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package zdns
import (
"fmt"
"io"
"log"
"net"
"net/http"
"net/url"
"os"
"sync"
"time"
"github.com/cenkalti/backoff/v4"
"github.com/mpolden/zdns/dns"
"github.com/mpolden/zdns/hosts"
)
const (
// HijackZero returns the zero IP address to matching requests.
HijackZero = iota
// HijackEmpty returns an empty answer to matching requests.
HijackEmpty
// HijackHosts returns the value of the hoss entry to matching request.
HijackHosts
)
// A Server defines parameters for running a DNS server.
type Server struct {
Config Config
hosts hosts.Hosts
proxy *dns.Proxy
done chan bool
mu sync.RWMutex
httpClient *http.Client
}
// NewServer returns a new server configured according to config.
func NewServer(proxy *dns.Proxy, config Config) (*Server, error) {
server := &Server{
Config: config,
done: make(chan bool, 1),
proxy: proxy,
httpClient: &http.Client{Timeout: 10 * time.Second},
}
proxy.Handler = server.hijack
// Periodically refresh hosts
if interval := config.DNS.refreshInterval; interval > 0 {
go server.reloadHosts(interval)
}
// Load initial hosts
go server.loadHosts()
return server, nil
}
func (s *Server) httpGet(url string) (io.ReadCloser, error) {
var body io.ReadCloser
policy := backoff.NewExponentialBackOff()
policy.MaxInterval = 2 * time.Second
policy.MaxElapsedTime = 30 * time.Second
err := backoff.Retry(func() error {
res, err := s.httpClient.Get(url)
if err == nil {
body = res.Body
}
return err
}, policy)
if err != nil {
return nil, err
}
return body, nil
}
func (s *Server) readHosts(name string) (hosts.Hosts, error) {
url, err := url.Parse(name)
if err != nil {
return nil, err
}
var rc io.ReadCloser
switch url.Scheme {
case "file":
f, err := os.Open(url.Path)
if err != nil {
return nil, err
}
rc = f
case "http", "https":
rc, err = s.httpGet(url.String())
if err != nil {
return nil, err
}
default:
return nil, fmt.Errorf("%s: invalid scheme: %s", url, url.Scheme)
}
hosts, err := hosts.Parse(rc)
if err1 := rc.Close(); err == nil {
err = err1
}
return hosts, err
}
func nonFqdn(s string) string {
sz := len(s)
if sz > 0 && s[sz-1:] == "." {
return s[:sz-1]
}
return s
}
func (s *Server) reloadHosts(interval time.Duration) {
for {
select {
case <-s.done:
return
case <-time.After(interval):
s.loadHosts()
}
}
}
func (s *Server) loadHosts() {
hs := make(hosts.Hosts)
for _, h := range s.Config.Hosts {
src := "inline hosts"
hs1 := h.hosts
if h.URL != "" {
src = h.URL
var err error
hs1, err = s.readHosts(h.URL)
if err != nil {
log.Printf("failed to read hosts from %s: %s", h.URL, err)
continue
}
}
if h.Hijack {
for name, ipAddrs := range hs1 {
hs[name] = ipAddrs
}
log.Printf("loaded %d hosts from %s", len(hs1), src)
} else {
removed := 0
for hostToRemove := range hs1 {
if _, ok := hs.Get(hostToRemove); ok {
removed++
hs.Del(hostToRemove)
}
}
if removed > 0 {
log.Printf("removed %d hosts from %s", removed, src)
}
}
}
s.mu.Lock()
s.hosts = hs
s.mu.Unlock()
log.Printf("loaded %d hosts in total", len(hs))
}
// Reload updates hosts entries of Server s.
func (s *Server) Reload() { s.loadHosts() }
// Close terminates all active operations and shuts down the DNS server.
func (s *Server) Close() error {
s.done <- true
return nil
}
func (s *Server) hijack(r *dns.Request) *dns.Reply {
if r.Type != dns.TypeA && r.Type != dns.TypeAAAA {
return nil // Type not applicable
}
s.mu.RLock()
ipAddrs, ok := s.hosts.Get(nonFqdn(r.Name))
s.mu.RUnlock()
if !ok {
return nil // No match
}
switch s.Config.DNS.hijackMode {
case HijackZero:
switch r.Type {
case dns.TypeA:
return dns.ReplyA(r.Name, net.IPv4zero)
case dns.TypeAAAA:
return dns.ReplyAAAA(r.Name, net.IPv6zero)
}
case HijackEmpty:
return &dns.Reply{}
case HijackHosts:
var ipv4Addr []net.IP
var ipv6Addr []net.IP
for _, ipAddr := range ipAddrs {
if ipAddr.IP.To4() == nil {
ipv6Addr = append(ipv6Addr, ipAddr.IP)
} else {
ipv4Addr = append(ipv4Addr, ipAddr.IP)
}
}
switch r.Type {
case dns.TypeA:
return dns.ReplyA(r.Name, ipv4Addr...)
case dns.TypeAAAA:
return dns.ReplyAAAA(r.Name, ipv6Addr...)
}
}
return nil
}
// ListenAndServe starts a server on configured address and protocol.
func (s *Server) ListenAndServe() error {
log.Printf("dns server listening on %s [%s]", s.Config.DNS.Listen, s.Config.DNS.Protocol)
return s.proxy.ListenAndServe(s.Config.DNS.Listen, s.Config.DNS.Protocol)
}
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