@ -22,22 +22,85 @@ import (
"sync/atomic"
"time"
"golang.org/x/exp/slices"
"tailscale.com/atomicfile"
"tailscale.com/envknob"
"tailscale.com/net/dns/recursive"
"tailscale.com/net/netmon"
"tailscale.com/net/netns"
"tailscale.com/net/tlsdial"
"tailscale.com/net/tshttpproxy"
"tailscale.com/tailcfg"
"tailscale.com/types/logger"
"tailscale.com/util/clientmetric"
"tailscale.com/util/slicesx"
)
var disableRecursiveResolver = envknob . RegisterBool ( "TS_DNSFALLBACK_DISABLE_RECURSIVE_RESOLVER" )
// MakeLookupFunc creates a function that can be used to resolve hostnames
// (e.g. as a LookupIPFallback from dnscache.Resolver).
// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
func MakeLookupFunc ( logf logger . Logf , netMon * netmon . Monitor ) func ( ctx context . Context , host string ) ( [ ] netip . Addr , error ) {
return func ( ctx context . Context , host string ) ( [ ] netip . Addr , error ) {
return lookup ( ctx , host , logf , netMon )
if disableRecursiveResolver ( ) {
return lookup ( ctx , host , logf , netMon )
}
addrsCh := make ( chan [ ] netip . Addr , 1 )
// Run the recursive resolver in the background so we can
// compare the results.
go func ( ) {
logf := logger . WithPrefix ( logf , "recursive: " )
// Ensure that we catch panics while we're testing this
// code path; this should never panic, but we don't
// want to take down the process by having the panic
// propagate to the top of the goroutine's stack and
// then terminate.
defer func ( ) {
if r := recover ( ) ; r != nil {
logf ( "bootstrap DNS: recovered panic: %v" , r )
metricRecursiveErrors . Add ( 1 )
}
} ( )
resolver := recursive . Resolver {
Dialer : netns . NewDialer ( logf , netMon ) ,
Logf : logf ,
}
addrs , minTTL , err := resolver . Resolve ( ctx , host )
if err != nil {
logf ( "error using recursive resolver: %v" , err )
metricRecursiveErrors . Add ( 1 )
return
}
slices . SortFunc ( addrs , func ( a , b netip . Addr ) bool { return a . Less ( b ) } )
// Wait for a response from the main function
oldAddrs := <- addrsCh
slices . SortFunc ( oldAddrs , func ( a , b netip . Addr ) bool { return a . Less ( b ) } )
matches := slices . Equal ( addrs , oldAddrs )
logf ( "bootstrap DNS comparison: matches=%v oldAddrs=%v addrs=%v minTTL=%v" , matches , oldAddrs , addrs , minTTL )
if matches {
metricRecursiveMatches . Add ( 1 )
} else {
metricRecursiveMismatches . Add ( 1 )
}
} ( )
addrs , err := lookup ( ctx , host , logf , netMon )
if err != nil {
addrsCh <- nil
return nil , err
}
addrsCh <- slices . Clone ( addrs )
return addrs , nil
}
}
@ -254,3 +317,9 @@ func SetCachePath(path string, logf logger.Logf) {
cachedDERPMap . Store ( dm )
logf ( "[v2] dnsfallback: SetCachePath loaded cached DERP map" )
}
var (
metricRecursiveMatches = clientmetric . NewCounter ( "dnsfallback_recursive_matches" )
metricRecursiveMismatches = clientmetric . NewCounter ( "dnsfallback_recursive_mismatches" )
metricRecursiveErrors = clientmetric . NewCounter ( "dnsfallback_recursive_errors" )
)
Andrew Dunham
12 months ago