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dns: Added miekgdns resolver as a hidden option to query and ruler.
Fixes: #1015 Signed-off-by: Bartek Plotka <bwplotka@gmail.com>
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,148 @@ | ||
| package miekgdns | ||
|
|
||
| import ( | ||
| "bytes" | ||
| "net" | ||
|
|
||
| "github.com/miekg/dns" | ||
| "github.com/pkg/errors" | ||
| ) | ||
|
|
||
| // Copied and slightly adjusted from Prometheus DNS SD: | ||
| // https://github.com/prometheus/prometheus/blob/be3c082539d85908ce03b6d280f83343e7c930eb/discovery/dns/dns.go#L212 | ||
|
|
||
| // lookupWithSearchPath tries to get an answer for various permutations of | ||
| // the given name, appending the system-configured search path as necessary. | ||
| // | ||
| // There are three possible outcomes: | ||
| // | ||
| // 1. One of the permutations of the given name is recognized as | ||
| // "valid" by the DNS, in which case we consider ourselves "done" | ||
| // and that answer is returned. Note that, due to the way the DNS | ||
| // handles "name has resource records, but none of the specified type", | ||
| // the answer received may have an empty set of results. | ||
| // | ||
| // 2. All of the permutations of the given name are responded to by one of | ||
| // the servers in the "nameservers" list with the answer "that name does | ||
| // not exist" (NXDOMAIN). In that case, it can be considered | ||
| // pseudo-authoritative that there are no records for that name. | ||
| // | ||
| // 3. One or more of the names was responded to by all servers with some | ||
| // sort of error indication. In that case, we can't know if, in fact, | ||
| // there are records for the name or not, so whatever state the | ||
| // configuration is in, we should keep it that way until we know for | ||
| // sure (by, presumably, all the names getting answers in the future). | ||
| // | ||
| // Outcomes 1 and 2 are indicated by a valid response message (possibly an | ||
| // empty one) and no error. Outcome 3 is indicated by an error return. The | ||
| // error will be generic-looking, because trying to return all the errors | ||
| // returned by the combination of all name permutations and servers is a | ||
| // nightmare. | ||
| func (r *Resolver) lookupWithSearchPath(name string, qtype dns.Type) (*dns.Msg, error) { | ||
| conf, err := dns.ClientConfigFromFile(r.ResolvConf) | ||
| if err != nil { | ||
| return nil, errors.Wrapf(err, "could not load resolv.conf: %s") | ||
| } | ||
|
|
||
| var errs []error | ||
| for _, lname := range conf.NameList(name) { | ||
| response, err := lookupFromAnyServer(lname, qtype, conf) | ||
| if err != nil { | ||
| // We can't go home yet, because a later name | ||
| // may give us a valid, successful answer. However | ||
| // we can no longer say "this name definitely doesn't | ||
| // exist", because we did not get that answer for | ||
| // at least one name. | ||
| errs = append(errs, err) | ||
| continue | ||
| } | ||
|
|
||
| if response.Rcode == dns.RcodeSuccess { | ||
| // Outcome 1: GOLD! | ||
| return response, nil | ||
| } | ||
| } | ||
|
|
||
| if len(errs) == 0 { | ||
| // Outcome 2: everyone says NXDOMAIN. | ||
| return &dns.Msg{}, nil | ||
| } | ||
| // Outcome 3: boned. | ||
| return nil, errors.Errorf("could not resolve %q: all servers responded with errors to at least one search domain. Errs %s", name, fmtErrs(errs)) | ||
| } | ||
|
|
||
| // lookupFromAnyServer uses all configured servers to try and resolve a specific | ||
| // name. If a viable answer is received from a server, then it is | ||
| // immediately returned, otherwise the other servers in the config are | ||
| // tried, and if none of them return a viable answer, an error is returned. | ||
| // | ||
| // A "viable answer" is one which indicates either: | ||
| // | ||
| // 1. "yes, I know that name, and here are its records of the requested type" | ||
| // (RCODE==SUCCESS, ANCOUNT > 0); | ||
| // 2. "yes, I know that name, but it has no records of the requested type" | ||
| // (RCODE==SUCCESS, ANCOUNT==0); or | ||
| // 3. "I know that name doesn't exist" (RCODE==NXDOMAIN). | ||
| // | ||
| // A non-viable answer is "anything else", which encompasses both various | ||
| // system-level problems (like network timeouts) and also | ||
| // valid-but-unexpected DNS responses (SERVFAIL, REFUSED, etc). | ||
| func lookupFromAnyServer(name string, qtype dns.Type, conf *dns.ClientConfig) (*dns.Msg, error) { | ||
| client := &dns.Client{} | ||
|
|
||
| var errs []error | ||
|
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| // TODO(bwplotka): Worth to do fanout and grab fastest as golang native lib? | ||
