osbase/net/dns/forward: add DNS forward handler
This adds a DNS server handler for forwarding queries to upstream DNS
resolvers, with a built-in cache. The implementation is partially based
on CoreDNS. The proxy, cache and up packages are only lightly modified.
The forward package itself however is mostly new code. Unlike CoreDNS,
it supports changing upstreams at runtime, and has integrated caching
and answer order randomization.
Some improvements over CoreDNS:
- Concurrent identical queries only result in one upstream query.
- In case of errors, Extended DNS Errors are added to replies.
- Very large replies are not stored in the cache to avoid using too much
memory.
Change-Id: I42294ae4997d621a6e55c98e46a04874eab75c99
Reviewed-on: https://review.monogon.dev/c/monogon/+/3258
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/osbase/net/dns/forward/proxy/health.go b/osbase/net/dns/forward/proxy/health.go
new file mode 100644
index 0000000..4630cdc
--- /dev/null
+++ b/osbase/net/dns/forward/proxy/health.go
@@ -0,0 +1,127 @@
+package proxy
+
+// Taken and modified from CoreDNS, under Apache 2.0.
+
+import (
+ "crypto/tls"
+ "sync/atomic"
+ "time"
+
+ "github.com/miekg/dns"
+)
+
+// HealthChecker checks the upstream health.
+type HealthChecker interface {
+ Check(*Proxy) error
+ SetTLSConfig(*tls.Config)
+ GetTLSConfig() *tls.Config
+ SetRecursionDesired(bool)
+ GetRecursionDesired() bool
+ SetDomain(domain string)
+ GetDomain() string
+ SetTCPTransport()
+ GetReadTimeout() time.Duration
+ SetReadTimeout(time.Duration)
+ GetWriteTimeout() time.Duration
+ SetWriteTimeout(time.Duration)
+}
+
+// dnsHc is a health checker for a DNS endpoint (DNS, and DoT).
+type dnsHc struct {
+ c *dns.Client
+ recursionDesired bool
+ domain string
+}
+
+// NewHealthChecker returns a new HealthChecker.
+func NewHealthChecker(recursionDesired bool, domain string) HealthChecker {
+ c := new(dns.Client)
+ c.Net = "udp"
+ c.ReadTimeout = 1 * time.Second
+ c.WriteTimeout = 1 * time.Second
+
+ return &dnsHc{
+ c: c,
+ recursionDesired: recursionDesired,
+ domain: domain,
+ }
+}
+
+func (h *dnsHc) SetTLSConfig(cfg *tls.Config) {
+ h.c.Net = "tcp-tls"
+ h.c.TLSConfig = cfg
+}
+
+func (h *dnsHc) GetTLSConfig() *tls.Config {
+ return h.c.TLSConfig
+}
+
+func (h *dnsHc) SetRecursionDesired(recursionDesired bool) {
+ h.recursionDesired = recursionDesired
+}
+func (h *dnsHc) GetRecursionDesired() bool {
+ return h.recursionDesired
+}
+
+func (h *dnsHc) SetDomain(domain string) {
+ h.domain = domain
+}
+func (h *dnsHc) GetDomain() string {
+ return h.domain
+}
+
+func (h *dnsHc) SetTCPTransport() {
+ h.c.Net = "tcp"
+}
+
+func (h *dnsHc) GetReadTimeout() time.Duration {
+ return h.c.ReadTimeout
+}
+
+func (h *dnsHc) SetReadTimeout(t time.Duration) {
+ h.c.ReadTimeout = t
+}
+
+func (h *dnsHc) GetWriteTimeout() time.Duration {
+ return h.c.WriteTimeout
+}
+
+func (h *dnsHc) SetWriteTimeout(t time.Duration) {
+ h.c.WriteTimeout = t
+}
+
+// For HC, we send to . IN NS +[no]rec message to the upstream.
+// Dial timeouts and empty replies are considered fails,
+// basically anything else constitutes a healthy upstream.
+
+// Check is used as the up.Func in the up.Probe.
+func (h *dnsHc) Check(p *Proxy) error {
+ err := h.send(p.addr)
+ if err != nil {
+ healthcheckFailureCount.WithLabelValues(p.addr).Inc()
+ p.incrementFails()
+ return err
+ }
+
+ atomic.StoreUint32(&p.fails, 0)
+ return nil
+}
+
+func (h *dnsHc) send(addr string) error {
+ ping := new(dns.Msg)
+ ping.SetQuestion(h.domain, dns.TypeNS)
+ ping.MsgHdr.RecursionDesired = h.recursionDesired
+ ping.SetEdns0(AdvertiseUDPSize, false)
+
+ m, _, err := h.c.Exchange(ping, addr)
+ // If we got a header, we're alright,
+ // basically only care about I/O errors 'n stuff.
+ if err != nil && m != nil {
+ // Silly check, something sane came back.
+ if m.Response || m.Opcode == dns.OpcodeQuery {
+ err = nil
+ }
+ }
+
+ return err
+}