Groundwork for monitoring and updating the dns

dns/dns_proxy.go

@@ -741,6 +741,16 @@ func (p *DNSProxy) SetJITHandler(handler func(siteId int)) {
 	p.jitHandler = handler
 }
 
+// SetUpstreamDNS replaces the list of upstream DNS servers used to forward
+// queries that are not served by local records.  The servers must be in
+// "host:port" format (e.g. "8.8.8.8:53").
+func (p *DNSProxy) SetUpstreamDNS(servers []string) {
+	if len(servers) == 0 {
+		return
+	}
+	p.upstreamDNS = servers
+}
+
 // AddDNSRecord adds a DNS record to the local store
 // domain should be a domain name (e.g., "example.com" or "example.com.")
 // ip should be a valid IPv4 or IPv6 address

dns/sysresolver.go

@@ -0,0 +1,177 @@
+package dns
+
+import (
+	"context"
+	"net"
+	"net/netip"
+	"sort"
+	"sync"
+	"time"
+
+	"github.com/fosrl/newt/logger"
+)
+
+const defaultPollInterval = 30 * time.Second
+
+// SystemDNSMonitor monitors the host system's DNS configuration and notifies
+// callers when it changes. The reported servers are in "host:port" format
+// (e.g. "8.8.8.8:53") and can be used directly as UpstreamDNS and PublicDNS.
+//
+// Platform behaviour:
+//   - Linux: reads /run/systemd/resolve/resolv.conf when present (updated by
+//     systemd-resolved on every DHCP change), then falls back to
+//     /etc/resolv.conf.olm.backup (written before olm overrides DNS), and
+//     finally /etc/resolv.conf.
+//   - macOS: reads /etc/resolv.conf, which is never modified by olm's
+//     supplemental scutil DNS override.
+//   - Windows: enumerates DHCP-assigned DNS servers from every network adapter
+//     in the registry.
+//   - Other platforms: returns an empty list (no-op monitor).
+type SystemDNSMonitor struct {
+	mu         sync.RWMutex
+	current    []string
+	onChange   func(servers []string)
+	interval   time.Duration
+	stopCh     chan struct{}
+	excludeMu  sync.RWMutex
+	excludeIPs map[netip.Addr]bool
+}
+
+// NewSystemDNSMonitor creates a new monitor. onChange is called with the new
+// server list whenever a change is detected; it is also called once from Start
+// with the initial values. A zero interval uses the 30-second default.
+func NewSystemDNSMonitor(interval time.Duration, onChange func(servers []string)) *SystemDNSMonitor {
+	if interval <= 0 {
+		interval = defaultPollInterval
+	}
+	return &SystemDNSMonitor{
+		interval:   interval,
+		onChange:   onChange,
+		stopCh:     make(chan struct{}),
+		excludeIPs: make(map[netip.Addr]bool),
+	}
+}
+
+// SetExcludeIP registers an IP address that must never appear in the reported
+// DNS server list. Call this after olm's DNS proxy is created to prevent the
+// proxy's own IP from being returned as an upstream server when the OS DNS has
+// been overridden to point at the proxy.
+func (m *SystemDNSMonitor) SetExcludeIP(ip netip.Addr) {
+	m.excludeMu.Lock()
+	m.excludeIPs[ip.Unmap()] = true
+	m.excludeMu.Unlock()
+}
+
+// Start reads the current system DNS immediately, fires onChange, then polls
+// in the background until Stop is called or ctx is cancelled.
+func (m *SystemDNSMonitor) Start(ctx context.Context) {
+	servers := m.readFiltered()
+	m.mu.Lock()
+	m.current = servers
+	m.mu.Unlock()
+
+	if m.onChange != nil && len(servers) > 0 {
+		m.onChange(servers)
+	}
+
+	go m.run(ctx)
+}
+
+// Stop halts the background polling goroutine.
+func (m *SystemDNSMonitor) Stop() {
+	select {
+	case <-m.stopCh:
+	default:
+		close(m.stopCh)
+	}
+}
+
+// Current returns the most recently observed system DNS servers.
+func (m *SystemDNSMonitor) Current() []string {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	out := make([]string, len(m.current))
+	copy(out, m.current)
+	return out
+}
+
+// readFiltered calls the platform-specific readSystemDNS and removes any
+// addresses that have been excluded via SetExcludeIP. If all addresses are
+// excluded the function returns nil so the caller can retain the last
+// known-good value.
+func (m *SystemDNSMonitor) readFiltered() []string {
+	raw := readSystemDNS()
+
+	m.excludeMu.RLock()
+	excludeIPs := m.excludeIPs
+	m.excludeMu.RUnlock()
+
+	if len(excludeIPs) == 0 {
+		return raw
+	}
+
+	var filtered []string
+	for _, s := range raw {
+		host, _, err := net.SplitHostPort(s)
+		if err != nil {
+			filtered = append(filtered, s)
+			continue
+		}
+		addr, err := netip.ParseAddr(host)
+		if err != nil || excludeIPs[addr.Unmap()] {
+			continue
+		}
+		filtered = append(filtered, s)
+	}
+	return filtered
+}
+
+func (m *SystemDNSMonitor) run(ctx context.Context) {
+	ticker := time.NewTicker(m.interval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-m.stopCh:
+			return
+		case <-ticker.C:
+			servers := m.readFiltered()
+			if len(servers) == 0 {
+				continue
+			}
+
+			m.mu.Lock()
+			changed := !dnsSlicesEqual(m.current, servers)
+			if changed {
+				m.current = servers
+			}
+			m.mu.Unlock()
+
+			if changed && m.onChange != nil {
+				logger.Info("System DNS changed: %v", servers)
+				m.onChange(servers)
+			}
+		}
+	}
+}
+
+// dnsSlicesEqual reports whether two server lists are equal regardless of order.
+func dnsSlicesEqual(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	ac := make([]string, len(a))
+	bc := make([]string, len(b))
+	copy(ac, a)
+	copy(bc, b)
+	sort.Strings(ac)
+	sort.Strings(bc)
+	for i := range ac {
+		if ac[i] != bc[i] {
+			return false
+		}
+	}
+	return true
+}

