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net: add Resolver.StrictErrors
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When LookupIP is performing multiple subqueries, this option causes a
timeout/servfail affecting a single query to abort the whole operation,
instead of returning a partial (IPv4/IPv6-only) result.

Similarly, operations that walk the DNS search list will also abort when
encountering one of these errors.

Fixes #17448

Change-Id: Ice22e4aceb555c5a80d19bd1fde8b8fe87ac9517
Reviewed-on: https://go-review.googlesource.com/32572
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
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@bradfitz
Paul Marks authored and bradfitz committed Mar 9, 2017
1 parent b0e91d8 commit bfc164c
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Showing 5 changed files with 393 additions and 42 deletions.
50 changes: 36 additions & 14 deletions src/net/dnsclient_unix.go
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Original file line number Diff line number Diff line change
Expand Up @@ -200,6 +200,11 @@ func tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype uint16)
if nerr, ok := err.(Error); ok && nerr.Timeout() {
lastErr.(*DNSError).IsTimeout = true
}
// Set IsTemporary for socket-level errors. Note that this flag
// may also be used to indicate a SERVFAIL response.
if _, ok := err.(*OpError); ok {
lastErr.(*DNSError).IsTemporary = true
}
continue
}
// libresolv continues to the next server when it receives
Expand Down Expand Up @@ -314,7 +319,7 @@ func (conf *resolverConfig) releaseSema() {
<-conf.ch
}

func lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
func (r *Resolver) lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
if !isDomainName(name) {
// We used to use "invalid domain name" as the error,
// but that is a detail of the specific lookup mechanism.
Expand All @@ -332,6 +337,11 @@ func lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs [
if err == nil {
break
}
if nerr, ok := err.(Error); ok && nerr.Temporary() && r.StrictErrors {
// If we hit a temporary error with StrictErrors enabled,
// stop immediately instead of trying more names.
break
}
}
if err, ok := err.(*DNSError); ok {
// Show original name passed to lookup, not suffixed one.
Expand Down Expand Up @@ -432,19 +442,19 @@ func (o hostLookupOrder) String() string {
// Normally we let cgo use the C library resolver instead of
// depending on our lookup code, so that Go and C get the same
// answers.
func goLookupHost(ctx context.Context, name string) (addrs []string, err error) {
return goLookupHostOrder(ctx, name, hostLookupFilesDNS)
func (r *Resolver) goLookupHost(ctx context.Context, name string) (addrs []string, err error) {
return r.goLookupHostOrder(ctx, name, hostLookupFilesDNS)
}

func goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []string, err error) {
func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []string, err error) {
if order == hostLookupFilesDNS || order == hostLookupFiles {
// Use entries from /etc/hosts if they match.
addrs = lookupStaticHost(name)
if len(addrs) > 0 || order == hostLookupFiles {
return
}
}
ips, _, err := goLookupIPCNAMEOrder(ctx, name, order)
ips, _, err := r.goLookupIPCNAMEOrder(ctx, name, order)
if err != nil {
return
}
Expand All @@ -470,13 +480,13 @@ func goLookupIPFiles(name string) (addrs []IPAddr) {

// goLookupIP is the native Go implementation of LookupIP.
// The libc versions are in cgo_*.go.
func goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
func (r *Resolver) goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
order := systemConf().hostLookupOrder(host)
addrs, _, err = goLookupIPCNAMEOrder(ctx, host, order)
addrs, _, err = r.goLookupIPCNAMEOrder(ctx, host, order)
return
}

func goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, cname string, err error) {
func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, cname string, err error) {
if order == hostLookupFilesDNS || order == hostLookupFiles {
addrs = goLookupIPFiles(name)
if len(addrs) > 0 || order == hostLookupFiles {
Expand Down Expand Up @@ -506,11 +516,16 @@ func goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrde
lane <- racer{cname, rrs, err}
}(qtype)
}
hitStrictError := false
for range qtypes {
racer := <-lane
if racer.error != nil {
// Prefer error for original name.
if lastErr == nil || fqdn == name+"." {
if nerr, ok := racer.error.(Error); ok && nerr.Temporary() && r.StrictErrors {
// This error will abort the nameList loop.
hitStrictError = true
lastErr = racer.error
} else if lastErr == nil || fqdn == name+"." {
// Prefer error for original name.
lastErr = racer.error
}
continue
Expand All @@ -520,6 +535,13 @@ func goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrde
cname = racer.cname
}
}
if hitStrictError {
// If either family hit an error with StrictErrors enabled,
// discard all addresses. This ensures that network flakiness
// cannot turn a dualstack hostname IPv4/IPv6-only.
addrs = nil
break
}
if len(addrs) > 0 {
break
}
Expand All @@ -543,9 +565,9 @@ func goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrde
}

// goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
func goLookupCNAME(ctx context.Context, host string) (cname string, err error) {
func (r *Resolver) goLookupCNAME(ctx context.Context, host string) (cname string, err error) {
order := systemConf().hostLookupOrder(host)
_, cname, err = goLookupIPCNAMEOrder(ctx, host, order)
_, cname, err = r.goLookupIPCNAMEOrder(ctx, host, order)
return
}

Expand All @@ -554,7 +576,7 @@ func goLookupCNAME(ctx context.Context, host string) (cname string, err error) {
// only if cgoLookupPTR is the stub in cgo_stub.go).
// Normally we let cgo use the C library resolver instead of depending
// on our lookup code, so that Go and C get the same answers.
func goLookupPTR(ctx context.Context, addr string) ([]string, error) {
func (r *Resolver) goLookupPTR(ctx context.Context, addr string) ([]string, error) {
names := lookupStaticAddr(addr)
if len(names) > 0 {
return names, nil
Expand All @@ -563,7 +585,7 @@ func goLookupPTR(ctx context.Context, addr string) ([]string, error) {
if err != nil {
return nil, err
}
_, rrs, err := lookup(ctx, arpa, dnsTypePTR)
_, rrs, err := r.lookup(ctx, arpa, dnsTypePTR)
if err != nil {
return nil, err
}
Expand Down
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