@ -21,6 +21,7 @@ import (
"time"
dns "golang.org/x/net/dns/dnsmessage"
"tailscale.com/control/controlknobs"
"tailscale.com/envknob"
"tailscale.com/net/dns/publicdns"
"tailscale.com/net/dnscache"
@ -68,6 +69,10 @@ const (
// DNS queries to the "fallback" DNS server IP for a known provider
// (e.g. how long to wait to query Google's 8.8.4.4 after 8.8.8.8).
wellKnownHostBackupDelay = 200 * time . Millisecond
// tcpQueryTimeout is the timeout for a DNS query performed over TCP.
// It matches the default 5sec timeout of the 'dig' utility.
tcpQueryTimeout = 5 * time . Second
)
// txid identifies a DNS transaction.
@ -180,6 +185,8 @@ type forwarder struct {
linkSel ForwardLinkSelector // TODO(bradfitz): remove this when tsdial.Dialer absorbs it
dialer * tsdial . Dialer
controlKnobs * controlknobs . Knobs // or nil
ctx context . Context // good until Close
ctxCancel context . CancelFunc // closes ctx
@ -206,12 +213,13 @@ func init() {
rand . Seed ( time . Now ( ) . UnixNano ( ) )
}
func newForwarder ( logf logger . Logf , netMon * netmon . Monitor , linkSel ForwardLinkSelector , dialer * tsdial . Dialer * forwarder {
func newForwarder ( logf logger . Logf , netMon * netmon . Monitor , linkSel ForwardLinkSelector , dialer * tsdial . Dialer , knobs * controlknobs . Knobs ) * forwarder {
f := & forwarder {
logf : logger . WithPrefix ( logf , "forward: " ) ,
netMon : netMon ,
linkSel : linkSel ,
dialer : dialer ,
controlKnobs : knobs ,
}
f . ctx , f . ctxCancel = context . WithCancel ( context . Background ( ) )
return f
@ -443,7 +451,10 @@ func (f *forwarder) sendDoH(ctx context.Context, urlBase string, c *http.Client,
return res , err
}
var verboseDNSForward = envknob . RegisterBool ( "TS_DEBUG_DNS_FORWARD_SEND" )
var (
verboseDNSForward = envknob . RegisterBool ( "TS_DEBUG_DNS_FORWARD_SEND" )
skipTCPRetry = envknob . RegisterBool ( "TS_DNS_FORWARD_SKIP_TCP_RETRY" )
)
// send sends packet to dst. It is best effort.
//
@ -477,10 +488,49 @@ func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDe
return nil , fmt . Errorf ( "tls:// resolvers not supported yet" )
}
return f . sendUDP ( ctx , fq , rr )
ret , err = f . sendUDP ( ctx , fq , rr )
if err != nil {
return nil , err
}
if ! truncatedFlagSet ( ret ) {
// Successful, non-truncated response; return it.
return ret , nil
}
if fq . family == "udp" {
// If this is a UDP query, return it regardless of whether the
// response is truncated or not; the client can retry
// communicating with tailscaled over TCP. There's no point
// falling back to TCP for a truncated query if we can't return
// the results to the client.
return ret , nil
}
if skipTCPRetry ( ) || ( f . controlKnobs != nil && f . controlKnobs . DisableDNSForwarderTCPRetries . Load ( ) ) {
// Envknob or control knob disabled the TCP retry behaviour;
// just return what we have.
return ret , nil
}
// Don't retry if our context is done.
if err := ctx . Err ( ) ; err != nil {
return nil , err
}
// Retry over TCP, best-effort; return the truncated UDP response if we
// cannot query via TCP.
