Add tooling to enable strict TLS.

.golangci.yml

@@ -247,11 +247,12 @@ linters:
       # Default: false
       check-type-assertions: true
       exclude-functions:
-        - (*git.wntrmute.dev/kyle/goutils/sbuf.Buffer).Write
+        - (*git.wntrmute.dev/kyle/goutils/dbg.DebugPrinter).Write
         - git.wntrmute.dev/kyle/goutils/lib.Warn
         - git.wntrmute.dev/kyle/goutils/lib.Warnx
         - git.wntrmute.dev/kyle/goutils/lib.Err
         - git.wntrmute.dev/kyle/goutils/lib.Errx
+        - (*git.wntrmute.dev/kyle/goutils/sbuf.Buffer).Write
 
     exhaustive:
       # Program elements to check for exhaustiveness.

cmd/certdump/main.go

@@ -7,6 +7,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rsa"
 	"crypto/sha256"
+	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"flag"
@@ -350,14 +351,11 @@ func main() {
 	flag.BoolVar(&leafOnly, "l", false, "only show the leaf certificate")
 	flag.Parse()
 
-	opts := &lib.FetcherOpts{
+	tlsCfg := &tls.Config{InsecureSkipVerify: true} // #nosec G402 - tool intentionally inspects broken TLS
-		SkipVerify: true,
-		Roots:      nil,
-	}
 
 	for _, filename := range flag.Args() {
 		fmt.Fprintf(os.Stdout, "--%s ---%s", filename, "\n")
-		certs, err := lib.GetCertificateChain(filename, opts)
+		certs, err := lib.GetCertificateChain(filename, tlsCfg)
 		if err != nil {
 			_, _ = lib.Warn(err, "couldn't read certificate")
 			continue

cmd/certexpiry/main.go

@@ -63,6 +63,7 @@ func checkCert(cert *x509.Certificate) {
 	warn := inDanger(cert)
 	name := displayName(cert.Subject)
 	name = fmt.Sprintf("%s/SN=%s", name, cert.SerialNumber)
+
 	expiry := expires(cert)
 	if warnOnly {
 		if warn {
@@ -74,15 +75,22 @@ func checkCert(cert *x509.Certificate) {
 }
 
 func main() {
-	opts := &lib.FetcherOpts{}
+	var skipVerify bool
+	var strictTLS bool
+	lib.StrictTLSFlag(&strictTLS)
 
-	flag.BoolVar(&opts.SkipVerify, "k", false, "skip server verification")
+	flag.BoolVar(&skipVerify, "k", false, "skip server verification") // #nosec G402
 	flag.BoolVar(&warnOnly, "q", false, "only warn about expiring certs")
 	flag.DurationVar(&leeway, "t", leeway, "warn if certificates are closer than this to expiring")
 	flag.Parse()
 
+	tlsCfg, err := lib.BaselineTLSConfig(skipVerify, strictTLS)
+	die.If(err)
+
 	for _, file := range flag.Args() {
-		certs, err := lib.GetCertificateChain(file, opts)
+		var certs []*x509.Certificate
+
+		certs, err = lib.GetCertificateChain(file, tlsCfg)
 		if err != nil {
 			_, _ = lib.Warn(err, "while parsing certificates")
 			continue

cmd/certser/main.go

@@ -31,16 +31,23 @@ func serialString(cert *x509.Certificate, mode lib.HexEncodeMode) string {
 }
 
 func main() {
-	opts := &lib.FetcherOpts{}
+	var skipVerify bool
+	var strictTLS bool
+	lib.StrictTLSFlag(&strictTLS)
 	displayAs := flag.String("d", "int", "display mode (int, hex, uhex)")
 	showExpiry := flag.Bool("e", false, "show expiry date")
-	flag.BoolVar(&opts.SkipVerify, "k", false, "skip server verification")
+	flag.BoolVar(&skipVerify, "k", false, "skip server verification") // #nosec G402
 	flag.Parse()
 