| for _, server := range conf.Servers { | ||
| servAddr := net.JoinHostPort(server, conf.Port) | ||
| msg, err := askServerForName(name, qtype, client, servAddr, true) | ||
| if err != nil { | ||
| errs = append(errs, errors.Wrapf(err, "resolution against server %s for %s", server, name)) | ||
| continue | ||
| } | ||
|
|
||
| if msg.Rcode == dns.RcodeSuccess || msg.Rcode == dns.RcodeNameError { | ||
| return msg, nil | ||
| } | ||
| } | ||
|
|
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| return nil, errors.Errorf("could not resolve %s: no servers returned a viable answer. Errs %v", name, fmtErrs(errs)) | ||
| } | ||
|
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||
| func fmtErrs(errs []error) string { | ||
| b := bytes.Buffer{} | ||
| for _, err := range errs { | ||
| b.WriteString(";") | ||
| b.WriteString(err.Error()) | ||
| } | ||
| return b.String() | ||
| } | ||
|
|
||
| // askServerForName makes a request to a specific DNS server for a specific | ||
| // name (and qtype). Retries with TCP in the event of response truncation, | ||
| // but otherwise just sends back whatever the server gave, whether that be a | ||
| // valid-looking response, or an error. | ||
| func askServerForName(name string, qType dns.Type, client *dns.Client, servAddr string, edns bool) (*dns.Msg, error) { | ||
| msg := &dns.Msg{} | ||
|
|
||
| msg.SetQuestion(dns.Fqdn(name), uint16(qType)) | ||
| if edns { | ||
| msg.SetEdns0(dns.DefaultMsgSize, false) | ||
| } | ||
|
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| response, _, err := client.Exchange(msg, servAddr) | ||
| if err != nil { | ||
| return nil, errors.Wrapf(err, "exchange") | ||
| } | ||
|
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| if response.Truncated { | ||
| if client.Net == "tcp" { | ||
| return nil, errors.Errorf("got truncated message on TCP (64kiB limit exceeded?)") | ||
| } | ||
|
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| client.Net = "tcp" | ||
| return askServerForName(name, qType, client, servAddr, false) | ||
| } | ||
|
|
||
| return response, nil | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,71 @@ | ||
| package miekgdns | ||
|
|
||
| import ( | ||
| "context" | ||
| "net" | ||
|
|
||
| "github.com/miekg/dns" | ||
| "github.com/pkg/errors" | ||
| ) | ||
|
|
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| // DefaultResolvConfPath is a common, default resolv.conf file present on linux server. | ||
| const DefaultResolvConfPath = "/etc/resolv.conf" | ||
|
|
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| // Resolver is a drop-in Resolver for *part* of std lib Golang net.DefaultResolver methods. | ||
| type Resolver struct { | ||
| ResolvConf string | ||
| } | ||
|
|
||
| func (r *Resolver) LookupSRV(ctx context.Context, service, proto, name string) (cname string, addrs []*net.SRV, err error) { | ||
| var target string | ||
| if service == "" && proto == "" { | ||
| target = name | ||
| } else { | ||
| target = "_" + service + "._" + proto + "." + name | ||
| } | ||
|
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| response, err := r.lookupWithSearchPath(target, dns.Type(dns.TypeSRV)) | ||
| if err != nil { | ||
| return "", nil, err | ||
| } | ||
|
|
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| for _, record := range response.Answer { | ||
| switch addr := record.(type) { | ||
| case *dns.SRV: | ||
| addrs = append(addrs, &net.SRV{ | ||
| Weight: addr.Weight, | ||
| Target: addr.Target, | ||
| Priority: addr.Priority, | ||
| Port: addr.Port, | ||
| }) | ||
| default: | ||
| return "", nil, errors.Errorf("invalid SRV response record %s", record) | ||
| } | ||
| } | ||
|
|
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| return "", addrs, nil | ||
| } | ||
|
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| func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]net.IPAddr, error) { | ||
| response, err := r.lookupWithSearchPath(host, dns.Type(dns.TypeAAAA)) | ||
| if err != nil { | ||
| // Ugly fallback to A lookup. | ||
| response, err = r.lookupWithSearchPath(host, dns.Type(dns.TypeA)) | ||
| if err != nil { | ||
| return nil, err | ||
| } | ||
| } | ||
|
|
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| var resp []net.IPAddr | ||
| for _, record := range response.Answer { | ||
| switch addr := record.(type) { | ||
| case *dns.A: | ||
| resp = append(resp, net.IPAddr{IP: addr.A}) | ||
| case *dns.AAAA: | ||
| resp = append(resp, net.IPAddr{IP: addr.AAAA}) | ||
| default: | ||
| return nil, errors.Errorf("invalid A or AAAA response record %s", record) | ||
| } | ||
| } | ||
| return resp, nil | ||
| } |
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