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tailscale/net/dnscache/dnscache.go

653 lines
17 KiB
Go

// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// TODO(bradfitz): update this code to use netaddr more
// Package dnscache contains a minimal DNS cache that makes a bunch of
// assumptions that are only valid for us. Not recommended for general use.
package dnscache
import (
"context"
"crypto/tls"
"errors"
"fmt"
"log"
"net"
"net/netip"
"runtime"
"sync"
"time"
"tailscale.com/envknob"
ipn/ipnlocal, net/dns*, util/cloudenv: specialize DNS config on Google Cloud This does three things: * If you're on GCP, it adds a *.internal DNS split route to the metadata server, so we never break GCP DNS names. This lets people have some Tailscale nodes on GCP and some not (e.g. laptops at home) without having to add a Tailnet-wide *.internal DNS route. If you already have such a route, though, it won't overwrite it. * If the 100.100.100.100 DNS forwarder has nowhere to forward to, it forwards it to the GCP metadata IP, which forwards to 8.8.8.8. This means there are never errNoUpstreams ("upstream nameservers not set") errors on GCP due to e.g. mangled /etc/resolv.conf (GCP default VMs don't have systemd-resolved, so it's likely a DNS supremacy fight) * makes the DNS fallback mechanism use the GCP metadata IP as a fallback before our hosted HTTP-based fallbacks I created a default GCP VM from their web wizard. It has no systemd-resolved. I then made its /etc/resolv.conf be empty and deleted its GCP hostnames in /etc/hosts. I then logged in to a tailnet with no global DNS settings. With this, tailscaled writes /etc/resolv.conf (direct mode, as no systemd-resolved) and sets it to 100.100.100.100, which then has regular DNS via the metadata IP and *.internal DNS via the metadata IP as well. If the tailnet configures explicit DNS servers, those are used instead, except for *.internal. This also adds a new util/cloudenv package based on version/distro where the cloud type is only detected once. We'll likely expand it in the future for other clouds, doing variants of this change for other popular cloud environments. Fixes #4911 RELNOTES=Google Cloud DNS improvements Change-Id: I19f3c2075983669b2b2c0f29a548da8de373c7cf Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2 years ago
"tailscale.com/util/cloudenv"
"tailscale.com/util/singleflight"
)
var zaddr netip.Addr
var single = &Resolver{
Forward: &net.Resolver{PreferGo: preferGoResolver()},
}
func preferGoResolver() bool {
// There does not appear to be a local resolver running
// on iOS, and NetworkExtension is good at isolating DNS.
// So do not use the Go resolver on macOS/iOS.
if runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
return false
}
// The local resolver is not available on Android.
if runtime.GOOS == "android" {
return false
}
// Otherwise, the Go resolver is fine and slightly preferred
// since it's lighter, not using cgo calls & threads.
return true
}
// Get returns a caching Resolver singleton.
func Get() *Resolver { return single }
// Resolver is a minimal DNS caching resolver.
//
// The TTL is always fixed for now. It's not intended for general use.
// Cache entries are never cleaned up so it's intended that this is
// only used with a fixed set of hostnames.
type Resolver struct {
// Forward is the resolver to use to populate the cache.
// If nil, net.DefaultResolver is used.
Forward *net.Resolver
// LookupIPFallback optionally provides a backup DNS mechanism
// to use if Forward returns an error or no results.
LookupIPFallback func(ctx context.Context, host string) ([]netip.Addr, error)
// TTL is how long to keep entries cached
//
// If zero, a default (currently 10 minutes) is used.
TTL time.Duration
// UseLastGood controls whether a cached entry older than TTL is used
// if a refresh fails.
UseLastGood bool
// SingleHostStaticResult, if non-nil, is the static result of IPs that is returned
// by Resolver.LookupIP for any hostname. When non-nil, SingleHost must also be
// set with the expected name.
SingleHostStaticResult []netip.Addr
// SingleHost is the hostname that SingleHostStaticResult is for.
// It is required when SingleHostStaticResult is present.