+	tlsCfg, err := lib.BaselineTLSConfig(skipVerify, strictTLS)
+	die.If(err)
+
 	displayMode := parseDisplayMode(*displayAs)
 
 	for _, arg := range flag.Args() {
-		cert, err := lib.GetCertificate(arg, opts)
+		var cert *x509.Certificate
+
+		cert, err = lib.GetCertificate(arg, tlsCfg)
 		die.If(err)
 
 		fmt.Printf("%s: %s", arg, serialString(cert, displayMode))

cmd/certverify/main.go

@@ -32,6 +32,7 @@ type appConfig struct {
 	caFile, intFile             string
 	forceIntermediateBundle     bool
 	revexp, skipVerify, verbose bool
+	strictTLS                   bool
 }
 
 func parseFlags() appConfig {
@@ -43,6 +44,7 @@ func parseFlags() appConfig {
 	flag.BoolVar(&cfg.skipVerify, "k", false, "skip CA verification")
 	flag.BoolVar(&cfg.revexp, "r", false, "print revocation and expiry information")
 	flag.BoolVar(&cfg.verbose, "v", false, "verbose")
+	lib.StrictTLSFlag(&cfg.strictTLS)
 	flag.Parse()
 	return cfg
 }
@@ -108,12 +110,13 @@ func run(cfg appConfig) error {
 		return fmt.Errorf("failed to build combined pool: %w", err)
 	}
 
-	opts := &lib.FetcherOpts{
+	tlsCfg, err := lib.BaselineTLSConfig(cfg.skipVerify, cfg.strictTLS)
-		Roots:      combinedPool,
+	if err != nil {
-		SkipVerify: cfg.skipVerify,
+		return err
 	}
+	tlsCfg.RootCAs = combinedPool
 
-	chain, err := lib.GetCertificateChain(flag.Arg(0), opts)
+	chain, err := lib.GetCertificateChain(flag.Arg(0), tlsCfg)
 	if err != nil {
 		return err
 	}

cmd/stealchain/main.go

@@ -3,50 +3,47 @@ package main
 import (
 	"context"
 	"crypto/tls"
-	"crypto/x509"
 	"encoding/pem"
 	"flag"
 	"fmt"
 	"net"
 	"os"
 
+	"git.wntrmute.dev/kyle/goutils/certlib"
 	"git.wntrmute.dev/kyle/goutils/die"
 	"git.wntrmute.dev/kyle/goutils/lib"
 )
 
 func main() {
-	var cfg = &tls.Config{} // #nosec G402
-
 	var sysRoot, serverName string
+	var skipVerify bool
+	var strictTLS bool
+	lib.StrictTLSFlag(&strictTLS)
 	flag.StringVar(&sysRoot, "ca", "", "provide an alternate CA bundle")
-	flag.StringVar(&cfg.ServerName, "sni", cfg.ServerName, "provide an SNI name")
+	flag.StringVar(&serverName, "sni", "", "provide an SNI name")
-	flag.BoolVar(&cfg.InsecureSkipVerify, "noverify", false, "don't verify certificates")
+	flag.BoolVar(&skipVerify, "noverify", false, "don't verify certificates")
 	flag.Parse()
 
+	tlsCfg, err := lib.BaselineTLSConfig(skipVerify, strictTLS)
+	die.If(err)
+
 	if sysRoot != "" {
-		pemList, err := os.ReadFile(sysRoot)
+		tlsCfg.RootCAs, err = certlib.LoadPEMCertPool(sysRoot)
 		die.If(err)
-
-		roots := x509.NewCertPool()
-		if !roots.AppendCertsFromPEM(pemList) {
-			fmt.Printf("[!] no valid roots found")
-			roots = nil
-		}
-
-		cfg.RootCAs = roots
 	}
 
 	if serverName != "" {
-		cfg.ServerName = serverName
+		tlsCfg.ServerName = serverName
 	}
 
 	for _, site := range flag.Args() {
-		_, _, err := net.SplitHostPort(site)
+		_, _, err = net.SplitHostPort(site)
 		if err != nil {
 			site += ":443"
 		}
-		// Use proxy-aware TLS dialer
+
-		conn, err := lib.DialTLS(context.Background(), site, lib.DialerOpts{TLSConfig: cfg})
+		var conn *tls.Conn
+		conn, err = lib.DialTLS(context.Background(), site, lib.DialerOpts{TLSConfig: tlsCfg})
 		die.If(err)
 
