feat(client): sign presigned URLs against a public endpoint

go.mod

@@ -2,6 +2,8 @@ module github.com/txn2/mcp-s3
 
 go 1.25.0
 
+toolchain go1.26.4
+
 require (
 	github.com/aws/aws-sdk-go-v2 v1.41.9
 	github.com/aws/aws-sdk-go-v2/config v1.32.20

pkg/client/client.go

@@ -185,8 +185,19 @@ func New(ctx context.Context, cfg *Config) (*Client, error) {
 	// Create S3 client
 	s3Client := s3.NewFromConfig(awsCfg, s3Opts...)
 
-	// Create presign client
-	presignClient := s3.NewPresignClient(s3Client)
+	// Create presign client. When a dedicated presign endpoint is configured,
+	// sign URLs against a client pointed at that public-facing endpoint so the
+	// URLs are reachable outside the cluster; data operations keep using
+	// s3Client (the internal Endpoint). With no presign endpoint the two are the
+	// same client, preserving prior behavior.
+	presignSource := s3Client
+	if cfg.PresignEndpoint != "" {
+		presignSource = s3.NewFromConfig(awsCfg, func(o *s3.Options) {
+			o.BaseEndpoint = aws.String(cfg.PresignEndpoint)
+			o.UsePathStyle = cfg.UsePathStyle
+		})
+	}
+	presignClient := s3.NewPresignClient(presignSource)
 
 	// Create the streaming/multipart uploader. It shares the same underlying
 	// S3 client so it honors the configured endpoint, credentials, and region.

pkg/client/client_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"io"
+	neturl "net/url"
 	"os"
 	"strings"
 	"testing"
@@ -720,6 +721,59 @@ func TestClient_PresignGetURL(t *testing.T) {
 	})
 }
 
+// TestNew_PresignEndpoint verifies that a configured PresignEndpoint signs
+// presigned URLs against the public endpoint while data operations keep using
+// Endpoint, and that an empty PresignEndpoint falls back to Endpoint. Presigning
+// is a local operation (no network), so a real client built via New is used.
+func TestNew_PresignEndpoint(t *testing.T) {
+	ctx := context.Background()
+
+	cfgWith := func(presign string) *Config {
+		return &Config{
+			Region:          "us-east-1",
+			Endpoint:        "http://internal:8333",
+			PresignEndpoint: presign,
+			AccessKeyID:     "test",
+			SecretAccessKey: "secret",
+			UsePathStyle:    true,
+		}
+	}
+	presignHost := func(t *testing.T, c *Client) (scheme, host string) {
+		t.Helper()
+		got, err := c.PresignGetURL(ctx, "my-bucket", "file.txt", time.Hour)
+		if err != nil {
+			t.Fatalf("PresignGetURL: %v", err)
+		}
+		u, err := neturl.Parse(got.URL)
+		if err != nil {
+			t.Fatalf("parse %q: %v", got.URL, err)
+		}
+		return u.Scheme, u.Host
+	}
+
+	t.Run("signs against the public presign endpoint", func(t *testing.T) {
+		c, err := New(ctx, cfgWith("https://s3.public.example.com"))
+		if err != nil {
+			t.Fatalf("New: %v", err)
+		}
+		scheme, host := presignHost(t, c)
+		if scheme != "https" || host != "s3.public.example.com" {
+			t.Errorf("presigned URL = %s://%s, want https://s3.public.example.com", scheme, host)
+		}
+	})
+
+	t.Run("empty presign endpoint falls back to the data endpoint", func(t *testing.T) {
+		c, err := New(ctx, cfgWith(""))
+		if err != nil {
+			t.Fatalf("New: %v", err)
+		}
+		_, host := presignHost(t, c)
+		if host != "internal:8333" {
+			t.Errorf("presigned URL host = %s, want internal:8333", host)
+		}
+	})
+}
+
 func TestClient_PresignPutURL(t *testing.T) {
 	t.Run("success", func(t *testing.T) {
 		presignMock := &mockPresignAPI{

pkg/client/config.go

@@ -22,6 +22,13 @@ type Config struct {
 	// Endpoint is an optional custom endpoint URL for S3-compatible services (SeaweedFS, LocalStack, etc.).
 	Endpoint string
 
+	// PresignEndpoint is an optional public-facing endpoint used only when
+	// signing presigned URLs. When set, presigned URLs are signed against it so
+	// they are reachable from outside the cluster, while data operations keep
+	// using Endpoint (typically an internal/cluster address). Empty falls back
+	// to Endpoint, preserving prior behavior.
+	PresignEndpoint string
+
 	// AccessKeyID is the AWS access key ID. If empty, the SDK credential chain is used.
 	AccessKeyID string
 
@@ -57,6 +64,7 @@ type Config struct {
 //   - AWS_SESSION_TOKEN: Session token (optional)
 //   - AWS_PROFILE: Profile name (optional)
 //   - S3_ENDPOINT: Custom endpoint URL (optional)
+//   - S3_PRESIGN_ENDPOINT: Public endpoint for presigned URLs (optional)
 //   - S3_USE_PATH_STYLE: Use path-style URLs (default: false)
 //   - S3_TIMEOUT: Operation timeout (default: 30s)
 //   - S3_CONNECTION_NAME: Connection name (optional)
@@ -65,6 +73,7 @@ func FromEnv() Config {
 	cfg := Config{
 		Region:          getEnvOrDefault("AWS_REGION", DefaultRegion),
 		Endpoint:        getEnvSanitized("S3_ENDPOINT"),
+		PresignEndpoint: getEnvSanitized("S3_PRESIGN_ENDPOINT"),
 		AccessKeyID:     getEnvSanitized("AWS_ACCESS_KEY_ID"),
 		SecretAccessKey: getEnvSanitized("AWS_SECRET_ACCESS_KEY"),
 		SessionToken:    getEnvSanitized("AWS_SESSION_TOKEN"),
@@ -107,6 +116,7 @@ func (c *Config) Clone() *Config {
 	return &Config{
 		Region:          c.Region,
 		Endpoint:        c.Endpoint,
+		PresignEndpoint: c.PresignEndpoint,
 		AccessKeyID:     c.AccessKeyID,
 		SecretAccessKey: c.SecretAccessKey,
 		SessionToken:    c.SessionToken,