dns/sysresolver_darwin.go

@@ -0,0 +1,58 @@
+//go:build darwin && !ios
+
+package dns
+
+import (
+	"bufio"
+	"net"
+	"net/netip"
+	"os"
+	"strings"
+)
+
+// readSystemDNS returns the current system DNS servers in "host:53" format.
+//
+// On macOS, olm adds supplemental DNS entries via scutil without modifying
+// /etc/resolv.conf or the primary network service DNS.  Reading /etc/resolv.conf
+// therefore always yields the physical-network DNS supplied by DHCP or the user.
+// /etc/resolv.conf on macOS is a regular file managed by mDNSResponder and is
+// updated whenever the network configuration changes.
+func readSystemDNS() []string {
+	return parseMacResolvConf("/etc/resolv.conf")
+}
+
+func parseMacResolvConf(path string) []string {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil
+	}
+	defer f.Close()
+
+	var result []string
+	seen := make(map[string]bool)
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if !strings.HasPrefix(line, "nameserver") {
+			continue
+		}
+		fields := strings.Fields(line)
+		if len(fields) < 2 {
+			continue
+		}
+		addr, err := netip.ParseAddr(fields[1])
+		if err != nil {
+			continue
+		}
+		if addr.IsLoopback() || addr.IsLinkLocalUnicast() {
+			continue
+		}
+		s := net.JoinHostPort(addr.String(), "53")
+		if !seen[s] {
+			seen[s] = true
+			result = append(result, s)
+		}
+	}
+	return result
+}

dns/sysresolver_linux.go

@@ -0,0 +1,84 @@
+//go:build linux && !android
+
+package dns
+
+import (
+	"bufio"
+	"net"
+	"net/netip"
+	"os"
+	"strings"
+)
+
+// readSystemDNS returns the current system DNS servers in "host:53" format.
+//
+// Resolution order:
+//  1. /run/systemd/resolve/resolv.conf — maintained by systemd-resolved with
+//     the real per-link DNS servers; updated on every DHCP change and never
+//     touched by olm's D-Bus DNS override.
+//  2. /etc/resolv.conf.olm.backup — written by olm before it overrides
+//     /etc/resolv.conf on non-systemd systems.
+//  3. /etc/resolv.conf — plain fallback.
+//
+// Loopback and link-local addresses (e.g. 127.0.0.53, ::1) are excluded
+// because they are stub resolver addresses, not real upstream servers.
+func readSystemDNS() []string {
+	// Prefer systemd-resolved's resolved (non-stub) resolv.conf.
+	if servers := parseResolvConf("/run/systemd/resolve/resolv.conf"); len(servers) > 0 {
+		return servers
+	}
+
+	// If olm has already overridden /etc/resolv.conf the backup holds the
+	// original pre-override DNS servers.
+	if _, err := os.Stat("/etc/resolv.conf.olm.backup"); err == nil {
+		if servers := parseResolvConf("/etc/resolv.conf.olm.backup"); len(servers) > 0 {
+			return servers
+		}
+	}
+
+	return parseResolvConf("/etc/resolv.conf")
+}
+
+// parseResolvConf reads nameserver lines from a resolv.conf-style file,
+// skipping loopback and link-local addresses.
+func parseResolvConf(path string) []string {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil
+	}
+	defer f.Close()
+
+	var result []string
+	seen := make(map[string]bool)
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if !strings.HasPrefix(line, "nameserver") {
+			continue
+		}
+		fields := strings.Fields(line)
+		if len(fields) < 2 {
+			continue
+		}
+		addr, err := netip.ParseAddr(fields[1])
+		if err != nil {
+			continue
+		}
+		if addr.IsLoopback() || addr.IsLinkLocalUnicast() {
+			continue
+		}
+		s := addrToHostPort(addr)
+		if !seen[s] {
+			seen[s] = true
+			result = append(result, s)
+		}
+	}
+	return result
+}
+
+// addrToHostPort converts a netip.Addr to "addr:53" format, wrapping IPv6
+// addresses in brackets as required by net.JoinHostPort.
+func addrToHostPort(addr netip.Addr) string {
+	return net.JoinHostPort(addr.String(), "53")
+}

