cloudflared-mirror/connection/discovery.go

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package connection
import (
"context"
"crypto/tls"
"fmt"
"net"
"time"
"github.com/pkg/errors"
log "github.com/sirupsen/logrus"
)
const (
// Used to discover HA Warp servers
srvService = "warp"
srvProto = "tcp"
srvName = "cloudflarewarp.com"
// Used to fallback to DoT when we can't use the default resolver to
// discover HA Warp servers (GitHub issue #75).
dotServerName = "cloudflare-dns.com"
dotServerAddr = "1.1.1.1:853"
dotTimeout = time.Duration(15 * time.Second)
)
var friendlyDNSErrorLines = []string{
`Please try the following things to diagnose this issue:`,
` 1. ensure that cloudflarewarp.com is returning "warp" service records.`,
` Run your system's equivalent of: dig srv _warp._tcp.cloudflarewarp.com`,
` 2. ensure that your DNS resolver is not returning compressed SRV records.`,
` See GitHub issue https://github.com/golang/go/issues/27546`,
` For example, you could use Cloudflare's 1.1.1.1 as your resolver:`,
` https://developers.cloudflare.com/1.1.1.1/setting-up-1.1.1.1/`,
}
func ResolveEdgeIPs(logger *log.Logger, addresses []string) ([]*net.TCPAddr, error) {
if len(addresses) > 0 {
var tcpAddrs []*net.TCPAddr
for _, address := range addresses {
// Addresses specified (for testing, usually)
tcpAddr, err := net.ResolveTCPAddr("tcp", address)
if err != nil {
return nil, err
}
tcpAddrs = append(tcpAddrs, tcpAddr)
}
return tcpAddrs, nil
}
// HA service discovery lookup
_, addrs, err := net.LookupSRV(srvService, srvProto, srvName)
if err != nil {
// Try to fall back to DoT from Cloudflare directly.
//
// Note: Instead of DoT, we could also have used DoH. Either of these:
// - directly via the JSON API (https://1.1.1.1/dns-query?ct=application/dns-json&name=_warp._tcp.cloudflarewarp.com&type=srv)
// - indirectly via `tunneldns.NewUpstreamHTTPS()`
// But both of these cases miss out on a key feature from the stdlib:
// "The returned records are sorted by priority and randomized by weight within a priority."
// (https://golang.org/pkg/net/#Resolver.LookupSRV)
// Does this matter? I don't know. It may someday. Let's use DoT so we don't need to worry about it.
// See also: Go feature request for stdlib-supported DoH: https://github.com/golang/go/issues/27552
r := fallbackResolver(dotServerName, dotServerAddr)
ctx, cancel := context.WithTimeout(context.Background(), dotTimeout)
defer cancel()
_, fallbackAddrs, fallbackErr := r.LookupSRV(ctx, srvService, srvProto, srvName)
if fallbackErr != nil || len(fallbackAddrs) == 0 {
// use the original DNS error `err` in messages, not `fallbackErr`
logger.Errorln("Error looking up Cloudflare edge IPs: the DNS query failed:", err)
for _, s := range friendlyDNSErrorLines {
logger.Errorln(s)
}
return nil, errors.Wrap(err, "Could not lookup srv records on _warp._tcp.cloudflarewarp.com")
}
// Accept the fallback results and keep going
addrs = fallbackAddrs
}
var resolvedIPsPerCNAME [][]*net.TCPAddr
var lookupErr error
for _, addr := range addrs {
ips, err := ResolveSRVToTCP(addr)
if err != nil || len(ips) == 0 {
// don't return early, we might be able to resolve other addresses
lookupErr = err
continue
}
resolvedIPsPerCNAME = append(resolvedIPsPerCNAME, ips)
}
ips := FlattenServiceIPs(resolvedIPsPerCNAME)
if lookupErr == nil && len(ips) == 0 {
return nil, fmt.Errorf("Unknown service discovery error")
}
return ips, lookupErr
}
func ResolveSRVToTCP(srv *net.SRV) ([]*net.TCPAddr, error) {
ips, err := net.LookupIP(srv.Target)
if err != nil {
return nil, err
}
addrs := make([]*net.TCPAddr, len(ips))
for i, ip := range ips {
addrs[i] = &net.TCPAddr{IP: ip, Port: int(srv.Port)}
}
return addrs, nil
}
// FlattenServiceIPs transposes and flattens the input slices such that the
// first element of the n inner slices are the first n elements of the result.
func FlattenServiceIPs(ipsByService [][]*net.TCPAddr) []*net.TCPAddr {
var result []*net.TCPAddr
for len(ipsByService) > 0 {
filtered := ipsByService[:0]
for _, ips := range ipsByService {
if len(ips) == 0 {
// sanity check
continue
}
result = append(result, ips[0])
if len(ips) > 1 {
filtered = append(filtered, ips[1:])
}
}
ipsByService = filtered
}
return result
}
// Inspiration: https://github.com/artyom/dot/blob/master/dot.go
func fallbackResolver(serverName, serverAddress string) *net.Resolver {
return &net.Resolver{
PreferGo: true,
Dial: func(ctx context.Context, _ string, _ string) (net.Conn, error) {
var dialer net.Dialer
conn, err := dialer.DialContext(ctx, "tcp", serverAddress)
if err != nil {
return nil, err
}
tlsConfig := &tls.Config{ServerName: serverName}
return tls.Client(conn, tlsConfig), nil
},
}
}