SingleHost string
sf singleflight.Group[string, ipRes]
mu sync.Mutex
ipCache map[string]ipCacheEntry
}
// ipRes is the type used by the Resolver.sf singleflight group.
type ipRes struct {
ip, ip6 netip.Addr
allIPs []netip.Addr
}
type ipCacheEntry struct {
ip netip.Addr // either v4 or v6
ip6 netip.Addr // nil if no v4 or no v6
allIPs []netip.Addr // 1+ v4 and/or v6
expires time.Time
}
func (r *Resolver) fwd() *net.Resolver {
if r.Forward != nil {
return r.Forward
}
return net.DefaultResolver
}
ipn/ipnlocal, net/dns*, util/cloudenv: specialize DNS config on Google Cloud This does three things: * If you're on GCP, it adds a *.internal DNS split route to the metadata server, so we never break GCP DNS names. This lets people have some Tailscale nodes on GCP and some not (e.g. laptops at home) without having to add a Tailnet-wide *.internal DNS route. If you already have such a route, though, it won't overwrite it. * If the 100.100.100.100 DNS forwarder has nowhere to forward to, it forwards it to the GCP metadata IP, which forwards to 8.8.8.8. This means there are never errNoUpstreams ("upstream nameservers not set") errors on GCP due to e.g. mangled /etc/resolv.conf (GCP default VMs don't have systemd-resolved, so it's likely a DNS supremacy fight) * makes the DNS fallback mechanism use the GCP metadata IP as a fallback before our hosted HTTP-based fallbacks I created a default GCP VM from their web wizard. It has no systemd-resolved. I then made its /etc/resolv.conf be empty and deleted its GCP hostnames in /etc/hosts. I then logged in to a tailnet with no global DNS settings. With this, tailscaled writes /etc/resolv.conf (direct mode, as no systemd-resolved) and sets it to 100.100.100.100, which then has regular DNS via the metadata IP and *.internal DNS via the metadata IP as well. If the tailnet configures explicit DNS servers, those are used instead, except for *.internal. This also adds a new util/cloudenv package based on version/distro where the cloud type is only detected once. We'll likely expand it in the future for other clouds, doing variants of this change for other popular cloud environments. Fixes #4911 RELNOTES=Google Cloud DNS improvements Change-Id: I19f3c2075983669b2b2c0f29a548da8de373c7cf Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2 years ago
// cloudHostResolver returns a Resolver for the current cloud hosting environment.
// It currently only supports Google Cloud.
func (r *Resolver) cloudHostResolver() (v *net.Resolver, ok bool) {
switch runtime.GOOS {
case "android", "ios", "darwin":
return nil, false
case "windows":
// TODO(bradfitz): remove this restriction once we're using Go 1.19
// which supports net.Resolver.PreferGo on Windows.
return nil, false
}
ip := cloudenv.Get().ResolverIP()
if ip == "" {
ipn/ipnlocal, net/dns*, util/cloudenv: specialize DNS config on Google Cloud This does three things: * If you're on GCP, it adds a *.internal DNS split route to the metadata server, so we never break GCP DNS names. This lets people have some Tailscale nodes on GCP and some not (e.g. laptops at home) without having to add a Tailnet-wide *.internal DNS route. If you already have such a route, though, it won't overwrite it. * If the 100.100.100.100 DNS forwarder has nowhere to forward to, it forwards it to the GCP metadata IP, which forwards to 8.8.8.8. This means there are never errNoUpstreams ("upstream nameservers not set") errors on GCP due to e.g. mangled /etc/resolv.conf (GCP default VMs don't have systemd-resolved, so it's likely a DNS supremacy fight) * makes the DNS fallback mechanism use the GCP metadata IP as a fallback before our hosted HTTP-based fallbacks I created a default GCP VM from their web wizard. It has no systemd-resolved. I then made its /etc/resolv.conf be empty and deleted its GCP hostnames in /etc/hosts. I then logged in to a tailnet with no global DNS settings. With this, tailscaled writes /etc/resolv.conf (direct mode, as