In preparation for our upcoming Long Term Support (LTS) release, we have decided to restore post-quantum TLS curve support for SimpleStreams image server connections.
Background and context
In March 2025, we introduced a restriction that forced SimpleStreams connections to use legacy TLS curves. This was done to accommodate a small number of users operating behind broken network middleboxes.
Because modern post-quantum cryptography (PQC) hybrid curves—such as X25519MLKEM768 and X25519Kyber768Draft00—require larger keys, the TLS ClientHello message can exceed the size of a single TCP packet. Broken middleboxes that fail to properly reassemble fragmented TCP packets often abruptly terminate these handshakes (see tldr.fail for an in-depth technical explanation).
Why restore post-quantom curves now
While this workaround helps affected environments download images, disabling post-quantum curves indefinitely poses broader security and compliance risks:
- Security audits and compliance: Disabling modern post-quantum curves risks complicating security audits and can prevent LXD from being used in highly regulated or strict compliance environments.
- Moving the ecosystem forward: The broader HTTPS/TLS ecosystem is actively transitioning to defend against future quantum threats. LXD should not permanently weaken its TLS posture for a small number of broken network environments.
- LTS readiness: As we stabilize for the next LTS, we want to ensure our default cryptographic configurations align with modern security standards.
User-visible impact
For the vast majority of users, this change will be completely transparent.
However, if your LXD deployment is behind a non-compliant firewall, proxy, or middlebox, you may experience aborted TLS/HTTPS connections when interacting with public SimpleStreams image servers (such as images: or ubuntu:). This typically surfaces as:
read: connection reset by peer
How to resolve it: If you encounter this error after upgrading, the root cause lies in your local network infrastructure. You will need to contact your network administrator to update or fix the middlebox/firewall firmware so it can correctly handle fragmented TLS ClientHello packets.