diff --git a/checker/fetch.go b/checker/fetch.go
new file mode 100644
index 0000000..1cdd816
--- /dev/null
+++ b/checker/fetch.go
@@ -0,0 +1,108 @@
+// This file is part of the happyDomain (R) project.
+// Copyright (c) 2020-2026 happyDomain
+// Authors: Pierre-Olivier Mercier, et al.
+//
+// This program is offered under a commercial and under the AGPL license.
+// For commercial licensing, contact us at <contact@happydomain.org>.
+//
+// For AGPL licensing:
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package checker
+
+import (
+	"context"
+	"crypto/tls"
+	"crypto/x509"
+	"fmt"
+	"net"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// FetchChain dials host:port, optionally upgrades the connection via STARTTLS,
+// completes a TLS handshake (InsecureSkipVerify so callers receive the chain
+// even when PKIX would reject it), and returns the peer certificates leaf
+// first.
+//
+// starttls is the protocol name as registered (smtp, submission, imap, pop3,
+// ldap, xmpp-client, ...); pass "" for direct TLS. AutoSTARTTLS provides
+// well-known port defaults.
+func FetchChain(ctx context.Context, host string, port uint16, starttls string, timeout time.Duration) ([]*x509.Certificate, error) {
+	host = strings.TrimSuffix(host, ".")
+	addr := net.JoinHostPort(host, strconv.Itoa(int(port)))
+
+	dialCtx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	conn, err := (&net.Dialer{}).DialContext(dialCtx, "tcp", addr)
+	if err != nil {
+		return nil, fmt.Errorf("dial %s: %w", addr, err)
+	}
+	defer conn.Close()
+	if dl, ok := dialCtx.Deadline(); ok {
+		_ = conn.SetDeadline(dl)
+	}
+
+	if starttls != "" {
+		up, ok := starttlsUpgraders[starttls]
+		if !ok {
+			return nil, fmt.Errorf("unsupported starttls protocol %q", starttls)
+		}
+		if err := up(conn, host); err != nil {
+			return nil, fmt.Errorf("starttls-%s: %w", starttls, err)
+		}
+	}
+
+	tlsConn := tls.Client(conn, &tls.Config{
+		ServerName:         host,
+		InsecureSkipVerify: true,
+	})
+	if err := tlsConn.HandshakeContext(dialCtx); err != nil {
+		return nil, fmt.Errorf("tls handshake: %w", err)
+	}
+	state := tlsConn.ConnectionState()
+	if len(state.PeerCertificates) == 0 {
+		return nil, fmt.Errorf("server presented no certificate")
+	}
+	return state.PeerCertificates, nil
+}
+
+// BuildChain produces a CertInfo per peer certificate (leaf first), with the
+// four (selector, matching_type) DANE hash pairs precomputed. This is the
+// same projection probe() applies internally; exported so HTTP handlers can
+// reuse it without re-deriving the format.
+func BuildChain(certs []*x509.Certificate) []CertInfo {
+	return buildChain(certs)
+}
+
+// AutoSTARTTLS maps a well-known port to the STARTTLS dialect FetchChain
+// should drive. Returns "" when the port has no auto-mapping (caller should
+// then use direct TLS or pass an explicit dialect).
+func AutoSTARTTLS(port uint16) string {
+	switch port {
+	case 25, 587:
+		return "smtp"
+	case 143:
+		return "imap"
+	case 110:
+		return "pop3"
+	case 389:
+		return "ldap"
+	case 5222:
+		return "xmpp-client"
+	}
+	return ""
+}