no systemd-resolved) and sets it to 100.100.100.100, which then has regular DNS via the metadata IP and *.internal DNS via the metadata IP as well. If the tailnet configures explicit DNS servers, those are used instead, except for *.internal. This also adds a new util/cloudenv package based on version/distro where the cloud type is only detected once. We'll likely expand it in the future for other clouds, doing variants of this change for other popular cloud environments. Fixes #4911 RELNOTES=Google Cloud DNS improvements Change-Id: I19f3c2075983669b2b2c0f29a548da8de373c7cf Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2 years ago
return nil, false
}
return &net.Resolver{
PreferGo: true,
Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
var d net.Dialer
return d.DialContext(ctx, network, net.JoinHostPort(ip, "53"))
ipn/ipnlocal, net/dns*, util/cloudenv: specialize DNS config on Google Cloud This does three things: * If you're on GCP, it adds a *.internal DNS split route to the metadata server, so we never break GCP DNS names. This lets people have some Tailscale nodes on GCP and some not (e.g. laptops at home) without having to add a Tailnet-wide *.internal DNS route. If you already have such a route, though, it won't overwrite it. * If the 100.100.100.100 DNS forwarder has nowhere to forward to, it forwards it to the GCP metadata IP, which forwards to 8.8.8.8. This means there are never errNoUpstreams ("upstream nameservers not set") errors on GCP due to e.g. mangled /etc/resolv.conf (GCP default VMs don't have systemd-resolved, so it's likely a DNS supremacy fight) * makes the DNS fallback mechanism use the GCP metadata IP as a fallback before our hosted HTTP-based fallbacks I created a default GCP VM from their web wizard. It has no systemd-resolved. I then made its /etc/resolv.conf be empty and deleted its GCP hostnames in /etc/hosts. I then logged in to a tailnet with no global DNS settings. With this, tailscaled writes /etc/resolv.conf (direct mode, as no systemd-resolved) and sets it to 100.100.100.100, which then has regular DNS via the metadata IP and *.internal DNS via the metadata IP as well. If the tailnet configures explicit DNS servers, those are used instead, except for *.internal. This also adds a new util/cloudenv package based on version/distro where the cloud type is only detected once. We'll likely expand it in the future for other clouds, doing variants of this change for other popular cloud environments. Fixes #4911 RELNOTES=Google Cloud DNS improvements Change-Id: I19f3c2075983669b2b2c0f29a548da8de373c7cf Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2 years ago
},
}, true
}
func (r *Resolver) ttl() time.Duration {
if r.TTL > 0 {
return r.TTL
}
return 10 * time.Minute
}
var debug = envknob.Bool("TS_DEBUG_DNS_CACHE")
// LookupIP returns the host's primary IP address (either IPv4 or
// IPv6, but preferring IPv4) and optionally its IPv6 address, if
// there is both IPv4 and IPv6.
//
// If err is nil, ip will be non-nil. The v6 address may be nil even
// with a nil error.
func (r *Resolver) LookupIP(ctx context.Context, host string) (ip, v6 netip.Addr, allIPs []netip.Addr, err error) {
if r.SingleHostStaticResult != nil {
if r.SingleHost != host {
return zaddr, zaddr, nil, fmt.Errorf("dnscache: unexpected hostname %q doesn't match expected %q", host, r.SingleHost)
}
for _, naIP := range r.SingleHostStaticResult {
if !ip.IsValid() && naIP.Is4() {
ip = naIP
}
if !v6.IsValid() && naIP.Is6() {
v6 = naIP
}
allIPs = append(allIPs, naIP)
}
return
}
if ip, err := netip.ParseAddr(host); err == nil {
ip = ip.Unmap()
if debug {
log.Printf("dnscache: %q is an IP", host)
}
return ip, zaddr, []netip.Addr{ip}, nil
}
if ip, ip6, allIPs, ok := r.lookupIPCache(host); ok {
if debug {
log.Printf("dnscache: %q = %v (cached)", host, ip)
}
return ip, ip6, allIPs, nil
}
ch := r.sf.DoChan(host, func() (ret ipRes, _ error) {
ip, ip6, allIPs, err := r.lookupIP(host)
