fix(net): use pure-Go DNS fallback to prevent thread exhaustion

When no custom resolver or host mapper is configured, xnet.Resolve now
resolves hostnames via Go's native pure-Go resolver (PreferGo: true)
instead of returning the hostname unchanged.

Previously, the unresolved hostname would reach net.Dialer.DialContext,
which on CGO_ENABLED=1 builds used cgo-based DNS — blocking a dedicated
OS thread per concurrent lookup. At scale (1000+ listeners), this
exhausted Go's 10000-thread limit.

Fixes go-gost/gost#550

internal/net/resovle.go

@@ -13,7 +13,8 @@ import (
 
 // Resolve resolves addr to a concrete IP address. If hosts is non-nil, it is
 // consulted first. If r is non-nil and no host mapping matched, the resolver
-// is used. If neither is available, addr is returned unchanged.
+// is used. If neither is available, Go's native pure-Go resolver is used as a
+// fallback to avoid cgo-based DNS lookups that would block OS threads.
 func Resolve(ctx context.Context, network, addr string, r resolver.Resolver, hosts hosts.HostMapper, log logger.Logger) (string, error) {
 	if addr == "" {
 		return addr, nil
@@ -51,5 +52,23 @@ func Resolve(ctx context.Context, network, addr string, r resolver.Resolver, hos
 		}
 		return net.JoinHostPort(ips[0].String(), port), nil
 	}
-	return addr, nil
+
+	// A literal IP address needs no resolution.
+	if net.ParseIP(host) != nil {
+		return addr, nil
+	}
+
+	// Fallback to Go's native (pure-Go) resolver when no custom resolver or
+	// host mapper is configured. This avoids cgo-based DNS lookups in
+	// downstream net.Dialer.DialContext which would block OS threads and
+	// cause thread exhaustion under high connection concurrency.
+	lr := &net.Resolver{PreferGo: true}
+	ips, err := lr.LookupIP(ctx, "ip", host)
+	if err != nil {
+		return "", err
+	}
+	if len(ips) == 0 {
+		return "", fmt.Errorf("resolver: domain %s does not exist", host)
+	}
+	return net.JoinHostPort(ips[0].String(), port), nil
 }

internal/net/resovle_test.go

@@ -115,13 +115,18 @@ func TestResolve_ResolverEmptyResult(t *testing.T) {
 }
 
 func TestResolve_NoResolverNoHosts(t *testing.T) {
+	// When no resolver and no host mapper are configured, the function now
+	// resolves via Go's native DNS resolver (PreferGo: true). This test
+	// uses an IP address which is short-circuited at the top of Resolve,
+	// avoiding the need for actual DNS resolution in unit tests.
+	// DNS fallback behavior is verified by the e2e tests.
 	ctx := context.Background()
-	addr, err := Resolve(ctx, "tcp", "example.com:8080", nil, nil, nil)
+	addr, err := Resolve(ctx, "tcp", "1.2.3.4:8080", nil, nil, nil)
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
-	if addr != "example.com:8080" {
-		t.Errorf("expected example.com:8080, got %s", addr)
+	if addr != "1.2.3.4:8080" {
+		t.Errorf("expected 1.2.3.4:8080, got %s", addr)
 	}
 }