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/*
*
* Copyright 2018 gRPC authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*/
// Package dns implements a dns resolver to be installed as the default resolver
// in grpc.
package dns
import (
"context"
"encoding/json"
"errors"
"fmt"
"net"
"os"
"strconv"
"strings"
"sync"
"time"
"google.golang.org/grpc/grpclog"
"google.golang.org/grpc/internal/backoff"
"google.golang.org/grpc/internal/grpcrand"
"google.golang.org/grpc/resolver"
)
func init() {
resolver.Register(NewBuilder())
}
const (
defaultPort = "443"
defaultFreq = time.Minute * 30
defaultDNSSvrPort = "53"
golang = "GO"
// txtPrefix is the prefix string to be prepended to the host name for txt record lookup.
txtPrefix = "_grpc_config."
// In DNS, service config is encoded in a TXT record via the mechanism
// described in RFC-1464 using the attribute name grpc_config.
txtAttribute = "grpc_config="
)
var (
errMissingAddr = errors.New("dns resolver: missing address")
// Addresses ending with a colon that is supposed to be the separator
// between host and port is not allowed. E.g. "::" is a valid address as
// it is an IPv6 address (host only) and "[::]:" is invalid as it ends with
// a colon as the host and port separator
errEndsWithColon = errors.New("dns resolver: missing port after port-separator colon")
)
var (
defaultResolver netResolver = net.DefaultResolver
)
var customAuthorityDialler = func(authority string) func(ctx context.Context, network, address string) (net.Conn, error) {
return func(ctx context.Context, network, address string) (net.Conn, error) {
var dialer net.Dialer
return dialer.DialContext(ctx, network, authority)
}
}
var customAuthorityResolver = func(authority string) (netResolver, error) {
host, port, err := parseTarget(authority, defaultDNSSvrPort)
if err != nil {
return nil, err
}
authorityWithPort := net.JoinHostPort(host, port)
return &net.Resolver{
PreferGo: true,
Dial: customAuthorityDialler(authorityWithPort),
}, nil
}
// NewBuilder creates a dnsBuilder which is used to factory DNS resolvers.
func NewBuilder() resolver.Builder {
return &dnsBuilder{minFreq: defaultFreq}
}
type dnsBuilder struct {
// minimum frequency of polling the DNS server.
minFreq time.Duration
}
// Build creates and starts a DNS resolver that watches the name resolution of the target.
func (b *dnsBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOption) (resolver.Resolver, error) {
host, port, err := parseTarget(target.Endpoint, defaultPort)
if err != nil {
return nil, err
}
// IP address.
if net.ParseIP(host) != nil {
host, _ = formatIP(host)
addr := []resolver.Address{{Addr: host + ":" + port}}
i := &ipResolver{
cc: cc,
ip: addr,
rn: make(chan struct{}, 1),
q: make(chan struct{}),
}
cc.NewAddress(addr)
go i.watcher()
return i, nil
}
// DNS address (non-IP).
ctx, cancel := context.WithCancel(context.Background())
d := &dnsResolver{
freq: b.minFreq,
backoff: backoff.Exponential{MaxDelay: b.minFreq},
host: host,
port: port,
ctx: ctx,
cancel: cancel,
cc: cc,
t: time.NewTimer(0),
rn: make(chan struct{}, 1),
disableServiceConfig: opts.DisableServiceConfig,
}
if target.Authority == "" {
d.resolver = defaultResolver
} else {
d.resolver, err = customAuthorityResolver(target.Authority)
if err != nil {
return nil, err
}
}
d.wg.Add(1)
go d.watcher()
return d, nil
}
// Scheme returns the naming scheme of this resolver builder, which is "dns".
func (b *dnsBuilder) Scheme() string {
return "dns"
}
type netResolver interface {
LookupHost(ctx context.Context, host string) (addrs []string, err error)
LookupSRV(ctx context.Context, service, proto, name string) (cname string, addrs []*net.SRV, err error)
LookupTXT(ctx context.Context, name string) (txts []string, err error)
}
// ipResolver watches for the name resolution update for an IP address.
type ipResolver struct {
cc resolver.ClientConn
ip []resolver.Address
// rn channel is used by ResolveNow() to force an immediate resolution of the target.