if err != nil {
return ret, err
}
return ipRes{ip, ip6, allIPs}, nil
})
select {
case res := <-ch:
if res.Err != nil {
if r.UseLastGood {
if ip, ip6, allIPs, ok := r.lookupIPCacheExpired(host); ok {
if debug {
log.Printf("dnscache: %q using %v after error", host, ip)
}
return ip, ip6, allIPs, nil
}
}
if debug {
log.Printf("dnscache: error resolving %q: %v", host, res.Err)
}
return zaddr, zaddr, nil, res.Err
}
r := res.Val
return r.ip, r.ip6, r.allIPs, nil
case <-ctx.Done():
if debug {
log.Printf("dnscache: context done while resolving %q: %v", host, ctx.Err())
}
return zaddr, zaddr, nil, ctx.Err()
}
}
func (r *Resolver) lookupIPCache(host string) (ip, ip6 netip.Addr, allIPs []netip.Addr, ok bool) {
r.mu.Lock()
defer r.mu.Unlock()
if ent, ok := r.ipCache[host]; ok && ent.expires.After(time.Now()) {
return ent.ip, ent.ip6, ent.allIPs, true
}
return zaddr, zaddr, nil, false
}
func (r *Resolver) lookupIPCacheExpired(host string) (ip, ip6 netip.Addr, allIPs []netip.Addr, ok bool) {
r.mu.Lock()
defer r.mu.Unlock()
if ent, ok := r.ipCache[host]; ok {
return ent.ip, ent.ip6, ent.allIPs, true
}
return zaddr, zaddr, nil, false
}
func (r *Resolver) lookupTimeoutForHost(host string) time.Duration {
if r.UseLastGood {
if _, _, _, ok := r.lookupIPCacheExpired(host); ok {
// If we have some previous good value for this host,
// don't give this DNS lookup much time. If we're in a
// situation where the user's DNS server is unreachable
// (e.g. their corp DNS server is behind a subnet router
// that can't come up due to Tailscale needing to
// connect to itself), then we want to fail fast and let
// our caller (who set UseLastGood) fall back to using
// the last-known-good IP address.
return 3 * time.Second
}
}
return 10 * time.Second
}
func (r *Resolver) lookupIP(host string) (ip, ip6 netip.Addr, allIPs []netip.Addr, err error) {
if ip, ip6, allIPs, ok := r.lookupIPCache(host); ok {
if debug {
log.Printf("dnscache: %q found in cache as %v", host, ip)
}
return ip, ip6, allIPs, nil
}
ctx, cancel := context.WithTimeout(context.Background(), r.lookupTimeoutForHost(host))
defer cancel()
ips, err := r.fwd().LookupNetIP(ctx, "ip", host)
ipn/ipnlocal, net/dns*, util/cloudenv: specialize DNS config on Google Cloud This does three things: * If you're on GCP, it adds a *.internal DNS split route to the metadata server, so we never break GCP DNS names. This lets people have some Tailscale nodes on GCP and some not (e.g. laptops at home) without having to add a Tailnet-wide *.internal DNS route. If you already have such a route, though, it won't overwrite it. * If the 100.100.100.100 DNS forwarder has nowhere to forward to, it forwards it to the GCP metadata IP, which forwards to 8.8.8.8. This means there are never errNoUpstreams ("upstream nameservers not set") errors on GCP due to e.g. mangled /etc/resolv.conf (GCP default VMs don't have systemd-resolved, so it's likely a DNS supremacy fight) * makes the DNS fallback mechanism use the GCP metadata IP as a fallback before our hosted HTTP-based fallbacks I created a default GCP VM from their web wizard. It has no systemd-resolved. I then made its /etc/resolv.conf be empty and deleted its GCP hostnames in /etc/hosts. I then logged in to a tailnet with no global DNS settings. With this, tailscaled writes /etc/resolv.conf (direct mode, as no systemd-resolved) and sets it to 100.100.100.100, which then has regular DNS via the metadata IP and *.internal DNS via the metadata IP as well. If the tailnet configures explicit DNS servers, those are used instead, except for *.internal. This also adds a new util/cloudenv package based on version/distro where the cloud type is only detected once. We'll likely expand it in the future for other clouds, doing variants of this change for other popular cloud environments. Fixes #4911 RELNOTES=Google Cloud DNS improvements Change-Id: I19f3c2075983669b2b2c0f29a548da8de373c7cf Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2 years ago
