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Encrypted ClientHello

fetch uses Go's crypto/tls implementation for Encrypted ClientHello (ECH). Control it with --ech off|auto|on.

  • off is the default. It does not query HTTPS/SVCB records only for ECH and does not send ECH or GREASE.
  • auto discovers HTTPS/SVCB records with address resolution. It uses a valid advertised ECH configuration when available. When no real configuration is usable, it may send structurally valid randomized GREASE and falls back to ordinary TLS after a verified rejection.
  • on requires a usable advertised configuration and an accepted ECH handshake. It fails on missing configuration, discovery failure, or rejection.

ECH requires TLS 1.3. Explicit TLS 1.2 bounds are rejected. --ech on cannot be combined with forced HTTP/3 because acceptance must be verified over the TCP TLS path. ECH is not used through proxies or cleartext HTTP/2. Automatic HTTP version selection uses TCP when hard ECH is requested.

Discovery and downgrade safety

HTTPS/SVCB discovery is scoped to the effective host and configured resolver. Validated ECH configurations keep DNS priority order. Authenticated resolver transport, parsing, and malformed-RRset failures are not silently downgraded. Authenticated no-data results may continue according to the selected mode. Unverified resolver failures may fall back with a verbose warning.

At -vvv, fetch warns once when discovery uses system or plaintext DNS, or an encrypted resolver whose certificate verification is disabled. --silent suppresses that warning. Redirects perform fresh host-scoped discovery.

Inspection

Use these commands to inspect the path before changing trust settings:

fetch --inspect-dns --dns-server https://1.1.1.1/dns-query example.com
fetch --inspect-tls --ech auto https://example.com

TLS inspection reports real ECH acceptance or rejection, GREASE offer and rejection, outer SNI, and final fallback when the rejection can be verified. The -v flag has no effect in TLS inspection mode. If QUIC cannot expose a rejected handshake's certificate state, inspection fails closed instead of downgrading (unless --insecure is explicit). DNS inspection displays ECH bytes as base64. Terminal diagnostics escape untrusted values.

See Advanced Features for resolver, TLS, proxy, and HTTP/3 details and Limits and safety for shared bounds.