rn chan struct{}
q chan struct{}
}
// ResolveNow resend the address it stores, no resolution is needed.
func (i *ipResolver) ResolveNow(opt resolver.ResolveNowOption) {
select {
case i.rn <- struct{}{}:
default:
}
}
// Close closes the ipResolver.
func (i *ipResolver) Close() {
close(i.q)
}
func (i *ipResolver) watcher() {
for {
select {
case <-i.rn:
i.cc.NewAddress(i.ip)
case <-i.q:
return
}
}
}
// dnsResolver watches for the name resolution update for a non-IP target.
type dnsResolver struct {
freq time.Duration
backoff backoff.Exponential
retryCount int
host string
port string
resolver netResolver
ctx context.Context
cancel context.CancelFunc
cc resolver.ClientConn
// rn channel is used by ResolveNow() to force an immediate resolution of the target.
rn chan struct{}
t *time.Timer
// wg is used to enforce Close() to return after the watcher() goroutine has finished.
// Otherwise, data race will be possible. [Race Example] in dns_resolver_test we
// replace the real lookup functions with mocked ones to facilitate testing.
// If Close() doesn't wait for watcher() goroutine finishes, race detector sometimes
// will warns lookup (READ the lookup function pointers) inside watcher() goroutine
// has data race with replaceNetFunc (WRITE the lookup function pointers).
wg sync.WaitGroup
disableServiceConfig bool
}
// ResolveNow invoke an immediate resolution of the target that this dnsResolver watches.
func (d *dnsResolver) ResolveNow(opt resolver.ResolveNowOption) {
select {
case d.rn <- struct{}{}:
default:
}
}
// Close closes the dnsResolver.
func (d *dnsResolver) Close() {
d.cancel()
d.wg.Wait()
d.t.Stop()
}
func (d *dnsResolver) watcher() {
defer d.wg.Done()
for {
select {
case <-d.ctx.Done():
return
case <-d.t.C:
case <-d.rn:
}
result, sc := d.lookup()
// Next lookup should happen within an interval defined by d.freq. It may be
// more often due to exponential retry on empty address list.
if len(result) == 0 {
d.retryCount++
d.t.Reset(d.backoff.Backoff(d.retryCount))
} else {
d.retryCount = 0
d.t.Reset(d.freq)
}
d.cc.NewServiceConfig(sc)
d.cc.NewAddress(result)
}
}
func (d *dnsResolver) lookupSRV() []resolver.Address {
var newAddrs []resolver.Address
_, srvs, err := d.resolver.LookupSRV(d.ctx, "grpclb", "tcp", d.host)
if err != nil {
grpclog.Infof("grpc: failed dns SRV record lookup due to %v.\n", err)
return nil
}
for _, s := range srvs {
lbAddrs, err := d.resolver.LookupHost(d.ctx, s.Target)
if err != nil {
grpclog.Infof("grpc: failed load balancer address dns lookup due to %v.\n", err)
continue
}
for _, a := range lbAddrs {
a, ok := formatIP(a)
if !ok {
grpclog.Errorf("grpc: failed IP parsing due to %v.\n", err)
continue
}
addr := a + ":" + strconv.Itoa(int(s.Port))
newAddrs = append(newAddrs, resolver.Address{Addr: addr, Type: resolver.GRPCLB, ServerName: s.Target})
}
}
return newAddrs
}
func (d *dnsResolver) lookupTXT() string {
ss, err := d.resolver.LookupTXT(d.ctx, txtPrefix+d.host)
if err != nil {
grpclog.Infof("grpc: failed dns TXT record lookup due to %v.\n", err)
return ""
}
var res string
for _, s := range ss {
res += s
}
// TXT record must have "grpc_config=" attribute in order to be used as service config.