if ret2 , err2 := f . sendTCP ( ctx , fq , rr ) ; err2 == nil {
if verboseDNSForward ( ) {
f . logf ( "forwarder.send(%q): successfully retried via TCP" , rr . name . Addr )
}
return ret2 , nil
} else if verboseDNSForward ( ) {
f . logf ( "forwarder.send(%q): could not retry via TCP: %v" , rr . name . Addr , err2 )
}
return ret , nil
}
var errServerFailure = errors . New ( "response code indicates server issue" )
var errTxIDMismatch = errors . New ( "txid doesn't match" )
func ( f * forwarder ) sendUDP ( ctx context . Context , fq * forwardQuery , rr resolverAndDelay ) ( ret [ ] byte , err error ) {
ipp , ok := rr . name . IPPort ( )
@ -545,7 +595,7 @@ func (f *forwarder) sendUDP(ctx context.Context, fq *forwardQuery, rr resolverAn
txid := getTxID ( out )
if txid != fq . txid {
metricDNSFwdUDPErrorTxID . Add ( 1 )
return nil , err ors. New ( "txid doesn't match" )
return nil , err TxIDMismatch
}
rcode := getRCode ( out )
// don't forward transient errors back to the client when the server fails
@ -577,6 +627,92 @@ func (f *forwarder) sendUDP(ctx context.Context, fq *forwardQuery, rr resolverAn
return out , nil
}
func ( f * forwarder ) sendTCP ( ctx context . Context , fq * forwardQuery , rr resolverAndDelay ) ( ret [ ] byte , err error ) {
ipp , ok := rr . name . IPPort ( )
if ! ok {
metricDNSFwdErrorType . Add ( 1 )
return nil , fmt . Errorf ( "unrecognized resolver type %q" , rr . name . Addr )
}
metricDNSFwdTCP . Add ( 1 )
ctx = sockstats . WithSockStats ( ctx , sockstats . LabelDNSForwarderTCP , f . logf )
// Specify the exact family to work around https://github.com/golang/go/issues/52264
tcpFam := "tcp4"
if ipp . Addr ( ) . Is6 ( ) {
tcpFam = "tcp6"
}
ctx , cancel := context . WithTimeout ( ctx , tcpQueryTimeout )
defer cancel ( )
conn , err := f . dialer . SystemDial ( ctx , tcpFam , ipp . String ( ) )
if err != nil {
return nil , err
}
defer conn . Close ( )
fq . closeOnCtxDone . Add ( conn )
defer fq . closeOnCtxDone . Remove ( conn )
ctxOrErr := func ( err2 error ) ( [ ] byte , error ) {
if err := ctx . Err ( ) ; err != nil {
return nil , err
}
return nil , err2
}
// Write the query to the server.
query := make ( [ ] byte , len ( fq . packet ) + 2 )
binary . BigEndian . PutUint16 ( query , uint16 ( len ( fq . packet ) ) )
copy ( query [ 2 : ] , fq . packet )
if _ , err := conn . Write ( query ) ; err != nil {
metricDNSFwdTCPErrorWrite . Add ( 1 )
return ctxOrErr ( err )
}
metricDNSFwdTCPWrote . Add ( 1 )
// Read the header length back from the server
var length uint16
if err := binary . Read ( conn , binary . BigEndian , & length ) ; err != nil {
metricDNSFwdTCPErrorRead . Add ( 1 )
return ctxOrErr ( err )
}
// Now read the response
out := make ( [ ] byte , length )
n , err := io . ReadFull ( conn , out )
if err != nil {
metricDNSFwdTCPErrorRead . Add ( 1 )
return ctxOrErr ( err )
}
if n < int ( length ) {
f . logf ( "sendTCP: packet too small (%d bytes)" , n )
return nil , io . ErrUnexpectedEOF
}
out = out [ : n ]
txid := getTxID ( out )
if txid != fq . txid {
metricDNSFwdTCPErrorTxID . Add ( 1 )
return nil , errTxIDMismatch
}
rcode := getRCode ( out )
// don't forward transient errors back to the client when the server fails
if rcode == dns . RCodeServerFailure {
f . logf ( "sendTCP: response code indicating server failure: %d" , rcode )
metricDNSFwdTCPErrorServer . Add ( 1 )
return nil , errServerFailure
}
// TODO(andrew): do we need to do this?
//clampEDNSSize(out, maxResponseBytes)
metricDNSFwdTCPSuccess . Add ( 1 )
return out , nil
}
// resolvers returns the resolvers to use for domain.
func ( f * forwarder ) resolvers ( domain dnsname . FQDN ) [ ] resolverAndDelay {
f . mu . Lock ( )
@ -601,6 +737,7 @@ func (f *forwarder) resolvers(domain dnsname.FQDN) []resolverAndDelay {
type forwardQuery struct {
txid txid
packet [ ] byte
family string // "tcp" or "udp"
// closeOnCtxDone lets send register values to Close if the
// caller's ctx expires. This avoids send from allocating its
@ -686,6 +823,7 @@ func (f *forwarder) forwardWithDestChan(ctx context.Context, query packet, respo
fq := & forwardQuery {
txid : getTxID ( query . bs ) ,
packet : query . bs ,
family : query . family ,
closeOnCtxDone : new ( closePool ) ,
}
defer fq . closeOnCtxDone . Close ( )
@ -727,7 +865,7 @@ func (f *forwarder) forwardWithDestChan(ctx context.Context, query packet, respo
case <- ctx . Done ( ) :
metricDNSFwdErrorContext . Add ( 1 )
return ctx . Err ( )
case responseChan <- packet { v , query . } :
case responseChan <- packet { v , query . family, query . addr} :
metricDNSFwdSuccess . Add ( 1 )
return nil
}
Andrew Dunham
1 year ago