简单的DNS服务器
提供一个简单的可以查询域名和反向查询的DNS服务器。
dig命令主要用来从 DNS 域名服务器查询主机地址信息。
查找www.baidu.com的ip (A记录):
命令:dig @127.0.0.1 www.baidu.com
根据ip查找对应域名 (PTR记录):
命令:dig @127.0.0.1 -x 220.181.38.150
源码 :
package main import ( "fmt" "net" "golang.org/x/net/dns/dnsmessage" ) func main() { conn, err := net.ListenUDP("udp", &net.UDPAddr{Port: 53}) if err != nil { panic(err) } defer conn.Close() fmt.Println("Listing ...") for { buf := make([]byte, 512) _, addr, _ := conn.ReadFromUDP(buf) var msg dnsmessage.Message if err := msg.Unpack(buf); err != nil { fmt.Println(err) continue } go ServerDNS(addr, conn, msg) } } // address books var ( addressBookOfA = map[string][4]byte{ "www.baidu.com.": [4]byte{220, 181, 38, 150}, } addressBookOfPTR = map[string]string{ "150.38.181.220.in-addr.arpa.": "www.baidu.com.", } ) // ServerDNS serve func ServerDNS(addr *net.UDPAddr, conn *net.UDPConn, msg dnsmessage.Message) { // query info if len(msg.Questions) < 1 { return } question := msg.Questions[0] var ( queryTypeStr = question.Type.String() queryNameStr = question.Name.String() queryType = question.Type queryName, _ = dnsmessage.NewName(queryNameStr) ) fmt.Printf("[%s] queryName: [%s]\n", queryTypeStr, queryNameStr) // find record var resource dnsmessage.Resource switch queryType { case dnsmessage.TypeA: if rst, ok := addressBookOfA[queryNameStr]; ok { resource = NewAResource(queryName, rst) } else { fmt.Printf("not fount A record queryName: [%s] \n", queryNameStr) Response(addr, conn, msg) return } case dnsmessage.TypePTR: if rst, ok := addressBookOfPTR[queryName.String()]; ok { resource = NewPTRResource(queryName, rst) } else { fmt.Printf("not fount PTR record queryName: [%s] \n", queryNameStr) Response(addr, conn, msg) return } default: fmt.Printf("not support dns queryType: [%s] \n", queryTypeStr) return } // send response msg.Response = true msg.Answers = append(msg.Answers, resource) Response(addr, conn, msg) } // Response return func Response(addr *net.UDPAddr, conn *net.UDPConn, msg dnsmessage.Message) { packed, err := msg.Pack() if err != nil { fmt.Println(err) return } if _, err := conn.WriteToUDP(packed, addr); err != nil { fmt.Println(err) } } // NewAResource A record func NewAResource(query dnsmessage.Name, a [4]byte) dnsmessage.Resource { return dnsmessage.Resource{ Header: dnsmessage.ResourceHeader{ Name: query, Class: dnsmessage.ClassINET, TTL: 600, }, Body: &dnsmessage.AResource{ A: a, }, } } // NewPTRResource PTR record func NewPTRResource(query dnsmessage.Name, ptr string) dnsmessage.Resource { name, _ := dnsmessage.NewName(ptr) return dnsmessage.Resource{ Header: dnsmessage.ResourceHeader{ Name: query, Class: dnsmessage.ClassINET, }, Body: &dnsmessage.PTRResource{ PTR: name, }, } }
补充:Golang自定义DNS Nameserver
某些情况下我们希望程序通过自定义Nameserver去查询域名,而不希望通过操作系统给定的Nameserver,本文介绍如何在Golang中实现自定义Nameserver。
DNS解析过程
Golang中一般通过net.Resolver的LookupHost(ctx context.Context, host string) (addrs []string, err error)去实现域名解析,
解析过程如下:
检查本地hosts文件是否存在解析记录,存在即返回解析地址
不存在即根据resolv.conf中读取的nameserver发起递归查询
nameserver不断的向上级nameserver发起迭代查询
nameserver最终返回查询结果给请求者
用户可以通过修改/etc/resolv.conf来添加特定的nameserver,但某些场景下我们不希望更改系统配置。比如在kubernetes中,作为sidecar服务需要通过service去访问其他集群内服务,必须更改dnsPolicy为ClusterFirst,但这可能会影响其他容器的DNS查询效率。
自定义Nameserver
在Golang中自定义Nameserver,需要我们自己实现一个Resolver,如果是httpClient需要自定义DialContext()
Resolver实现如下:
// 默认dialer dialer := &net.Dialer{ Timeout: 1 * time.Second, } // 定义resolver resolver := &net.Resolver{ Dial: func(ctx context.Context, network, address string) (net.Conn, error) { return dialer.DialContext(ctx, "tcp", nameserver) // 通过tcp请求nameserver解析域名 }, }
自定义Dialer如下:
type Dialer struct { dialer *net.Dialer resolver *net.Resolver nameserver string } // NewDialer create a Dialer with user's nameserver. func NewDialer(dialer *net.Dialer, nameserver string) (*Dialer, error) { conn, err := dialer.Dial("tcp", nameserver) if err != nil { return nil, err } defer conn.Close() return &Dialer{ dialer: dialer, resolver: &net.Resolver{ Dial: func(ctx context.Context, network, address string) (net.Conn, error) { return dialer.DialContext(ctx, "tcp", nameserver) }, }, nameserver: nameserver, // 用户设置的nameserver }, nil } // DialContext connects to the address on the named network using // the provided context. func (d *Dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) { host, port, err := net.SplitHostPort(address) if err != nil { return nil, err } ips, err := d.resolver.LookupHost(ctx, host) // 通过自定义nameserver查询域名 for _, ip := range ips { // 创建链接 conn, err := d.dialer.DialContext(ctx, network, ip+":"+port) if err == nil { return conn, nil } } return d.dialer.DialContext(ctx, network, address) }
httpClient中自定义DialContext()如下:
ndialer, _ := NewDialer(dialer, nameserver) client := &http.Client{ Transport: &http.Transport{ DialContext: ndialer.DialContext, TLSHandshakeTimeout: 10 * time.Second, }, Timeout: timeout, }
总结
通过以上实现可解决自定义Nameserver,也可以在Dailer中添加缓存,实现DNS缓存。
以上为个人经验,希望能给大家一个参考,也希望大家多多支持。如有错误或未考虑完全的地方,望不吝赐教。