if !strings.HasPrefix(res, txtAttribute) {
grpclog.Warningf("grpc: TXT record %v missing %v attribute", res, txtAttribute)
return ""
}
return strings.TrimPrefix(res, txtAttribute)
}
func (d *dnsResolver) lookupHost() []resolver.Address {
var newAddrs []resolver.Address
addrs, err := d.resolver.LookupHost(d.ctx, d.host)
if err != nil {
grpclog.Warningf("grpc: failed dns A record lookup due to %v.\n", err)
return nil
}
for _, a := range addrs {
a, ok := formatIP(a)
if !ok {
grpclog.Errorf("grpc: failed IP parsing due to %v.\n", err)
continue
}
addr := a + ":" + d.port
newAddrs = append(newAddrs, resolver.Address{Addr: addr})
}
return newAddrs
}
func (d *dnsResolver) lookup() ([]resolver.Address, string) {
newAddrs := d.lookupSRV()
// Support fallback to non-balancer address.
newAddrs = append(newAddrs, d.lookupHost()...)
if d.disableServiceConfig {
return newAddrs, ""
}
sc := d.lookupTXT()
return newAddrs, canaryingSC(sc)
}
// formatIP returns ok = false if addr is not a valid textual representation of an IP address.
// If addr is an IPv4 address, return the addr and ok = true.
// If addr is an IPv6 address, return the addr enclosed in square brackets and ok = true.
func formatIP(addr string) (addrIP string, ok bool) {
ip := net.ParseIP(addr)
if ip == nil {
return "", false
}
if ip.To4() != nil {
return addr, true
}
return "[" + addr + "]", true
}
// parseTarget takes the user input target string and default port, returns formatted host and port info.
// If target doesn't specify a port, set the port to be the defaultPort.
// If target is in IPv6 format and host-name is enclosed in square brackets, brackets
// are stripped when setting the host.
// examples:
// target: "www.google.com" defaultPort: "443" returns host: "www.google.com", port: "443"
// target: "ipv4-host:80" defaultPort: "443" returns host: "ipv4-host", port: "80"
// target: "[ipv6-host]" defaultPort: "443" returns host: "ipv6-host", port: "443"
// target: ":80" defaultPort: "443" returns host: "localhost", port: "80"
func parseTarget(target, defaultPort string) (host, port string, err error) {
if target == "" {
return "", "", errMissingAddr
}
if ip := net.ParseIP(target); ip != nil {
// target is an IPv4 or IPv6(without brackets) address
return target, defaultPort, nil
}
if host, port, err = net.SplitHostPort(target); err == nil {
if port == "" {
// If the port field is empty (target ends with colon), e.g. "[::1]:", this is an error.
return "", "", errEndsWithColon
}
// target has port, i.e ipv4-host:port, [ipv6-host]:port, host-name:port
if host == "" {
// Keep consistent with net.Dial(): If the host is empty, as in ":80", the local system is assumed.
host = "localhost"
}
return host, port, nil
}
if host, port, err = net.SplitHostPort(target + ":" + defaultPort); err == nil {
// target doesn't have port
return host, port, nil
}
return "", "", fmt.Errorf("invalid target address %v, error info: %v", target, err)
}
type rawChoice struct {
ClientLanguage *[]string `json:"clientLanguage,omitempty"`
Percentage *int `json:"percentage,omitempty"`
ClientHostName *[]string `json:"clientHostName,omitempty"`
ServiceConfig *json.RawMessage `json:"serviceConfig,omitempty"`
}
func containsString(a *[]string, b string) bool {
if a == nil {
return true
}
for _, c := range *a {
if c == b {
return true
}
}
return false
}
func chosenByPercentage(a *int) bool {
if a == nil {
return true
}
return grpcrand.Intn(100)+1 <= *a
}
func canaryingSC(js string) string {
if js == "" {
return ""
}
var rcs []rawChoice
err := json.Unmarshal([]byte(js), &rcs)
if err != nil {
grpclog.Warningf("grpc: failed to parse service config json string due to %v.\n", err)
return ""
}
cliHostname, err := os.Hostname()
if err != nil {
grpclog.Warningf("grpc: failed to get client hostname due to %v.\n", err)
return ""
}
var sc string
for _, c := range rcs {
if !containsString(c.ClientLanguage, golang) ||
!chosenByPercentage(c.Percentage) ||
!containsString(c.ClientHostName, cliHostname) ||
c.ServiceConfig == nil {
continue
}
sc = string(*c.ServiceConfig)
break
}
return sc
}
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