SkyDNS2源码分析

SkyDNS2是SkyDNS Version 2.x的统称，其官方文档只有README.md，网上能找到的资料也不多，因此需要我们自行对代码进行一定的分析，才能对其有更好的理解，这就是本文的工作，通过走读SkyDNS的代码，了解其内部架构及其工作原理。

说明

• SkyDNS2的github地址: https://github.com/skynetservices/skydns
• Version: v2.5.3a

SkyDNS架构

关于SkyDNS是什么？…. 这些知识，请前往官网了解。

下面我直接把我阅读代码后理解的SkyDNS架构贴出来：

SkyDNS工作原理

SkyDNS Server的工作，依赖后端Key-Value存储的支持。当前支持etcd或etcd3作为Backend（架构图中蓝色部分），为SkyDNS提供配置和数据的管理。

通过环境变量ETCD_MACHINES进行etcd cluster的配置，如果Backend为etcd3，还需要设置etcd中/v2/keys//skydns/config/etcd3为true。SkyDNS中有etcd client模块，负责与ETCD_MACHINES的通信。

SkyDNS主要对应的etcd key path如下：

/v2/keys/skydns/config
/v2/keys/skydns/local/skydns/east/production/rails
/v2/keys/skydns/local/skydns/dns/stub
/v2/keys/skydns/local/skydns/...

通过如下环境变量的配置，支持prometheus监控（架构图中棕色部分）。如果想disable prometheus监控，则配置环境变量PROMETHEUS_PORT的值为0即可。

Port      = os.Getenv("PROMETHEUS_PORT")
Path      = envOrDefault("PROMETHEUS_PATH", "/metrics")
Namespace = envOrDefault("PROMETHEUS_NAMESPACE", "skydns")
Subsystem = envOrDefault("PROMETHEUS_SUBSYSTEM", "skydns")

如果/v2/keys/skydns/config/nameservers有值，则SkyDNS解析不了的Domain，会forward到对应的这些IP:Port构成的nameservers，由它们进行解析（架构图中绿色部分）。

参考官方文档https://github.com/skynetservices/skydns/blob/master/README.md完成参数配置后，便可启动SkyDNS。

SkyDNS Server的启动过程如下：

• 创建etcd client对象；
• dns_addr 和 nameservers参数合法性检查；
• 加载启动参数到etcd，覆盖/v2/keys/skydns/config中原有数据；
• 配置SkyDNS Server参数的default值，并创建SkyDNS server对象；
• 去etcd中加载…/dns/stub//xx数据作为server的stub zones数据，并启动对…/dns/stub/的watcher，一旦有数据更新，就加载到server的stub zones数据中；
• 注册SkyDNS metrics到prometheus；
• 然后在/v2/keys/skydns/config/dns_addr配置的interface和port上开启tcp/udp监听服务并block住，由此开始提供DSN服务。

在github.com/skynetservices/skydns/server/server.go中的ServeDNS方法覆盖了miekg/dns/server中的ServeMux.ServeDNS方法，由自实现的ServeDNS提供来处理DNS client的请求。

github.com/skynetservices/skydns/server/server.go

// ServeDNS is the handler for DNS requests, responsible for parsing DNS request, possibly forwarding
// it to a real dns server and returning a response.
func (s *server) ServeDNS(w dns.ResponseWriter, req *dns.Msg) {
    ...
    // Check cache first.
    m1 := s.rcache.Hit(q, dnssec, tcp, m.Id)
    if m1 != nil {
        ...
        // Still round-robin even with hits from the cache.
        // Only shuffle A and AAAA records with each other.
        if q.Qtype == dns.TypeA || q.Qtype == dns.TypeAAAA {
            s.RoundRobin(m1.Answer)
        }

        ...
        return
    }

    for zone, ns := range *s.config.stub {
        if strings.HasSuffix(name, "." + zone) || name == zone {
            metrics.ReportRequestCount(req, metrics.Stub)

            resp := s.ServeDNSStubForward(w, req, ns)
            if resp != nil {
                s.rcache.InsertMessage(cache.Key(q, dnssec, tcp), resp)
            }

            metrics.ReportDuration(resp, start, metrics.Stub)
            metrics.ReportErrorCount(resp, metrics.Stub)
            return
        }
    }
    ...

