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ipn/localapi,tstest/natlab: fix debug derp TLS check for sha256-raw CertName

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serveDebugDERPRegion built its TLS config with
ServerName: cmp.Or(derpNode.CertName, derpNode.HostName), which for a
"sha256-raw:<hex>" CertName passed the raw fingerprint to Go's stock
verifier as a hostname; the handshake always failed with a hostname
mismatch. This is the second half of #15579; the first half (tailscaled
itself failing with "unexpected multiple certs presented") was fixed in

Extract a tlsConfigForNode helper that mirrors derphttp.Client.tlsClient
so that sha256-raw and domain-fronting CertName values are dispatched
to tlsdial.SetConfigExpectedCertHash and tlsdial.SetConfigExpectedCert
respectively, falling back to HostName when CertName is empty.

The core fix here was originally written by @imnuke in #19965; that PR
also added a unit test in ipn/localapi/debugderp_test.go which is
replaced in this commit by a new vmtest that exercises the whole stack:
vnet now serves a self-signed cert valid for each fake DERP node's
HostName and exposes its SHA-256 fingerprint, and vmtest grows a new
SelfSignedDERPCertPinning EnvOption that swaps the test DERP map's
nodes to CertName="sha256-raw:<hex>" with InsecureForTests cleared.
TestSelfSignedDERPHashPinning then stands up two hard-NAT'd nodes, has
them communicate over DERP, and calls DebugDERPRegion on each. Before
this fix the test fails with the exact x509 hostname-mismatch error
from the original bug; after, it passes.

Updates #15579

Change-Id: I61f38ffebc7ac5abc962639db1ae88f5cd8633b1
Co-authored-by: Nuke <nuke@imnuke.dev>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Brad Fitzpatrick
2026-06-02 12:02:40 -07:00
committed by Brad Fitzpatrick
co-authored by Nuke
parent 52400dc6f4
commit c91b7188e8
4 changed files with 206 additions and 24 deletions
Download Patch File Download Diff File Expand all files Collapse all files
ipn/localapi/debugderp.go
+20 -9
View File
@@ -15,6 +15,7 @@ import (
"net/http"
"net/netip"
"strconv"
"strings"
"time"
"tailscale.com/derp/derphttp"
@@ -22,11 +23,28 @@ import (
"tailscale.com/net/netaddr"
"tailscale.com/net/netns"
"tailscale.com/net/stun"
"tailscale.com/net/tlsdial"
"tailscale.com/tailcfg"
"tailscale.com/types/key"
"tailscale.com/types/nettype"
)
// tlsConfigForNode builds a *tls.Config for connecting to a DERP node,
// mirroring the logic in derphttp.Client.tlsClient so that sha256-raw cert
// pinning and domain-fronting CertName values are handled correctly.
func tlsConfigForNode(node *tailcfg.DERPNode) *tls.Config {
conf := tlsdial.Config(nil, nil)
conf.ServerName = node.HostName
if node.CertName != "" {
if suf, ok := strings.CutPrefix(node.CertName, "sha256-raw:"); ok {
tlsdial.SetConfigExpectedCertHash(conf, suf)
} else {
tlsdial.SetConfigExpectedCert(conf, node.CertName)
}
}
return conf
}
func (h *Handler) serveDebugDERPRegion(w http.ResponseWriter, r *http.Request) {
if !h.PermitWrite {
http.Error(w, "debug access denied", http.StatusForbidden)
@@ -100,9 +118,7 @@ func (h *Handler) serveDebugDERPRegion(w http.ResponseWriter, r *http.Request) {
defer conn.Close()
// Upgrade to TLS and verify that works properly.
tlsConn := tls.Client(conn, &tls.Config{
ServerName: cmp.Or(derpNode.CertName, derpNode.HostName),
})
tlsConn := tls.Client(conn, tlsConfigForNode(derpNode))
if err := tlsConn.HandshakeContext(ctx); err != nil {
st.Errors = append(st.Errors, fmt.Sprintf("Error upgrading connection to node %q @ %q to TLS over IPv4: %v", derpNode.HostName, addr, err))
} else {
@@ -119,12 +135,7 @@ func (h *Handler) serveDebugDERPRegion(w http.ResponseWriter, r *http.Request) {
defer conn.Close()
// Upgrade to TLS and verify that works properly.
tlsConn := tls.Client(conn, &tls.Config{
ServerName: cmp.Or(derpNode.CertName, derpNode.HostName),
// TODO(andrew-d): we should print more
// detailed failure information on if/why TLS
// verification fails
})
tlsConn := tls.Client(conn, tlsConfigForNode(derpNode))
if err := tlsConn.HandshakeContext(ctx); err != nil {
st.Errors = append(st.Errors, fmt.Sprintf("Error upgrading connection to node %q @ %q to TLS over IPv6: %v", derpNode.HostName, addr, err))
} else {