 		cs := conn.ConnectionState()

dbg/dbg.go

@@ -1,12 +1,34 @@
-// Package dbg implements a debug printer.
+// Package dbg implements a simple debug printer.
+//
+// There are two main ways to use it:
+//   - By using one of the constructors and calling flag.BoolVar(&debug.Enabled...)
+//   - By setting the environment variable GOUTILS_ENABLE_DEBUG to true or false and
+//     calling NewFromEnv().
+//
+// If enabled, any of the print statements will be written to stdout. Otherwise,
+// nothing will be emitted.
 package dbg
 
 import (
 	"fmt"
 	"io"
 	"os"
+	"runtime/debug"
+	"strings"
 )
 
+const DebugEnvKey = "GOUTILS_ENABLE_DEBUG"
+
+var enabledValues = map[string]bool{
+	"1":       true,
+	"true":    true,
+	"yes":     true,
+	"on":      true,
+	"y":       true,
+	"enable":  true,
+	"enabled": true,
+}
+
 // A DebugPrinter is a drop-in replacement for fmt.Print*, and also acts as
 // an io.WriteCloser when enabled.
 type DebugPrinter struct {
@@ -15,6 +37,23 @@ type DebugPrinter struct {
 	out     io.WriteCloser
 }
 
+// New returns a new DebugPrinter on os.Stdout.
+func New() *DebugPrinter {
+	return &DebugPrinter{
+		out: os.Stderr,
+	}
+}
+
+// NewFromEnv returns a new DebugPrinter based on the value of the environment
+// variable GOUTILS_ENABLE_DEBUG.
+func NewFromEnv() *DebugPrinter {
+	enabled := strings.ToLower(os.Getenv(DebugEnvKey))
+	return &DebugPrinter{
+		out:     os.Stderr,
+		Enabled: enabledValues[enabled],
+	}
+}
+
 // Close satisfies the Closer interface.
 func (dbg *DebugPrinter) Close() error {
 	return dbg.out.Close()
@@ -28,13 +67,6 @@ func (dbg *DebugPrinter) Write(p []byte) (int, error) {
 	return 0, nil
 }
 
-// New returns a new DebugPrinter on os.Stdout.
-func New() *DebugPrinter {
-	return &DebugPrinter{
-		out: os.Stdout,
-	}
-}
-
 // ToFile sets up a new DebugPrinter to a file, truncating it if it exists.
 func ToFile(path string) (*DebugPrinter, error) {
 	file, err := os.Create(path)
@@ -74,3 +106,7 @@ func (dbg *DebugPrinter) Printf(format string, v ...any) {
 		fmt.Fprintf(dbg.out, format, v...)
 	}
 }
+
+func (dbg *DebugPrinter) StackTrace() {
+	dbg.Write(debug.Stack())
+}

lib/dialer.go

@@ -20,6 +20,7 @@ import (
 	"crypto/tls"
 	"encoding/base64"
 	"errors"
+	"flag"
 	"fmt"
 	"net"
 	"net/http"
@@ -29,8 +30,42 @@ import (
 	"time"
 
 	xproxy "golang.org/x/net/proxy"
+
+	"git.wntrmute.dev/kyle/goutils/dbg"
 )
 
+// StrictBaselineTLSConfig returns a secure TLS config.
+// Many of the tools in this repo are designed to debug broken TLS systems
+// and therefore explicitly support old or insecure TLS setups.
+func StrictBaselineTLSConfig() *tls.Config {
+	return &tls.Config{
+		MinVersion:         tls.VersionTLS12,
+		InsecureSkipVerify: false, // explicitly set
+	}
+}
+
+func StrictTLSFlag(useStrict *bool) {
+	flag.BoolVar(useStrict, "strict-tls", false, "Use strict TLS configuration (disables certificate verification)")
+}
+
+func BaselineTLSConfig(skipVerify bool, secure bool) (*tls.Config, error) {
+	if secure && skipVerify {
+		return nil, errors.New("cannot skip verification and use secure TLS")
+	}
+
+	if skipVerify {
+		return &tls.Config{InsecureSkipVerify: true}, nil // #nosec G402 - intentional
+	}
+
+	if secure {
+		return StrictBaselineTLSConfig(), nil
+	}
+
+	return &tls.Config{}, nil // #nosec G402 - intentional
+}
+
+var debug = dbg.NewFromEnv()
+
 // DialerOpts controls creation of proxy-aware dialers.
 //
 // Timeout controls the maximum amount of time spent establishing the
@@ -94,24 +129,30 @@ func NewNetDialer(opts DialerOpts) (ContextDialer, error) {
 	}
 
