 25 09:19:32 raspberrypi openvpn[13707]: TUN/TAP device tun0 opened
May 25 09:19:32 raspberrypi openvpn[13707]: net_iface_mtu_set: mtu 1500 for tun0
May 25 09:19:32 raspberrypi openvpn[13707]: net_iface_up: set tun0 up
May 25 09:19:32 raspberrypi openvpn[13707]: net_addr_v6_add: 2001:a61:XXXX:XXXX::1:2/112 dev tun0
May 25 09:19:32 raspberrypi openvpn[13707]: Socket Buffers: R=[131072->131072] S=[16384->16384]
May 25 09:19:32 raspberrypi openvpn[13707]: setsockopt(IPV6_V6ONLY=0)
May 25 09:19:32 raspberrypi openvpn[13707]: Listening for incoming TCP connection on [AF_INET6][undef]:1194
May 25 09:19:32 raspberrypi openvpn[13707]: TCPv6_SERVER link local (bound): [AF_INET6][undef]:1194
May 25 09:19:32 raspberrypi openvpn[13707]: TCPv6_SERVER link remote: [AF_UNSPEC]
May 25 09:19:32 raspberrypi openvpn[13707]: MULTI: multi_init called, r=256 v=256

topology option should not be necessary for ipv6.

[ordex]

thanks for reporting - indeed the topology should matter only if an IPv4 was configured.

[schwabe]

Interfaces are configured differently depending on the subnet. I.e. broadcast/multicast flags on the tun interface. How much impact that has needs further investigation.

[cron2]

So, IPv6 always uses subnets, and never p2p mode. But depending on IPv4 topology, the tun interface might still be in p2p mode, confusing DCO (at least on FreeBSD).

So I think the safest approach here would be "if there is no topology in the config and no IPv4 ifconfig either, then just auto-configure topology subnet". And do not annoy IPv6 people with IPv4 archeology.