[转]go(golang) dns 解析源码 go/src/net/dnsclient_unix.go 分析

转：原文：https://blog.csdn.net/mumumuwudi/article/details/48722063

strace 命令不错，非常有用！！

使用strace命令分析一下，系统调用过程:

________________________________________________________________

关于go dns解析的一些说明参照另一篇文章http://blog.csdn.net/mumumuwudi/article/details/48200505
go dns 解析源码在go/src/net/dnsclient_unix.go, lookupHost()通过向本地dns server发送请求，获得IP和
域名的对应关系然后返回，函数调用关系如下：

// ---------------------------------------------------
// lookupHost()
//
// ->goLookupHostOrder()
//
// ---->goLookupIPOrder()
//
// -------->tryOneName()
//
// ----------->exchange()

其中的timeout 是 dns 超时时间是在dnsconfig_unix.go 文件中读取 /etc/reslove.conf 的配置决定的.
net.go中的DialTimeout函数也会走到DNS解析流程中，该函数最终会调用到 lookupIPDeadline 启用
一个新的协程去解析DNS，具体调用栈如下:

// ----------------------------------------------------------------
// DialTimeout()
//
// ->resolveAddrList()
//
// --->internetAddrList()
//
// ------>lookupIPDeadline()
//
// --------->lookupGroup.DoChan() 在新的协程中去做 dns解析
//
// ------------->lookupIP()
//
// ------------------>goLookupIPOrder()
// ------------------------------------------------------------------

总之，纯go语言的 DNS解析流程还是比较完善的~~

——————————————————————————————

之前使用过GO语言写了一个实时图片下载程序，主要考虑到GO语言的DNS解析对协程支持友好，即
DNS解析时不会阻塞执行线程，只会阻塞当前协程，顺便研究了一下GO的net.LookupHost/ResolveIPAddr
实现方式。下面一段描述翻译自go语言的官方文档 https://golang.org/pkg/net/域名解析：
域名解析函数，Dial函数会间接调用到，而LokupHost和LookupAddr则会直接调用域名解析函数，不同
的操作系统实现不同, 在Unix系统中有两种方法进行域名解析：
1）纯GO语言实现的域名解析,从/etc/resolv.conf中取出本地dns server地址列表，发送DNS请求(UDP
报文)并获得结果
2) 使用cgo方式，最终会调用到c标准库的getaddrinfo或getnameinfo函数(不建议使用对GO协程不
友好)

关于 cgo dns 解析的坑参照以下链接:
  https://jira.mongodb.org/browse/MGO-41
  https://github.com/golang/go/issues/8602#issuecomment-66098142

GO语言默认使用纯GO的域名解析，因为这样一个阻塞的DNS请求只会消耗一个协程，使用cgo的
方式则会阻塞一个系统线程, 只有某些特定条件下才会使用系统提供的cgo方式, 例如: 1) 在OS X系统中不
允许程序直接发送DNS请求; 2) LOCALDOMAINH环境变量存在，即使为空; 3) ES_OPTIONS或
HOSTALIASES或ASR_CONFIG环境变量非空; 4)/etc/resolv.conf或/etc/nsswitch.conf指定的使用
方式GO解析器没有实现;5) 当要解析的域名以.local结束，或者是一个mDNS域名
可以通过GODEBUG环境变量来设置go语言的默认DNS解析方式纯go或cgo,
-----------------------------------------------------------------------
export GODEBUG=netdns=go # force pure Go resolver 纯go 方式
export GODEBUG=netdns=cgo # force cgo resolver cgo 方式
-----------------------------------------------------------------------
也可以在编译时指定netgo或netcgo的编译tag来设置
在plan 9中域名解析只能通过 /net/cs和 /net/dns
在windows中域名解析只能通过windows提供的C标准库函数GetAddrInfo或DnsQuery

OK 官方说明看完了，我们写一个例子试一下:

package main

import (
	"net"
	"fmt"
	"os"
)


func main() {


	ns, err := net.LookupHost("www.baidu.com")
	if err != nil {
		fmt.Fprintf(os.Stderr, "Err: %s", err.Error())
		return
	}

	for _, n := range ns {
		fmt.Fprintf(os.Stdout, "--%s
", n) 
	}

}

1.　　编译上述程序：go build -o ss source-dns.go

2. 执行上述程序，得到进程号： 15097

root@fly:/home/fly/mm# ./ss &
[7] 15097

3. 使用strace命令分析一下，系统调用过程: strace -p 15097 -o aaa

使用strace命令分析一下，系统调用过程:

上面得到的aaa 文件内如如下：

restart_syscall(<... resuming interrupted futex ...>) = -1 ETIMEDOUT (Connection timed out)
futex(0x60ac70, FUTEX_WAKE_PRIVATE, 1)  = 1
openat(AT_FDCWD, "/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 3
epoll_create1(EPOLL_CLOEXEC)            = 4
epoll_ctl(4, EPOLL_CTL_ADD, 3, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=716087048, u64=139737477062408}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 3, 0xc000045abc) = -1 EPERM (Operation not permitted)
read(3, "# /etc/nsswitch.conf
#
# Example"..., 1024) = 513
read(3, "", 1024)                       = 0
close(3)                                = 0
openat(AT_FDCWD, "/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 3
epoll_ctl(4, EPOLL_CTL_ADD, 3, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=716087048, u64=139737477062408}}) = -1 EPERM (Operation not permitted)
epoll_ctl(4, EPOLL_CTL_DEL, 3, 0xc00004599c) = -1 EPERM (Operation not permitted)
fstat(3, {st_mode=S_IFREG|0644, st_size=726, ...}) = 0
read(3, "# This file is managed by man:sy"..., 65536) = 726
read(3, "", 64810)                      = 0
read(3, "", 65536)                      = 0
close(3)                                = 0
newfstatat(AT_FDCWD, "/etc/mdns.allow", 0xc000073078, 0) = -1 ENOENT (No such file or directory)
getrlimit(RLIMIT_NOFILE, {rlim_cur=1024, rlim_max=1024*1024}) = 0
socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
connect(3, {sa_family=AF_UNIX, sun_path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
close(3)                                = 0
socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
connect(3, {sa_family=AF_UNIX, sun_path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
close(3)                                = 0
openat(AT_FDCWD, "/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=513, ...}) = 0
read(3, "# /etc/nsswitch.conf
#
# Example"..., 4096) = 513
read(3, "", 4096)                       = 0
close(3)                                = 0
stat("/etc/resolv.conf", {st_mode=S_IFREG|0644, st_size=726, ...}) = 0
openat(AT_FDCWD, "/etc/host.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
read(3, "# The "order" line is only used "..., 4096) = 92
read(3, "", 4096)                       = 0
close(3)                                = 0
futex(0x7f172a74aba4, FUTEX_WAKE_PRIVATE, 2147483647) = 0
openat(AT_FDCWD, "/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=726, ...}) = 0
read(3, "# This file is managed by man:sy"..., 4096) = 726
read(3, "", 4096)                       = 0
close(3)                                = 0
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=30918, ...}) = 0
mmap(NULL, 30918, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f172ab8a000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "177ELF2113>1P#"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=47568, ...}) = 0
mmap(NULL, 2168632, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1725f34000
mprotect(0x7f1725f3f000, 2093056, PROT_NONE) = 0
mmap(0x7f172613e000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xa000) = 0x7f172613e000
mmap(0x7f1726140000, 22328, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f1726140000
close(3)                                = 0
mprotect(0x7f172613e000, 4096, PROT_READ) = 0
munmap(0x7f172ab8a000, 30918)           = 0
openat(AT_FDCWD, "/etc/hosts", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=218, ...}) = 0
read(3, "127.0.0.1 localhost
127.0.1.1 fl"..., 4096) = 218
read(3, "", 4096)                       = 0
close(3)                                = 0
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=30918, ...}) = 0
mmap(NULL, 30918, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f172ab8a000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "177ELF2113>120017"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=26936, ...}) = 0
mmap(NULL, 2121952, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1725d2d000
mprotect(0x7f1725d32000, 2097152, PROT_NONE) = 0
mmap(0x7f1725f32000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x5000) = 0x7f1725f32000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libresolv.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "177ELF2113>10008"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=101168, ...}) = 0
mmap(NULL, 2206336, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1725b12000
mprotect(0x7f1725b29000, 2097152, PROT_NONE) = 0
mmap(0x7f1725d29000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x17000) = 0x7f1725d29000
mmap(0x7f1725d2b000, 6784, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f1725d2b000
close(3)                                = 0
mprotect(0x7f1725d29000, 4096, PROT_READ) = 0
mprotect(0x7f1725f32000, 4096, PROT_READ) = 0
munmap(0x7f172ab8a000, 30918)           = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.53")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0)    = 1 ([{fd=3, revents=POLLOUT}])
sendmmsg(3, [{msg_hdr={msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="3t1113iot10vidagrid3com1"..., iov_len=45}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, msg_len=45}, {msg_hdr={msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="3152021113iot10vidagrid3com34"..., iov_len=45}], msg_iovlen=1, msg_controllen=0, msg_flags=MSG_TRUNC|MSG_WAITALL|MSG_NOSIGNAL|MSG_MORE|MSG_WAITFORONE}, msg_len=45}], 2, MSG_NOSIGNAL) = 2
poll([{fd=3, events=POLLIN}], 1, 5000)  = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [61])                = 0
 recvfrom(3, "3t2012001113iot10vidagrid3com1"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.53")}, [28->16]) = 61
 poll([{fd=3, events=POLLIN}], 1, 4999)  = 1 ([{fd=3, revents=POLLIN}])
 ioctl(3, FIONREAD, [45])                = 0
 recvfrom(3, "315202201200113iot10vidagrid3com34"..., 65536, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.53")}, [28->16]) = 45
 close(3)                                = 0
 futex(0x60ac70, FUTEX_WAKE_PRIVATE, 1)  = 1
 futex(0x60ab70, FUTEX_WAKE_PRIVATE, 1)  = 1
 write(1, "--120.78.15.160
", 16)       = 16
 exit_group(0)                           = ?
 +++ exited with 0 +++

# coding=UTF-8
import threading
import socket
import socketserver


class Handler(socketserver.BaseRequestHandler):
    def handle(self):
        request_data = self.request[0]
        # 将请求转发到 114 DNS
        redirect_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        redirect_socket.sendto(request_data, ('114.114.114.114', 53))
        response_data, address = redirect_socket.recvfrom(1024)

        # 将114响应响应给客户
        client_socket = self.request[1]
        client_socket.sendto(response_data, self.client_address)


class Server(socketserver.ThreadingMixIn, socketserver.UDPServer):
    pass


if __name__ == "__main__":
    # 一下ip需换成自己电脑的ip
    server = Server(('0.0.0.0',8088), Handler)
    with server:
        server_thread = threading.Thread(target=server.serve_forever)
        server_thread.daemon = True
        server_thread.start()
        print('The DNS server is running at 172.16.42.254...')
        server_thread.join()

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原文地址：https://www.cnblogs.com/oxspirt/p/14154610.html