 	if u := getProxyURLFromEnv("SOCKS5_PROXY"); u != nil {
+		debug.Printf("using SOCKS5 proxy %q\n", u)
 		return newSOCKS5Dialer(u, opts)
 	}
 
 	if u := getProxyURLFromEnv("HTTPS_PROXY"); u != nil {
+		// Respect the proxy URL scheme. Zscaler may set HTTPS_PROXY to an HTTP proxy
+		// running locally; in that case we must NOT TLS-wrap the proxy connection.
+		debug.Printf("using HTTPS proxy %q\n", u)
 		return &httpProxyDialer{
 			proxyURL: u,
 			timeout:  opts.Timeout,
-			secure:   true,
+			secure:   strings.EqualFold(u.Scheme, "https"),
 			config:   opts.TLSConfig,
 		}, nil
 	}
 
 	if u := getProxyURLFromEnv("HTTP_PROXY"); u != nil {
+		debug.Printf("using HTTP proxy %q\n", u)
 		return &httpProxyDialer{
 			proxyURL: u,
 			timeout:  opts.Timeout,
-			secure:   true,
+			// Only TLS-wrap the proxy connection if the URL scheme is https.
-			config:   opts.TLSConfig,
+			secure: strings.EqualFold(u.Scheme, "https"),
+			config: opts.TLSConfig,
 		}, nil
 	}
 
@@ -131,6 +172,7 @@ func NewTLSDialer(opts DialerOpts) (ContextDialer, error) {
 
 	// Prefer SOCKS5 if present.
 	if u := getProxyURLFromEnv("SOCKS5_PROXY"); u != nil {
+		debug.Printf("using SOCKS5 proxy %q\n", u)
 		base, err := newSOCKS5Dialer(u, opts)
 		if err != nil {
 			return nil, err
@@ -140,19 +182,22 @@ func NewTLSDialer(opts DialerOpts) (ContextDialer, error) {
 
 	// For TLS, prefer HTTPS proxy over HTTP if both set.
 	if u := getProxyURLFromEnv("HTTPS_PROXY"); u != nil {
+		debug.Printf("using HTTPS proxy %q\n", u)
 		base := &httpProxyDialer{
 			proxyURL: u,
 			timeout:  opts.Timeout,
-			secure:   true,
+			secure:   strings.EqualFold(u.Scheme, "https"),
 			config:   opts.TLSConfig,
 		}
 		return &tlsWrappingDialer{base: base, tcfg: opts.TLSConfig, timeout: opts.Timeout}, nil
 	}
+
 	if u := getProxyURLFromEnv("HTTP_PROXY"); u != nil {
+		debug.Printf("using HTTP proxy %q\n", u)
 		base := &httpProxyDialer{
 			proxyURL: u,
 			timeout:  opts.Timeout,
-			secure:   true,
+			secure:   strings.EqualFold(u.Scheme, "https"),
 			config:   opts.TLSConfig,
 		}
 		return &tlsWrappingDialer{base: base, tcfg: opts.TLSConfig, timeout: opts.Timeout}, nil
@@ -246,13 +291,8 @@ type httpProxyDialer struct {
 	config   *tls.Config
 }
 
