Resolve container-local upstream endpoints

internal/acp/execution.go

@@ -8,11 +8,13 @@ import (
 	"io"
 	"mime"
 	"net/http"
+	"net"
 	"net/url"
 	"os"
 	"path/filepath"
 	"sort"
 	"strings"
+	"strconv"
 	"time"
 
 	"github.com/gorilla/websocket"
@@ -201,7 +203,66 @@ func (s *Server) probeExternalProvider(
 }
 
 func resolveSingleAgentForwardEndpoint(provider syncedProvider) string {
-	return strings.TrimSpace(provider.Endpoint)
+	return resolveContainerReachableEndpoint(strings.TrimSpace(provider.Endpoint))
+}
+
+func resolveContainerReachableEndpoint(endpoint string) string {
+	trimmed := strings.TrimSpace(endpoint)
+	if trimmed == "" {
+		return ""
+	}
+	parsed, err := url.Parse(trimmed)
+	if err != nil {
+		return trimmed
+	}
+	host := strings.TrimSpace(parsed.Hostname())
+	if host != "127.0.0.1" && host != "localhost" {
+		return trimmed
+	}
+	gatewayHost := containerDefaultGatewayAddress()
+	if gatewayHost == "" {
+		return trimmed
+	}
+	parsed.Host = net.JoinHostPort(gatewayHost, parsed.Port())
+	return parsed.String()
+}
+
+func containerDefaultGatewayAddress() string {
+	data, err := os.ReadFile("/proc/net/route")
+	if err != nil {
+		return ""
+	}
+	lines := strings.Split(string(data), "\n")
+	for _, line := range lines[1:] {
+		fields := strings.Fields(line)
+		if len(fields) < 3 {
+			continue
+		}
+		if fields[1] != "00000000" {
+			continue
+		}
+		gatewayHex := fields[2]
+		if len(gatewayHex) != 8 {
+			continue
+		}
+		b0, err0 := parseHexByte(gatewayHex[6:8])
+		b1, err1 := parseHexByte(gatewayHex[4:6])
+		b2, err2 := parseHexByte(gatewayHex[2:4])
+		b3, err3 := parseHexByte(gatewayHex[0:2])
+		if err0 != nil || err1 != nil || err2 != nil || err3 != nil {
+			continue
+		}
+		return fmt.Sprintf("%d.%d.%d.%d", b0, b1, b2, b3)
+	}
+	return ""
+}
+
+func parseHexByte(raw string) (int, error) {
+	value, err := strconv.ParseUint(raw, 16, 8)
+	if err != nil {
+		return 0, err
+	}
+	return int(value), nil
 }
 
 func sanitizeExternalACPParams(method string, params map[string]any) map[string]any {