开启 DNS 缓存

  • 当前 GO SDK 总是会去请求 DNS 服务器，而没有使用到 nscd 的缓存，可以通过导出环境变量GODEBUG=netdns=cgo，或者go build编译时指定参数-tags 'netcgo'控制读取 nscd 缓存。

测试

package main

import (
	"net/http"
	"os"
)

func main() {
	http.DefaultClient.Get(os.Args[1])
}

go build -o goCli t.go
strace ./goCli https://i.cnblogs.com/   # 在输出结果中无法找到相关nscd关键字

① 使用GODEBUG=netdns=cgo
export GODEBUG=netdns=cgo
strace ./goCli https://i.cnblogs.com/    # 在输出结果中可以找到相关nscd关键字 
② 使用-tags 'netcgo'
go build -tags 'netcgo' -o goCli t.go
strace ./goCli https://i.cnblogs.com/   # 在输出结果中可以找到相关nscd关键字

为什么netcgo会走nscd缓存

在golang官网中对net包(http://golang.org/pkg/net)关于域名解析有如下解释:

On Unix systems, the resolver has two options for resolving names. It can use a pure Go resolver that sends DNS requests directly to the servers listed in /etc/resolv.conf, or it can use a cgo-based resolver that calls C library routines such as getaddrinfo and getnameinfo.
By default the pure Go resolver is used, because a blocked DNS request consumes only a goroutine, while a blocked C call consumes an operating system thread. When cgo is available, the cgo-based resolver is used instead under a variety of conditions: on systems that do not let programs make direct DNS requests (OS X), when the LOCALDOMAIN environment variable is present (even if empty), when the RES_OPTIONS or HOSTALIASES environment variable is non-empty, when the ASR_CONFIG environment variable is non-empty (OpenBSD only), when /etc/resolv.conf or /etc/nsswitch.conf specify the use of features that the Go resolver does not implement, and when the name being looked up ends in .local or is an mDNS name.

大致意思是,在Unix系统中，解析域名有两种选项：

  1. 使用纯Go解析器直接发送DNS请求给/etc/resolv.conf文件中的服务器
  2. 使用基于CGo的解析器调用C库程序，例如getaddrinfo和getnameinfo

Dockerfile

FROM golang:alpine
ENV GO111MODULE=on \
    CGO_ENABLED=0 \
    GOOS=linux \
    GOARCH=amd64 \
	GOPROXY="https://goproxy.cn,direct"

WORKDIR /slb_exporter
COPY ./ ./
RUN go mod tidy && go build -o app
FROM busybox
EXPOSE 9202
COPY --from=0 /etc/ssl/certs /etc/ssl/certs
COPY --from=0 /slb_exporter/app .
ENTRYPOINT ["./app"]