-func (d *httpProxyDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+// proxyAddress returns host:port for the proxy, applying defaults by scheme when missing.
-	if !strings.HasPrefix(network, "tcp") {
+func (d *httpProxyDialer) proxyAddress() string {
-		return nil, fmt.Errorf("http proxy dialer only supports TCP, got %q", network)
-	}
-
-	// Dial to proxy
-	var nd = &net.Dialer{Timeout: d.timeout}
 	proxyAddr := d.proxyURL.Host
 	if !strings.Contains(proxyAddr, ":") {
 		if d.secure {
@@ -261,7 +301,61 @@ func (d *httpProxyDialer) DialContext(ctx context.Context, network, address stri
 			proxyAddr += ":80"
 		}
 	}
-	conn, err := nd.DialContext(ctx, "tcp", proxyAddr)
+	return proxyAddr
+}
+
+// tlsWrapProxyConn performs a TLS handshake to the proxy when d.secure is true.
+// It clones the provided tls.Config (if any), ensures ServerName and a safe
+// minimum TLS version.
+func (d *httpProxyDialer) tlsWrapProxyConn(ctx context.Context, conn net.Conn) (net.Conn, error) {
+	host := d.proxyURL.Hostname()
+	// Clone provided config (if any) to avoid mutating caller's config.
+	cfg := &tls.Config{} // #nosec G402 - intentional
+	if d.config != nil {
+		cfg = d.config.Clone()
+	}
+
+	if cfg.ServerName == "" {
+		cfg.ServerName = host
+	}
+
+	tlsConn := tls.Client(conn, cfg)
+	if err := tlsConn.HandshakeContext(ctx); err != nil {
+		_ = conn.Close()
+		return nil, fmt.Errorf("tls handshake with https proxy failed: %w", err)
+	}
+	return tlsConn, nil
+}
+
+// readConnectResponse reads and validates the proxy's response to a CONNECT
+// request. It returns nil on a 200 status and an error otherwise.
+func readConnectResponse(br *bufio.Reader) error {
+	statusLine, err := br.ReadString('\n')
+	if err != nil {
+		return fmt.Errorf("failed to read CONNECT response: %w", err)
+	}
+
+	if !strings.HasPrefix(statusLine, "HTTP/") {
+		return fmt.Errorf("invalid proxy response: %q", strings.TrimSpace(statusLine))
+	}
+
+	if !strings.Contains(statusLine, " 200 ") && !strings.HasSuffix(strings.TrimSpace(statusLine), " 200") {
+		// Drain headers for context
+		_ = drainHeaders(br)
+		return fmt.Errorf("proxy CONNECT failed: %s", strings.TrimSpace(statusLine))
+	}
+
+	return drainHeaders(br)
+}
+
+func (d *httpProxyDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	if !strings.HasPrefix(network, "tcp") {
+		return nil, fmt.Errorf("http proxy dialer only supports TCP, got %q", network)
+	}
+
+	// Dial to proxy
+	var nd = &net.Dialer{Timeout: d.timeout}
+	conn, err := nd.DialContext(ctx, "tcp", d.proxyAddress())
 	if err != nil {
 		return nil, err
 	}
@@ -273,14 +367,11 @@ func (d *httpProxyDialer) DialContext(ctx context.Context, network, address stri
 
 	// If HTTPS proxy, wrap with TLS to the proxy itself.
 	if d.secure {
-		host := d.proxyURL.Hostname()
+		c, werr := d.tlsWrapProxyConn(ctx, conn)
-		d.config.ServerName = host
+		if werr != nil {
-		tlsConn := tls.Client(conn, d.config)
+			return nil, werr
-		if err = tlsConn.HandshakeContext(ctx); err != nil {
-			_ = conn.Close()
-			return nil, fmt.Errorf("tls handshake with https proxy failed: %w", err)
 		}
-		conn = tlsConn
+		conn = c
 	}
 
 	req := buildConnectRequest(d.proxyURL, address)
@@ -291,25 +382,7 @@ func (d *httpProxyDialer) DialContext(ctx context.Context, network, address stri
 
