internal/clientinfo: read mdns data from avahi-daemon cache

When avahi-daemon is avaibale, reading data from its cache help ctrld
populate the mdns data with already known services within local network,
allowing discover client info more quickly.

internal/clientinfo/mdns.go

@@ -1,11 +1,16 @@
 package clientinfo
 
 import (
+	"bufio"
+	"bytes"
 	"context"
 	"errors"
+	"io"
 	"net"
 	"net/netip"
 	"os"
+	"os/exec"
+	"strings"
 	"sync"
 	"syscall"
 	"time"
@@ -107,6 +112,7 @@ func (m *mdns) init(quitCh chan struct{}) error {
 
 	go m.probeLoop(v4ConnList, mdnsV4Addr, quitCh)
 	go m.probeLoop(v6ConnList, mdnsV6Addr, quitCh)
+	go m.getDataFromAvahiDaemonCache()
 
 	return nil
 }
@@ -212,6 +218,44 @@ func (m *mdns) probe(conns []*net.UDPConn, remoteAddr net.Addr) error {
 	return err
 }
 
+// getDataFromAvahiDaemonCache reads entries from avahi-daemon cache to update mdns data.
+func (m *mdns) getDataFromAvahiDaemonCache() {
+	if _, err := exec.LookPath("avahi-browse"); err != nil {
+		ctrld.ProxyLogger.Load().Debug().Err(err).Msg("could not find avahi-browse binary, skipping.")
+		return
+	}
+	// Run avahi-browse to discover services from cache:
+	//  - "-a" -> all services.
+	//  - "-r" -> resolve found services.
+	//  - "-p" -> parseable format.
+	//  - "-c" -> read from cache.
+	out, err := exec.Command("avahi-browse", "-a", "-r", "-p", "-c").Output()
+	if err != nil {
+		ctrld.ProxyLogger.Load().Debug().Err(err).Msg("could not browse services from avahi cache")
+		return
+	}
+	m.storeDataFromAvahiBrowseOutput(bytes.NewReader(out))
+}
+
+// storeDataFromAvahiBrowseOutput parses avahi-browse output from reader, then updating found data to mdns table.
+func (m *mdns) storeDataFromAvahiBrowseOutput(r io.Reader) {
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		fields := strings.FieldsFunc(scanner.Text(), func(r rune) bool {
+			return r == ';'
+		})
+		if len(fields) < 8 || fields[0] != "=" {
+			continue
+		}
+		ip := fields[7]
+		name := normalizeHostname(fields[6])
+		// Only using cache value if we don't have existed one.
+		if _, loaded := m.name.LoadOrStore(ip, name); !loaded {
+			ctrld.ProxyLogger.Load().Debug().Msgf("found hostname: %q, ip: %q via avahi cache", name, ip)
+		}
+	}
+}
+
 func multicastInterfaces() ([]net.Interface, error) {
 	ifaces, err := net.Interfaces()
 	if err != nil {

internal/clientinfo/mdns_test.go

@@ -0,0 +1,27 @@
+package clientinfo
+
+import (
+	"strings"
+	"testing"
+)
+
+func Test_mdns_storeDataFromAvahiBrowseOutput(t *testing.T) {
+	const content = `+;wlp0s20f3;IPv6;Foo\032\0402\041;_companion-link._tcp;local
++;wlp0s20f3;IPv4;Foo\032\0402\041;_companion-link._tcp;local
+=;wlp0s20f3;IPv6;Foo\032\0402\041;_companion-link._tcp;local;Foo-2.local;192.168.1.123;64842;"rpBA=00:00:00:00:00:01" "rpHI=e6ae2cbbca0e" "rpAD=36566f4d850f" "rpVr=510.71.1" "rpHA=0ddc20fdddc8" "rpFl=0x30000" "rpHN=1d4a03afdefa" "rpMac=0"
+=;wlp0s20f3;IPv4;Foo\032\0402\041;_companion-link._tcp;local;Foo-2.local;192.168.1.123;64842;"rpBA=00:00:00:00:00:01" "rpHI=e6ae2cbbca0e" "rpAD=36566f4d850f" "rpVr=510.71.1" "rpHA=0ddc20fdddc8" "rpFl=0x30000" "rpHN=1d4a03afdefa" "rpMac=0"
+`
+	m := &mdns{}
+	m.storeDataFromAvahiBrowseOutput(strings.NewReader(content))
+	ip := "192.168.1.123"
+	val, loaded := m.name.LoadOrStore(ip, "")
+	if !loaded {
+		t.Fatal("missing Foo-2 data from mdns table")
+	}
+
+	wantHostname := "Foo-2"
+	hostname := val.(string)
+	if hostname != wantHostname {
+		t.Fatalf("unexpected hostname, want: %q, got: %q", wantHostname, hostname)
+	}
+}