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net/dns/resolver: fall back to IPv6 for well-known DoH servers if v4 fails

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Should help with IPv6-only environments when the tailnet admin
only specified IPv4 DNS IPs.

See https://github.com/tailscale/tailscale/issues/2447#issuecomment-884188562

Co-Author: Adrian Dewhurst <adrian@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Brad Fitzpatrick 2021-07-21 12:43:06 -07:00
parent 74eee4de1c
commit 3daf27eaad
2 changed files with 39 additions and 2 deletions
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13
net/dns/resolver/doh_test.go
View File

@ -81,3 +81,16 @@ func TestDoH(t *testing.T) {
})
}
}
func TestDoHV6Fallback(t *testing.T) {
for ip, base := range knownDoH {
if ip.Is4() {
ip6, ok := dohV6(base)
if !ok {
t.Errorf("no v6 DoH known for %v", ip)
} else if !ip6.Is6() {
t.Errorf("dohV6(%q) returned non-v6 address %v", base, ip6)
}
}
}
}

28
net/dns/resolver/forwarder.go
View File

@ -243,7 +243,16 @@ func (f *forwarder) getDoHClient(ip netaddr.IP) (urlBase string, c *http.Client,
if !strings.HasPrefix(netw, "tcp") {
return nil, fmt.Errorf("unexpected network %q", netw)
}
return nsDialer.DialContext(ctx, "tcp", net.JoinHostPort(ip.String(), "443"))
c, err := nsDialer.DialContext(ctx, "tcp", net.JoinHostPort(ip.String(), "443"))
// If v4 failed, try an equivalent v6 also in the time remaining.
if err != nil && ctx.Err() == nil {
if ip6, ok := dohV6(urlBase); ok && ip.Is4() {
if c6, err := nsDialer.DialContext(ctx, "tcp", net.JoinHostPort(ip6.String(), "443")); err == nil {
return c6, nil
}
}
}
return c, err
},
},
}
@ -509,7 +518,22 @@ func (p *closePool) Close() error {
var knownDoH = map[netaddr.IP]string{}
func addDoH(ip, base string) { knownDoH[netaddr.MustParseIP(ip)] = base }
var dohIPsOfBase = map[string][]netaddr.IP{}
func addDoH(ipStr, base string) {
ip := netaddr.MustParseIP(ipStr)
knownDoH[ip] = base
dohIPsOfBase[base] = append(dohIPsOfBase[base], ip)
}
func dohV6(base string) (ip netaddr.IP, ok bool) {
for _, ip := range dohIPsOfBase[base] {
if ip.Is6() {
return ip, true
}
}
return ip, false
}
func init() {
// Cloudflare