if err != nil || len(ips) == 0 {
if resolver, ok := r.cloudHostResolver(); ok {
ips, err = resolver.LookupNetIP(ctx, "ip", host)
ipn/ipnlocal, net/dns*, util/cloudenv: specialize DNS config on Google Cloud This does three things: * If you're on GCP, it adds a *.internal DNS split route to the metadata server, so we never break GCP DNS names. This lets people have some Tailscale nodes on GCP and some not (e.g. laptops at home) without having to add a Tailnet-wide *.internal DNS route. If you already have such a route, though, it won't overwrite it. * If the 100.100.100.100 DNS forwarder has nowhere to forward to, it forwards it to the GCP metadata IP, which forwards to 8.8.8.8. This means there are never errNoUpstreams ("upstream nameservers not set") errors on GCP due to e.g. mangled /etc/resolv.conf (GCP default VMs don't have systemd-resolved, so it's likely a DNS supremacy fight) * makes the DNS fallback mechanism use the GCP metadata IP as a fallback before our hosted HTTP-based fallbacks I created a default GCP VM from their web wizard. It has no systemd-resolved. I then made its /etc/resolv.conf be empty and deleted its GCP hostnames in /etc/hosts. I then logged in to a tailnet with no global DNS settings. With this, tailscaled writes /etc/resolv.conf (direct mode, as no systemd-resolved) and sets it to 100.100.100.100, which then has regular DNS via the metadata IP and *.internal DNS via the metadata IP as well. If the tailnet configures explicit DNS servers, those are used instead, except for *.internal. This also adds a new util/cloudenv package based on version/distro where the cloud type is only detected once. We'll likely expand it in the future for other clouds, doing variants of this change for other popular cloud environments. Fixes #4911 RELNOTES=Google Cloud DNS improvements Change-Id: I19f3c2075983669b2b2c0f29a548da8de373c7cf Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2 years ago
}
}
if (err != nil || len(ips) == 0) && r.LookupIPFallback != nil {
ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
defer cancel()
ips, err = r.LookupIPFallback(ctx, host)
}
if err != nil {
return netip.Addr{}, netip.Addr{}, nil, err
}
if len(ips) == 0 {
return netip.Addr{}, netip.Addr{}, nil, fmt.Errorf("no IPs for %q found", host)
}
have4 := false
for _, ipa := range ips {
if ipa.Is4() {
if !have4 {
ip6 = ip
ip = ipa
have4 = true
}
} else {
if have4 {
ip6 = ipa
} else {
ip = ipa
}
}
}
r.addIPCache(host, ip, ip6, ips, r.ttl())
return ip, ip6, ips, nil
}
func (r *Resolver) addIPCache(host string, ip, ip6 netip.Addr, allIPs []netip.Addr, d time.Duration) {
if ip.IsPrivate() {
// Don't cache obviously wrong entries from captive portals.
// TODO: use DoH or DoT for the forwarding resolver?
if debug {
log.Printf("dnscache: %q resolved to private IP %v; using but not caching", host, ip)
}
return
}
if debug {
log.Printf("dnscache: %q resolved to IP %v; caching", host, ip)
}
r.mu.Lock()
defer r.mu.Unlock()
if r.ipCache == nil {
r.ipCache = make(map[string]ipCacheEntry)
}
r.ipCache[host] = ipCacheEntry{
ip: ip,
ip6: ip6,
allIPs: allIPs,
expires: time.Now().Add(d),
}
}
type DialContextFunc func(ctx context.Context, network, address string) (net.Conn, error)
// Dialer returns a wrapped DialContext func that uses the provided dnsCache.
func Dialer(fwd DialContextFunc, dnsCache *Resolver) DialContextFunc {
d := &dialer{
fwd: fwd,
dnsCache: dnsCache,
pastConnect: map[netip.Addr]time.Time{},
}
return d.DialContext
}
// dialer is the config and accumulated state for a dial func returned by Dialer.
type dialer struct {
fwd DialContextFunc
dnsCache *Resolver
mu sync.Mutex
pastConnect map[netip.Addr]time.Time
}
func (d *dialer) DialContext(ctx context.Context, network, address string) (retConn net.Conn, ret error) {
host, port, err := net.SplitHostPort(address)
if err != nil {
// Bogus. But just let the real dialer return an error rather than
// inventing a similar one.
