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7cf8ec8108
We still try the host's x509 roots first, but if that fails (like if the host is old), we fall back to using LetsEncrypt's root and retrying with that. tlsdial was used in the three main places: logs, control, DERP. But it was missing in dnsfallback. So added it there too, so we can run fine now on a machine with no DNS config and no root CAs configured. Also, move SSLKEYLOGFILE support out of DERP. tlsdial is the logical place for that support. Fixes #1609 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
142 lines
4.2 KiB
Go
142 lines
4.2 KiB
Go
// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:generate go run update-dns-fallbacks.go
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// Package dnsfallback contains a DNS fallback mechanism
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// for starting up Tailscale when the system DNS is broken or otherwise unavailable.
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package dnsfallback
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import (
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"context"
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_ "embed"
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"encoding/json"
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"errors"
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"fmt"
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"log"
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"math/rand"
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"net"
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"net/http"
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"net/url"
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"time"
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"inet.af/netaddr"
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"tailscale.com/net/netns"
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"tailscale.com/net/tlsdial"
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"tailscale.com/net/tshttpproxy"
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"tailscale.com/tailcfg"
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)
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func Lookup(ctx context.Context, host string) ([]netaddr.IP, error) {
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type nameIP struct {
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dnsName string
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ip netaddr.IP
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}
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dm := getDERPMap()
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var cands4, cands6 []nameIP
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for _, dr := range dm.Regions {
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for _, n := range dr.Nodes {
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if ip, err := netaddr.ParseIP(n.IPv4); err == nil {
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cands4 = append(cands4, nameIP{n.HostName, ip})
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}
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if ip, err := netaddr.ParseIP(n.IPv6); err == nil {
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cands6 = append(cands6, nameIP{n.HostName, ip})
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}
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}
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}
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rand.Shuffle(len(cands4), func(i, j int) { cands4[i], cands4[j] = cands4[j], cands4[i] })
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rand.Shuffle(len(cands6), func(i, j int) { cands6[i], cands6[j] = cands6[j], cands6[i] })
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const maxCands = 6
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var cands []nameIP // up to maxCands alternating v4/v6 as long as we have both
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for (len(cands4) > 0 || len(cands6) > 0) && len(cands) < maxCands {
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if len(cands4) > 0 {
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cands = append(cands, cands4[0])
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cands4 = cands4[1:]
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}
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if len(cands6) > 0 {
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cands = append(cands, cands6[0])
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cands6 = cands6[1:]
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}
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}
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if len(cands) == 0 {
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return nil, fmt.Errorf("no DNS fallback options for %q", host)
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}
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for _, cand := range cands {
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if err := ctx.Err(); err != nil {
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return nil, err
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}
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log.Printf("trying bootstrapDNS(%q, %q) for %q ...", cand.dnsName, cand.ip, host)
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ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
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defer cancel()
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dm, err := bootstrapDNSMap(ctx, cand.dnsName, cand.ip, host)
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if err != nil {
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log.Printf("bootstrapDNS(%q, %q) for %q error: %v", cand.dnsName, cand.ip, host, err)
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continue
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}
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if ips := dm[host]; len(ips) > 0 {
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log.Printf("bootstrapDNS(%q, %q) for %q = %v", cand.dnsName, cand.ip, host, ips)
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return ips, nil
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}
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}
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if err := ctx.Err(); err != nil {
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return nil, err
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}
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return nil, fmt.Errorf("no DNS fallback candidates remain for %q", host)
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}
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// serverName and serverIP of are, say, "derpN.tailscale.com".
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// queryName is the name being sought (e.g. "controlplane.tailscale.com"), passed as hint.
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func bootstrapDNSMap(ctx context.Context, serverName string, serverIP netaddr.IP, queryName string) (dnsMap, error) {
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dialer := netns.NewDialer()
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tr := http.DefaultTransport.(*http.Transport).Clone()
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tr.Proxy = tshttpproxy.ProxyFromEnvironment
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tr.DialContext = func(ctx context.Context, netw, addr string) (net.Conn, error) {
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return dialer.DialContext(ctx, "tcp", net.JoinHostPort(serverIP.String(), "443"))
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}
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tr.TLSClientConfig = tlsdial.Config(serverName, tr.TLSClientConfig)
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c := &http.Client{Transport: tr}
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req, err := http.NewRequestWithContext(ctx, "GET", "https://"+serverName+"/bootstrap-dns?q="+url.QueryEscape(queryName), nil)
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if err != nil {
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return nil, err
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}
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dm := make(dnsMap)
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res, err := c.Do(req)
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if err != nil {
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return nil, err
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}
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defer res.Body.Close()
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if res.StatusCode != 200 {
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return nil, errors.New(res.Status)
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}
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if err := json.NewDecoder(res.Body).Decode(&dm); err != nil {
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return nil, err
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}
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return dm, nil
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}
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// dnsMap is the JSON type returned by the DERP /bootstrap-dns handler:
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// https://derp10.tailscale.com/bootstrap-dns
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type dnsMap map[string][]netaddr.IP
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// getDERPMap returns some DERP map. The DERP servers also run a fallback
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// DNS server.
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func getDERPMap() *tailcfg.DERPMap {
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// TODO(bradfitz): try to read the last known DERP map from disk,
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// at say /var/lib/tailscale/derpmap.txt and write it when it changes,
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// and read it here.
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// But ultimately the fallback will be to use a copy baked into the binary,
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// which is this part:
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dm := new(tailcfg.DERPMap)
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if err := json.Unmarshal(staticDERPMapJSON, dm); err != nil {
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panic(err)
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}
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return dm
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}
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//go:embed dns-fallback-servers.json
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var staticDERPMapJSON []byte
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