default control_socket to /run/dnscrypt/control.sock, so configs need no key for it

dnscrypt-proxy/config.go

@@ -117,6 +117,7 @@ func newConfig() Config {
 		LogLevel:        int(dlog.LogLevel()),
 		LogFileLatest:   true,
 		ListenAddresses: []string{"127.0.0.1:53"},
+		ControlSocket:   DefaultControlSocket,
 		LocalDoH:        LocalDoHConfig{Path: "/dns-query"},
 		MonitoringUI: MonitoringUIConfig{
 			Enabled:        false,

dnscrypt-proxy/control.go

@@ -34,6 +34,12 @@ import (
 // Replies are "ok <removed entries>" or "error <reason>". There is no
 // authentication: the channel is only ever a unix socket (control_socket),
 // readable and writable by its owner only.
+//
+// control_socket defaults to DefaultControlSocket, so a config needs no key for
+// it and stays loadable by builds without the control channel; set it to '' to
+// turn the channel off. A missing default directory only disables the channel.
+
+const DefaultControlSocket = "/run/dnscrypt/control.sock"
 
 const (
 	controlReadTimeout = 2 * time.Second
@@ -147,7 +153,7 @@ func readControlLine(r io.Reader) (string, error) {
 // cache, prints the reply and returns the exit code.
 func Flush(path string, arg string) int {
 	if path == "" {
-		fmt.Fprintln(os.Stderr, "control_socket is not set in the configuration file")
+		fmt.Fprintln(os.Stderr, "the control channel is turned off (control_socket = '')")
 		return 1
 	}
 	command := "flush " + arg

dnscrypt-proxy/control_test.go

@@ -2,8 +2,10 @@ package main
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"net"
 	"net/netip"
 	"os"
@@ -13,6 +15,7 @@ import (
 
 	"codeberg.org/miekg/dns"
 	"codeberg.org/miekg/dns/rdata"
+	"github.com/BurntSushi/toml"
 )
 
 func resetControlState(t *testing.T) {
@@ -421,3 +424,22 @@ func TestControlFlushClient(t *testing.T) {
 		t.Fatalf("an unreachable proxy must exit 1, got %d", code)
 	}
 }
+
+func TestControlSocketDefault(t *testing.T) {
+	cfg := newConfig()
+	if cfg.ControlSocket != DefaultControlSocket {
+		t.Fatalf("default control_socket %q, want %q", cfg.ControlSocket, DefaultControlSocket)
+	}
+	// a config without the key keeps the default, an empty value turns it off
+	if _, err := toml.Decode("cache = true\n", &cfg); err != nil || cfg.ControlSocket != DefaultControlSocket {
+		t.Fatalf("without the key: %q, %v", cfg.ControlSocket, err)
+	}
+	if _, err := toml.Decode("control_socket = ''\n", &cfg); err != nil || cfg.ControlSocket != "" {
+		t.Fatalf("control_socket = '': %q, %v", cfg.ControlSocket, err)
+	}
+	// a missing directory is reported as such, so the default can be skipped quietly
+	err := (&Proxy{}).startControl(t.TempDir() + "/missing/c.sock")
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("missing directory: got %v", err)
+	}
+}

dnscrypt-proxy/example-dnscrypt-proxy.toml

@@ -517,6 +517,14 @@ cache_neg_min_ttl = 60
 
 cache_neg_max_ttl = 600
 
+## Unix socket for dropping names from the cache: one command per connection,
+## "flush <name>" or "flush_type <name> <type>". Also available from the
+## command line as `dnscrypt-proxy -config <file> -flush <name>[,<type>]`.
+## Readable and writable by the proxy's user only. If that directory does not
+## exist the channel stays off; set to '' to turn it off explicitly.
+
+# control_socket = '/run/dnscrypt/control.sock'
+
 
 ###############################################################################
 #                           Captive portal handling                            #

dnscrypt-proxy/proxy.go

@@ -3,8 +3,11 @@ package main
 import (
 	"context"
 	"encoding/binary"
+	"errors"
+	"io/fs"
 	"net"
 	"os"
+	"path/filepath"
 	"runtime"
 	"strings"
 	"sync"
@@ -286,7 +289,11 @@ func (proxy *Proxy) StartProxy() {
 
 	if proxy.controlSocket != "" {
 		if err := proxy.startControl(proxy.controlSocket); err != nil {
-			dlog.Errorf("Failed to start the control channel on %s: %v", proxy.controlSocket, err)
+			if proxy.controlSocket == DefaultControlSocket && errors.Is(err, fs.ErrNotExist) {
+				dlog.Noticef("Control channel off: %s does not exist", filepath.Dir(DefaultControlSocket))
+			} else {
+				dlog.Errorf("Failed to start the control channel on %s: %v", proxy.controlSocket, err)
+			}
 		}
 	}