dns/sysresolver_stub.go

@@ -0,0 +1,10 @@
+//go:build !linux && !darwin && !windows
+
+package dns
+
+// readSystemDNS returns nil on platforms where automatic DNS discovery is not
+// implemented (android, ios, freebsd, etc.).  Callers should fall back to a
+// statically configured DNS server.
+func readSystemDNS() []string {
+	return nil
+}

dns/sysresolver_windows.go

@@ -0,0 +1,103 @@
+//go:build windows
+
+package dns
+
+import (
+	"fmt"
+	"net"
+	"net/netip"
+
+	"golang.org/x/sys/windows/registry"
+)
+
+const (
+	tcpipInterfacesPath = `SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces`
+	dhcpNameServerKey   = "DhcpNameServer"
+	staticNameServerKey = "NameServer"
+)
+
+// readSystemDNS returns the current system DNS servers in "host:53" format by
+// enumerating every network adapter in the Windows registry.
+//
+// For each adapter olm reads the DHCP-assigned DNS servers (DhcpNameServer).
+// Static DNS (NameServer) is ignored on the assumption that it belongs to the
+// olm WireGuard adapter or another VPN; DHCP-assigned servers always reflect
+// the physical network's DNS.  Loopback and link-local addresses are excluded.
+func readSystemDNS() []string {
+	key, err := registry.OpenKey(registry.LOCAL_MACHINE, tcpipInterfacesPath, registry.ENUMERATE_SUB_KEYS)
+	if err != nil {
+		return nil
+	}
+	defer key.Close()
+
+	subkeys, err := key.ReadSubKeyNames(-1)
+	if err != nil {
+		return nil
+	}
+
+	seen := make(map[string]bool)
+	var result []string
+
+	for _, guid := range subkeys {
+		path := fmt.Sprintf(`%s\%s`, tcpipInterfacesPath, guid)
+		iKey, err := registry.OpenKey(registry.LOCAL_MACHINE, path, registry.QUERY_VALUE)
+		if err != nil {
+			continue
+		}
+
+		dhcp, _, err := iKey.GetStringValue(dhcpNameServerKey)
+		iKey.Close()
+		if err != nil || dhcp == "" {
+			continue
+		}
+
+		for _, s := range splitWinDNSList(dhcp) {
+			addr, err := netip.ParseAddr(s)
+			if err != nil {
+				continue
+			}
+			if addr.IsLoopback() || addr.IsLinkLocalUnicast() {
+				continue
+			}
+			hp := net.JoinHostPort(addr.String(), "53")
+			if !seen[hp] {
+				seen[hp] = true
+				result = append(result, hp)
+			}
+		}
+	}
+
+	return result
+}
+
+// splitWinDNSList splits a Windows DNS server list that may be comma- or
+// space-separated.
+func splitWinDNSList(s string) []string {
+	var out []string
+	for _, part := range splitByRunes(s, []rune{',', ' '}) {
+		if part != "" {
+			out = append(out, part)
+		}
+	}
+	return out
+}
+
+func splitByRunes(s string, delims []rune) []string {
+	var result []string
+	start := 0
+	for i, r := range s {
+		for _, d := range delims {
+			if r == d {
+				if i > start {
+					result = append(result, s[start:i])
+				}
+				start = i + len(string(r))
+				break
+			}
+		}
+	}
+	if start < len(s) {
+		result = append(result, s[start:])
+	}
+	return result
+}