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dns_resolver.go
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dns_resolver.go
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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"
"fmt"
"net"
"os"
"strconv"
"strings"
"sync"
"time"
grpclbstate "google.golang.org/grpc/balancer/grpclb/state"
"google.golang.org/grpc/grpclog"
"google.golang.org/grpc/internal/backoff"
"google.golang.org/grpc/internal/envconfig"
"google.golang.org/grpc/internal/grpcrand"
"google.golang.org/grpc/internal/resolver/dns/internal"
"google.golang.org/grpc/resolver"
"google.golang.org/grpc/serviceconfig"
)
// EnableSRVLookups controls whether the DNS resolver attempts to fetch gRPCLB
// addresses from SRV records. Must not be changed after init time.
var EnableSRVLookups = false
// ResolvingTimeout specifies the maximum duration for a DNS resolution request.
// If the timeout expires before a response is received, the request will be canceled.
//
// It is recommended to set this value at application startup. Avoid modifying this variable
// after initialization as it's not thread-safe for concurrent modification.
var ResolvingTimeout = 30 * time.Second
var logger = grpclog.Component("dns")
func init() {
resolver.Register(NewBuilder())
internal.TimeAfterFunc = time.After
internal.NewNetResolver = newNetResolver
internal.AddressDialer = addressDialer
}
const (
defaultPort = "443"
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 addressDialer = func(address string) func(context.Context, string, string) (net.Conn, error) {
return func(ctx context.Context, network, _ string) (net.Conn, error) {
var dialer net.Dialer
return dialer.DialContext(ctx, network, address)
}
}
var newNetResolver = func(authority string) (internal.NetResolver, error) {
if authority == "" {
return net.DefaultResolver, nil
}
host, port, err := parseTarget(authority, defaultDNSSvrPort)
if err != nil {
return nil, err
}
authorityWithPort := net.JoinHostPort(host, port)
return &net.Resolver{
PreferGo: true,
Dial: internal.AddressDialer(authorityWithPort),
}, nil
}
// NewBuilder creates a dnsBuilder which is used to factory DNS resolvers.
func NewBuilder() resolver.Builder {
return &dnsBuilder{}
}
type dnsBuilder struct{}
// 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.BuildOptions) (resolver.Resolver, error) {
host, port, err := parseTarget(target.Endpoint(), defaultPort)
if err != nil {
return nil, err
}
// IP address.
if ipAddr, ok := formatIP(host); ok {
addr := []resolver.Address{{Addr: ipAddr + ":" + port}}
cc.UpdateState(resolver.State{Addresses: addr})
return deadResolver{}, nil
}
// DNS address (non-IP).
ctx, cancel := context.WithCancel(context.Background())
d := &dnsResolver{
host: host,
port: port,
ctx: ctx,
cancel: cancel,
cc: cc,
rn: make(chan struct{}, 1),
disableServiceConfig: opts.DisableServiceConfig,
}
d.resolver, err = internal.NewNetResolver(target.URL.Host)
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"
}
// deadResolver is a resolver that does nothing.
type deadResolver struct{}
func (deadResolver) ResolveNow(resolver.ResolveNowOptions) {}
func (deadResolver) Close() {}
// dnsResolver watches for the name resolution update for a non-IP target.
type dnsResolver struct {
host string
port string
resolver internal.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{}
// 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(resolver.ResolveNowOptions) {
select {
case d.rn <- struct{}{}:
default:
}
}
// Close closes the dnsResolver.
func (d *dnsResolver) Close() {
d.cancel()
d.wg.Wait()
}
func (d *dnsResolver) watcher() {
defer d.wg.Done()
backoffIndex := 1
for {
state, err := d.lookup()
if err != nil {
// Report error to the underlying grpc.ClientConn.
d.cc.ReportError(err)
} else {
err = d.cc.UpdateState(*state)
}
var waitTime time.Duration
if err == nil {
// Success resolving, wait for the next ResolveNow. However, also wait 30
// seconds at the very least to prevent constantly re-resolving.
backoffIndex = 1
waitTime = internal.MinResolutionRate
select {
case <-d.ctx.Done():
return
case <-d.rn:
}
} else {
// Poll on an error found in DNS Resolver or an error received from
// ClientConn.
