这篇文章主要讲解了“SkyDNS工作原理是什么”,文中的讲解内容简单清晰,易于学习与理解,下面请大家跟着小编的思路慢慢深入,一起来研究和学习“SkyDNS工作原理是什么”吧!
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。
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/<domain>/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。
感谢各位的阅读,以上就是“SkyDNS工作原理是什么”的内容了,经过本文的学习后,相信大家对SkyDNS工作原理是什么这一问题有了更深刻的体会,具体使用情况还需要大家实践验证。这里是天达云,小编将为大家推送更多相关知识点的文章,欢迎关注!