Fix mDNS dial to downgrade to plaintext for insecure connections#182
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The mDNS branch of `create_channel` unconditionally forced TLS, so dialing a local/insecure viam-server — which serves plaintext gRPC on the loopback port it advertises via mDNS — failed the TLS handshake every time. This regressed in #180, which removed the previous loopback -> h2c branch. When the caller allows an insecure/downgraded connection (`DialOptions.allow_downgrade`, set from the FFI's `c_allow_insec` and in turn the SDK's `insecure` option), retry the mDNS connection over plaintext h2c after the TLS attempt fails — mirroring the direct-dial path and the golang dialer's "downgrading from TLS to plaintext" behavior. Secure dials are unchanged: TLS is still attempted first and only downgraded when the caller opted in. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
stuqdog
approved these changes
Jun 9, 2026
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Problem
Dialing a local/insecure viam-server by an mDNS-discoverable name fails.
viam-serveradvertises a loopback address over mDNS and serves plaintext gRPC there, but the mDNS branch ofcreate_channelunconditionally forces TLS:so the TLS handshake always failed against the plaintext server. This regressed in #180 (RSDK-13879), which removed the earlier
is_loopback -> h2cbranch.Fix
On the mDNS path, attempt TLS first (unchanged for cloud robots with real certs); if it fails and the caller allowed an insecure/downgraded connection, retry the local connection over plaintext.
Testing
cargo build/cargo clippy --all-features: clean (no new warnings).