waitTime = backoff.DefaultExponential.Backoff(backoffIndex)
backoffIndex++
}
select {
case <-d.ctx.Done():
return
case <-internal.TimeAfterFunc(waitTime):
}
}
}
func (d *dnsResolver) lookupSRV(ctx context.Context) ([]resolver.Address, error) {
if !EnableSRVLookups {
return nil, nil
}
var newAddrs []resolver.Address
_, srvs, err := d.resolver.LookupSRV(ctx, "grpclb", "tcp", d.host)
if err != nil {
err = handleDNSError(err, "SRV") // may become nil
return nil, err
}
for _, s := range srvs {
lbAddrs, err := d.resolver.LookupHost(ctx, s.Target)
if err != nil {
err = handleDNSError(err, "A") // may become nil
if err == nil {
// If there are other SRV records, look them up and ignore this
// one that does not exist.
continue
}
return nil, err
}
for _, a := range lbAddrs {
ip, ok := formatIP(a)
if !ok {
return nil, fmt.Errorf("dns: error parsing A record IP address %v", a)
}
addr := ip + ":" + strconv.Itoa(int(s.Port))
newAddrs = append(newAddrs, resolver.Address{Addr: addr, ServerName: s.Target})
}
}
return newAddrs, nil
}
func handleDNSError(err error, lookupType string) error {
dnsErr, ok := err.(*net.DNSError)
if ok && !dnsErr.IsTimeout && !dnsErr.IsTemporary {
// Timeouts and temporary errors should be communicated to gRPC to
// attempt another DNS query (with backoff). Other errors should be
// suppressed (they may represent the absence of a TXT record).
return nil
}
if err != nil {
err = fmt.Errorf("dns: %v record lookup error: %v", lookupType, err)
logger.Info(err)
}
return err
}
func (d *dnsResolver) lookupTXT(ctx context.Context) *serviceconfig.ParseResult {
ss, err := d.resolver.LookupTXT(ctx, txtPrefix+d.host)
if err != nil {
if envconfig.TXTErrIgnore {
return nil
}
if err = handleDNSError(err, "TXT"); err != nil {
return &serviceconfig.ParseResult{Err: err}
}
return nil
}
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) {
logger.Warningf("dns: TXT record %v missing %v attribute", res, txtAttribute)
// This is not an error; it is the equivalent of not having a service
// config.
return nil
}
sc := canaryingSC(strings.TrimPrefix(res, txtAttribute))
return d.cc.ParseServiceConfig(sc)
}
func (d *dnsResolver) lookupHost(ctx context.Context) ([]resolver.Address, error) {
addrs, err := d.resolver.LookupHost(ctx, d.host)
if err != nil {
err = handleDNSError(err, "A")
return nil, err
}
newAddrs := make([]resolver.Address, 0, len(addrs))
for _, a := range addrs {
ip, ok := formatIP(a)
if !ok {
return nil, fmt.Errorf("dns: error parsing A record IP address %v", a)
}
addr := ip + ":" + d.port
newAddrs = append(newAddrs, resolver.Address{Addr: addr})
}
return newAddrs, nil
}
func (d *dnsResolver) lookup() (*resolver.State, error) {
ctx, cancel := context.WithTimeout(d.ctx, ResolvingTimeout)
defer cancel()
srv, srvErr := d.lookupSRV(ctx)
addrs, hostErr := d.lookupHost(ctx)
if hostErr != nil && (srvErr != nil || len(srv) == 0) {
return nil, hostErr
}
state := resolver.State{Addresses: addrs}
if len(srv) > 0 {
state = grpclbstate.Set(state, &grpclbstate.State{BalancerAddresses: srv})
}
if !d.disableServiceConfig {
state.ServiceConfig = d.lookupTXT(ctx)
}
return &state, nil
}
// 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 "", "", internal.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 "", "", internal.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 {
logger.Warningf("dns: error parsing service config json: %v", err)
return ""
}
cliHostname, err := os.Hostname()
if err != nil {
logger.Warningf("dns: error getting client hostname: %v", 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
}