    if name == s.config.Domain {
        if q.Qtype == dns.TypeSOA {
            m.Answer = []dns.RR{s.NewSOA()}
            return
        }
        if q.Qtype == dns.TypeDNSKEY {
            if s.config.PubKey != nil {
                m.Answer = []dns.RR{s.config.PubKey}
                return
            }
        }
    }
    if q.Qclass == dns.ClassCHAOS {
        if q.Qtype == dns.TypeTXT {
            switch name {
            case "authors.bind.":
                fallthrough
            case s.config.Domain:
                hdr := dns.RR_Header{Name: q.Name, Rrtype: dns.TypeTXT, Class: dns.ClassCHAOS, Ttl: 0}
                authors := []string{"Erik St. Martin", "Brian Ketelsen", "Miek Gieben", "Michael Crosby"}
                for _, a := range authors {
                    m.Answer = append(m.Answer, &dns.TXT{Hdr: hdr, Txt: []string{a}})
                }
                for j := 0; j < len(authors)*(int(dns.Id())%4+1); j++ {
                    q := int(dns.Id()) % len(authors)
                    p := int(dns.Id()) % len(authors)
                    if q == p {
                        p = (p + 1) % len(authors)
                    }
                    m.Answer[q], m.Answer[p] = m.Answer[p], m.Answer[q]
                }
                return
            case "version.bind.":
                fallthrough
            case "version.server.":
                hdr := dns.RR_Header{Name: q.Name, Rrtype: dns.TypeTXT, Class: dns.ClassCHAOS, Ttl: 0}
                m.Answer = []dns.RR{&dns.TXT{Hdr: hdr, Txt: []string{Version}}}
                return
            case "hostname.bind.":
                fallthrough
            case "id.server.":
                // TODO(miek): machine name to return
                hdr := dns.RR_Header{Name: q.Name, Rrtype: dns.TypeTXT, Class: dns.ClassCHAOS, Ttl: 0}
                m.Answer = []dns.RR{&dns.TXT{Hdr: hdr, Txt: []string{"localhost"}}}
                return
            }
        }
        // still here, fail
        m.SetReply(req)
        m.SetRcode(req, dns.RcodeServerFailure)
        return
    }

    switch q.Qtype {
    case dns.TypeNS:
        if name != s.config.Domain {
            break
        }
        // Lookup s.config.DnsDomain
        records, extra, err := s.NSRecords(q, s.config.dnsDomain)
        if isEtcdNameError(err, s) {
            m = s.NameError(req)
            return
        }
        m.Answer = append(m.Answer, records...)
        m.Extra = append(m.Extra, extra...)
    case dns.TypeA, dns.TypeAAAA:
        records, err := s.AddressRecords(q, name, nil, bufsize, dnssec, false)
        if isEtcdNameError(err, s) {
            m = s.NameError(req)
            return
        }
        m.Answer = append(m.Answer, records...)
    case dns.TypeTXT:
        records, err := s.TXTRecords(q, name)
        if isEtcdNameError(err, s) {
            m = s.NameError(req)
            return
        }
        m.Answer = append(m.Answer, records...)
    case dns.TypeCNAME:
        records, err := s.CNAMERecords(q, name)
        if isEtcdNameError(err, s) {
            m = s.NameError(req)
            return
        }
        m.Answer = append(m.Answer, records...)
    case dns.TypeMX:
        records, extra, err := s.MXRecords(q, name, bufsize, dnssec)
        if isEtcdNameError(err, s) {
            m = s.NameError(req)
            return
        }
        m.Answer = append(m.Answer, records...)
        m.Extra = append(m.Extra, extra...)
    default:
        fallthrough // also catch other types, so that they return NODATA
    case dns.TypeSRV:
        records, extra, err := s.SRVRecords(q, name, bufsize, dnssec)
        if err != nil {
            if isEtcdNameError(err, s) {
                m = s.NameError(req)
                return
            }
            logf("got error from backend: %s", err)
            if q.Qtype == dns.TypeSRV { // Otherwise NODATA
                m = s.ServerFailure(req)
                return
            }
        }
        // if we are here again, check the types, because an answer may only
        // be given for SRV. All other types should return NODATA, the
        // NXDOMAIN part is handled in the above code. TODO(miek): yes this
        // can be done in a more elegant manor.
        if q.Qtype == dns.TypeSRV {
            m.Answer = append(m.Answer, records...)
            m.Extra = append(m.Extra, extra...)
        }
    }

    if len(m.Answer) == 0 { // NODATA response
        m.Ns = []dns.RR{s.NewSOA()}
        m.Ns[0].Header().Ttl = s.config.MinTtl
    }
}

上面代码逻辑比较复杂，细节上需要你慢慢去理解，简短的可以总结如下：

• 如架构图中标注的线路1：如果在SkyDNS维护的cache中找到对应Msg，则从cache中读取并返回Msg给DNS client;
• 如架构图中标注的线路2：如果在cache中没有对应的记录，并且是需要DNS forward的场景（比如name匹配到stub zones等），则将请求forward到对应的DNS servers进行处理；
• 如架构图中标注的线路3：如果在cache中没有对应的记录，并且Question Type为A/AAAA，SRV等类型时，就通过etcd client去etcd cluster中获取对应的Rule，并构造Msg返回。

总结

通过走读SkyDNS的代码，了解其内部架构及其工作原理。

Mark

SkyDNS2 Changes since version 1：

• Does away with Raft and uses etcd (which uses raft).
• Makes it possible to query arbitrary domain names.
• Is a thin layer above etcd, that translates etcd keys and values to the DNS.
• Does DNSSEC with NSEC3 instead of NSEC.