return d.fwd(ctx, network, address)
}
dc := &dialCall{
d: d,
network: network,
address: address,
host: host,
port: port,
}
defer func() {
// On failure, consider that our DNS might be wrong and ask the DNS fallback mechanism for
// some other IPs to try.
if ret == nil || ctx.Err() != nil || d.dnsCache.LookupIPFallback == nil || dc.dnsWasTrustworthy() {
return
}
ips, err := d.dnsCache.LookupIPFallback(ctx, host)
if err != nil {
// Return with original error
return
}
if c, err := dc.raceDial(ctx, ips); err == nil {
retConn = c
ret = nil
return
}
}()
ip, ip6, allIPs, err := d.dnsCache.LookupIP(ctx, host)
if err != nil {
return nil, fmt.Errorf("failed to resolve %q: %w", host, err)
}
i4s := v4addrs(allIPs)
if len(i4s) < 2 {
if debug {
log.Printf("dnscache: dialing %s, %s for %s", network, ip, address)
}
c, err := dc.dialOne(ctx, ip.Unmap())
if err == nil || ctx.Err() != nil {
return c, err
}
// Fall back to trying IPv6, if any.
return dc.dialOne(ctx, ip6)
}
// Multiple IPv4 candidates, and 0+ IPv6.
ipsToTry := append(i4s, v6addrs(allIPs)...)
return dc.raceDial(ctx, ipsToTry)
}
// dialCall is the state around a single call to dial.
type dialCall struct {
d *dialer
network, address, host, port string
mu sync.Mutex // lock ordering: dialer.mu, then dialCall.mu
fails map[netip.Addr]error // set of IPs that failed to dial thus far
}
// dnsWasTrustworthy reports whether we think the IP address(es) we
// tried (and failed) to dial were probably the correct IPs. Currently
// the heuristic is whether they ever worked previously.
func (dc *dialCall) dnsWasTrustworthy() bool {
dc.d.mu.Lock()
defer dc.d.mu.Unlock()
dc.mu.Lock()
defer dc.mu.Unlock()
if len(dc.fails) == 0 {
// No information.
return false
}
// If any of the IPs we failed to dial worked previously in
// this dialer, assume the DNS is fine.
for ip := range dc.fails {
if _, ok := dc.d.pastConnect[ip]; ok {
return true
}
}
return false
}
func (dc *dialCall) dialOne(ctx context.Context, ip netip.Addr) (net.Conn, error) {
c, err := dc.d.fwd(ctx, dc.network, net.JoinHostPort(ip.String(), dc.port))
dc.noteDialResult(ip, err)
return c, err
}
// noteDialResult records that a dial to ip either succeeded or
// failed.
func (dc *dialCall) noteDialResult(ip netip.Addr, err error) {
if err == nil {
d := dc.d
d.mu.Lock()
defer d.mu.Unlock()
d.pastConnect[ip] = time.Now()
return
}
dc.mu.Lock()
defer dc.mu.Unlock()
if dc.fails == nil {
dc.fails = map[netip.Addr]error{}
}
dc.fails[ip] = err
}
// uniqueIPs returns a possibly-mutated subslice of ips, filtering out
// dups and ones that have already failed previously.
func (dc *dialCall) uniqueIPs(ips []netip.Addr) (ret []netip.Addr) {
dc.mu.Lock()
defer dc.mu.Unlock()
seen := map[netip.Addr]bool{}
ret = ips[:0]
for _, ip := range ips {
if seen[ip] {
continue
}
seen[ip] = true
if dc.fails[ip] != nil {
continue
}
ret = append(ret, ip)
}
return ret
}
// fallbackDelay is how long to wait between trying subsequent
// addresses when multiple options are available.
// 300ms is the same as Go's Happy Eyeballs fallbackDelay value.
const fallbackDelay = 300 * time.Millisecond
// raceDial tries to dial port on each ip in ips, starting a new race
// dial every fallbackDelay apart, returning whichever completes first.
func (dc *dialCall) raceDial(ctx context.Context, ips []netip.Addr) (net.Conn, error) {
ctx, cancel := context.WithCancel(ctx)
defer cancel()
type res struct {
c net.Conn
err error
}
resc := make(chan res) // must be unbuffered
failBoost := make(chan struct{}) // best effort send on dial failure
// Remove IPs that we tried & failed to dial previously
// (such as when we're being called after a dnsfallback lookup and get
// the same results)
ips = dc.uniqueIPs(ips)
if len(ips) == 0 {
return nil, errors.New("no IPs")
}
// Partition candidate list and then merge such that an IPv6 address is
// in the first spot if present, and then addresses are interleaved.
