Added dns resolver that uses miekg/dns package.

srv/dnsresolver.go

@@ -0,0 +1,138 @@
+package srv
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// DefaultResolvConfPath is a default resolv.conf file path that is used if
+// NewDNSResolverFromResolvFile() resolvConfFilePath is set to an empty string
+const DefaultResolvConfPath = "/etc/resolv.conf"
+
+// NewDNSResolver is a resolver that uses github.com/miekg/dns dns client
+// with a given DNS server list
+func NewDNSResolver(defaultTTL uint32, dnsServers []string) Resolver {
+	client := &dns.Client{}
+	return &dnsResolver{
+		client:     client,
+		dnsServers: dnsServers,
+		defaultTTL: defaultTTL,
+	}
+}
+
+// NewDNSResolverFromResolvFile is a resolver that uses github.com/miekg/dns dns client
+// and a provided resolv.conf file path ("" defaults to /etc/resolv.conf) to retrieve
+// available DNS servers
+func NewDNSResolverFromResolvFile(defaultTTL uint32, resolvConfFilePath string) (Resolver, error) {
+	if resolvConfFilePath == "" {
+		resolvConfFilePath = DefaultResolvConfPath
+	}
+	cfg, err := dns.ClientConfigFromFile(resolvConfFilePath)
+	if err != nil {
+		return nil, err
+	}
+
+	servers := make([]string, 0, len(cfg.Servers))
+	for _, s := range cfg.Servers {
+		ip := net.ParseIP(s)
+
+		// handle IPv6
+		if ip != nil && ip.To4() != nil {
+			servers = append(servers, fmt.Sprintf("%s:%s", s, cfg.Port))
+		} else {
+			servers = append(servers, fmt.Sprintf("[%s]:%s", s, cfg.Port))
+		}
+	}
+
+	client := &dns.Client{}
+	return &dnsResolver{
+		client:     client,
+		dnsServers: servers,
+		defaultTTL: defaultTTL,
+	}, nil
+}
+
+type dnsResolver struct {
+	client     *dns.Client
+	dnsServers []string
+	defaultTTL uint32
+}
+
+func (r *dnsResolver) Lookup(name string) ([]*Target, error) {
+	var (
+		tgs []*Target
+		err error
+	)
+	for _, rs := range r.dnsServers {
+		tgs, err = r.resolve(rs, name)
+		if err != nil {
+			continue
+		}
+
+		if len(tgs) > 0 {
+			break
+		}
+	}
+
+	// got error during resolve (so return the last one)
+	if err != nil {
+		return nil, err
+	}
+
+	// no entries found
+	if len(tgs) == 0 {
+		return nil, errors.New("failed resolving hostnames for SRV entries")
+	}
+
+	return tgs, nil
+}
+
+func (r *dnsResolver) resolve(server string, name string) ([]*Target, error) {
+	msg := &dns.Msg{}
+	msg.SetQuestion(dns.Fqdn(name), dns.TypeSRV)
+
+	resp, _, err := r.client.Exchange(msg, server)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(resp.Answer) == 0 {
+		return nil, nil
+	}
+
+	// for fqdn to IP mapping
+	nim := make(map[string]net.IP)
+	for _, ra := range resp.Extra {
+		if a, ok := ra.(*dns.A); ok {
+			nim[a.Hdr.Name] = a.A
+		}
+	}
+
+	ttgs := make([]*Target, 0, len(resp.Answer))
+	for _, ra := range resp.Answer {
+		if srv, ok := ra.(*dns.SRV); ok {
+			t := Target{}
+			// try using IP address instead of hostname
+			if ip, ok := nim[srv.Target]; ok {
+				t.DialAddr = fmt.Sprintf("%v:%v", ip.String(), srv.Port)
+			} else {
+				t.DialAddr = fmt.Sprintf("%v:%v", srv.Target, srv.Port)
+			}
+
+			// we do want ttl do be > 0 for the LB updates
+			if srv.Hdr.Ttl == 0 {
+				t.Ttl = time.Duration(r.defaultTTL) * time.Second
+			} else {
+				t.Ttl = time.Duration(srv.Hdr.Ttl) * time.Second
+			}
+
+			ttgs = append(ttgs, &t)
+		}
+	}
+
+	return ttgs, err
+}