 	// Read proxy response until end of headers
 	br := bufio.NewReader(conn)
-	statusLine, err := br.ReadString('\n')
+	if err = readConnectResponse(br); err != nil {
-	if err != nil {
-		_ = conn.Close()
-		return nil, fmt.Errorf("failed to read CONNECT response: %w", err)
-	}
-
-	if !strings.HasPrefix(statusLine, "HTTP/") {
-		_ = conn.Close()
-		return nil, fmt.Errorf("invalid proxy response: %q", strings.TrimSpace(statusLine))
-	}
-
-	if !strings.Contains(statusLine, " 200 ") && !strings.HasSuffix(strings.TrimSpace(statusLine), " 200") {
-		// Drain headers for context
-		_ = drainHeaders(br)
-		_ = conn.Close()
-		return nil, fmt.Errorf("proxy CONNECT failed: %s", strings.TrimSpace(statusLine))
-	}
-
-	if err = drainHeaders(br); err != nil {
 		_ = conn.Close()
 		return nil, err
 	}
@@ -429,7 +502,7 @@ func (t *tlsWrappingDialer) DialContext(ctx context.Context, network, address st
 		}
 		cfg = c
 	} else {
-		cfg = &tls.Config{ServerName: host} // #nosec G402 - intentional
+		cfg = &tls.Config{ServerName: host, MinVersion: tls.VersionTLS12}
 	}
 
 	tlsConn := tls.Client(raw, cfg)

lib/fetch.go

@@ -14,18 +14,8 @@ import (
 	"git.wntrmute.dev/kyle/goutils/fileutil"
 )
 
-// FetcherOpts are options for fetching certificates. They are only applicable to ServerFetcher.
+// Note: Previously this package exposed a FetcherOpts type. It has been
-type FetcherOpts struct {
+// refactored to use *tls.Config directly for configuring TLS behavior.
-	SkipVerify bool
-	Roots      *x509.CertPool
-}
-
-func (fo *FetcherOpts) TLSConfig() *tls.Config {
-	return &tls.Config{
-		InsecureSkipVerify: fo.SkipVerify, // #nosec G402 - intentional
-		RootCAs:            fo.Roots,
-	}
-}
 
 // Fetcher is an interface for fetching certificates from a remote source. It
 // currently supports fetching from a server or a file.
@@ -143,7 +133,10 @@ func (ff *FileFetcher) Get() (*x509.Certificate, error) {
 }
 
 // GetCertificateChain fetches a certificate chain from a remote source.
-func GetCertificateChain(spec string, opts *FetcherOpts) ([]*x509.Certificate, error) {
+// If cfg is non-nil and spec refers to a TLS server, the provided TLS
+// configuration will be used to control verification behavior (e.g.,
+// InsecureSkipVerify, RootCAs).
+func GetCertificateChain(spec string, cfg *tls.Config) ([]*x509.Certificate, error) {
 	if fileutil.FileDoesExist(spec) {
 		return NewFileFetcher(spec).GetChain()
 	}
@@ -153,17 +146,17 @@ func GetCertificateChain(spec string, opts *FetcherOpts) ([]*x509.Certificate, e
 		return nil, err
 	}
 
-	if opts != nil {
+	if cfg != nil {
-		fetcher.insecure = opts.SkipVerify
+		fetcher.insecure = cfg.InsecureSkipVerify
-		fetcher.roots = opts.Roots
+		fetcher.roots = cfg.RootCAs
 	}
 
 	return fetcher.GetChain()
 }
 
 // GetCertificate fetches the first certificate from a certificate chain.
-func GetCertificate(spec string, opts *FetcherOpts) (*x509.Certificate, error) {
+func GetCertificate(spec string, cfg *tls.Config) (*x509.Certificate, error) {
-	certs, err := GetCertificateChain(spec, opts)
+	certs, err := GetCertificateChain(spec, cfg)
 	if err != nil {
 		return nil, err
 	}

release-docker.sh

@@ -45,7 +45,7 @@ fi
 
 # Use the first tag if multiple are present; warn the user.
 # Avoid readarray for broader Bash compatibility (e.g., macOS Bash 3.2).
-TAG_ARRAY=($TAGS)
+TAG_ARRAY=("$TAGS")
 TAG="${TAG_ARRAY[0]}"
 
 if (( ${#TAG_ARRAY[@]} > 1 )); then