// This ensures that we're trying an IPv6 address first, then
// alternating between v4 and v6 in case one of the two networks is
// broken.
var iv4, iv6 []netip.Addr
for _, ip := range ips {
if ip.Is6() {
iv6 = append(iv6, ip)
} else {
iv4 = append(iv4, ip)
}
}
ips = interleaveSlices(iv6, iv4)
go func() {
for i, ip := range ips {
if i != 0 {
timer := time.NewTimer(fallbackDelay)
select {
case <-timer.C:
case <-failBoost:
timer.Stop()
case <-ctx.Done():
timer.Stop()
return
}
}
go func(ip netip.Addr) {
c, err := dc.dialOne(ctx, ip)
if err != nil {
// Best effort wake-up a pending dial.
// e.g. IPv4 dials failing quickly on an IPv6-only system.
// In that case we don't want to wait 300ms per IPv4 before
// we get to the IPv6 addresses.
select {
case failBoost <- struct{}{}:
default:
}
}
select {
case resc <- res{c, err}:
case <-ctx.Done():
if c != nil {
c.Close()
}
}
}(ip)
}
}()
var firstErr error
var fails int
for {
select {
case r := <-resc:
if r.c != nil {
return r.c, nil
}
fails++
if firstErr == nil {
firstErr = r.err
}
if fails == len(ips) {
return nil, firstErr
}
case <-ctx.Done():
return nil, ctx.Err()
}
}
}
// interleaveSlices combines two slices of the form [a, b, c] and [x, y, z]
// into a slice with elements interleaved; i.e. [a, x, b, y, c, z].
func interleaveSlices[T any](a, b []T) []T {
var (
i int
ret = make([]T, 0, len(a)+len(b))
)
for i = 0; i < len(a) && i < len(b); i++ {
ret = append(ret, a[i], b[i])
}
ret = append(ret, a[i:]...)
ret = append(ret, b[i:]...)
return ret
}
func v4addrs(aa []netip.Addr) (ret []netip.Addr) {
for _, a := range aa {
a = a.Unmap()
if a.Is4() {
ret = append(ret, a)
}
}
return ret
}
func v6addrs(aa []netip.Addr) (ret []netip.Addr) {
for _, a := range aa {
if a.Is6() && !a.Is4In6() {
ret = append(ret, a)
}
}
return ret
}
var errTLSHandshakeTimeout = errors.New("timeout doing TLS handshake")
// TLSDialer is like Dialer but returns a func suitable for using with net/http.Transport.DialTLSContext.
// It returns a *tls.Conn type on success.
// On TLS cert validation failure, it can invoke a backup DNS resolution strategy.
func TLSDialer(fwd DialContextFunc, dnsCache *Resolver, tlsConfigBase *tls.Config) DialContextFunc {
tcpDialer := Dialer(fwd, dnsCache)
return func(ctx context.Context, network, address string) (net.Conn, error) {
host, _, err := net.SplitHostPort(address)
if err != nil {
return nil, err
}
tcpConn, err := tcpDialer(ctx, network, address)
if err != nil {
return nil, err
}
cfg := cloneTLSConfig(tlsConfigBase)
if cfg.ServerName == "" {
cfg.ServerName = host
}
tlsConn := tls.Client(tcpConn, cfg)
handshakeCtx, handshakeTimeoutCancel := context.WithTimeout(ctx, 5*time.Second)
defer handshakeTimeoutCancel()
if err := tlsConn.HandshakeContext(handshakeCtx); err != nil {
tcpConn.Close()
// TODO: if err != errTLSHandshakeTimeout,
// assume it might be some captive portal or
// otherwise incorrect DNS and try the backup
// DNS mechanism.
return nil, err
}
return tlsConn, nil
}
}
func cloneTLSConfig(cfg *tls.Config) *tls.Config {
if cfg == nil {
return &tls.Config{}
}
return cfg.Clone()
}