mholt/caddy-l4

Feature Request: Support for StartTLS protocols

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#384 opened on Feb 17, 2026

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enhancementhelp wanted

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Description

This is a generic tracking issue based on this earlier discussion.

The feature would allow for protocols that use StartTLS to be handled by Caddy instead of only forwarded via passthrough, with either Caddy proxying the negotiation if it could detect SNI (would presumably require successfully upgrading to TLS), or only handling StartTLS as an outbound connection the proxied service similar to HTTPS => HTTP.

I assume this is relevant to the caddy-l4 project, but it may also be suited as a separate plugin for L4 if not?

I don't actually expect anyone to tackle this. I am just opening / documenting the feature more generically for users to subscribe to and express interest via 👍 (instead of the various separate issues already on the topic) Maybe someone will give it a go, or can chime in with advice on approach/suitability to anyone that is interested in implementing

Contributor guide

Research direction
Study the current TLS handling in caddy l4 and understand how StartTLS protocols (like SMTP, IMAP) negotiate upgrades. Look at the existing passthrough mechanism and consider implementing a proxy that can detect and handle StartTLS negotiations. Refer to the linked discussion for context.
Tech stack
go
Domain
backend
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Needs investigation
Prerequisites
GoTLS
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50