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ipn/ipnlocal,net/dns/resolver: use the user dialer and routes for DNS forwarding by default, except on iOS and Android

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In this PR, we make the "user-dial-routes" behavior default on all platforms except for iOS and Android.
It can be disabled by setting the TS_DNS_FORWARD_USE_ROUTES envknob to 0 or false.

Updates #12027
Updates #13837

Signed-off-by: Nick Khyl <nickk@tailscale.com>
This commit is contained in:
Nick Khyl
2025-05-09 10:25:28 -05:00
committed by Nick Khyl
parent 3177e50b14
commit a9be049c19
5 changed files with 36 additions and 16 deletions
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42
net/dns/resolver/forwarder.go
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@@ -17,6 +17,7 @@ import (
"net/http"
"net/netip"
"net/url"
"runtime"
"sort"
"strings"
"sync"
@@ -740,18 +741,37 @@ func (f *forwarder) sendUDP(ctx context.Context, fq *forwardQuery, rr resolverAn
return out, nil
}
var optDNSForwardUseRoutes = envknob.RegisterOptBool("TS_DNS_FORWARD_USE_ROUTES")
// ShouldUseRoutes reports true if the DNS resolver should use the peer or system dialer
// for forwarding DNS queries to upstream nameservers via TCP, based on the destination
// address and configured routes. Currently, this requires maintaining a [bart.Table],
// resulting in a slightly higher memory usage.
//
// It reports false if the system dialer should always be used, regardless of the
// destination address.
//
// TODO(nickkhyl): Update [tsdial.Dialer] to reuse the bart.Table we create in net/tstun.Wrapper
// to avoid having two bart tables in memory, especially on iOS. Once that's done,
// we can get rid of the nodeAttr/control knob and always use UserDial for DNS.
//
// See https://github.com/tailscale/tailscale/issues/12027.
func ShouldUseRoutes(knobs *controlknobs.Knobs) bool {
switch runtime.GOOS {
case "android", "ios":
// On mobile platforms with lower memory limits (e.g., 50MB on iOS),
// this behavior is still gated by the "user-dial-routes" nodeAttr.
return knobs != nil && knobs.UserDialUseRoutes.Load()
default:
// On all other platforms, it is the default behavior,
// but it can be overridden with the "TS_DNS_FORWARD_USE_ROUTES" env var.
doNotUseRoutes := optDNSForwardUseRoutes().EqualBool(false)
return !doNotUseRoutes
}
}
func (f *forwarder) getDialerType() netx.DialFunc {
if f.controlKnobs != nil && f.controlKnobs.UserDialUseRoutes.Load() {
// It is safe to use UserDial as it dials external servers without going through Tailscale
// and closes connections on interface change in the same way as SystemDial does,
// thus preventing DNS resolution issues when switching between WiFi and cellular,
// but can also dial an internal DNS server on the Tailnet or via a subnet router.
//
// TODO(nickkhyl): Update tsdial.Dialer to reuse the bart.Table we create in net/tstun.Wrapper
// to avoid having two bart tables in memory, especially on iOS. Once that's done,
// we can get rid of the nodeAttr/control knob and always use UserDial for DNS.
//
// See https://github.com/tailscale/tailscale/issues/12027.
if ShouldUseRoutes(f.controlKnobs) {
return f.dialer.UserDial
}
return f.dialer.SystemDial