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package main
import (
	"fmt"
	"net/http"
)
func main() {
	client := &http.Client{}
	_, err := client.Get("http://localhost:8080")
	fmt.Println(err)
}

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GODEBUG="netdns=go+2" go run main.go 
go package net: GODEBUG setting forcing use of Go's resolver
go package net: hostLookupOrder(localhost) = files,dns
Get "http://localhost:8080": dial tcp [::1]:8080: connect: connection refused

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FROM golang:1.15 as builder
WORKDIR /app
COPY main.go main.go
COPY run.sh run.sh
ENV CGO_ENABLED=0
ENV GOOS=linux
RUN go build main.go
FROM alpine:3
WORKDIR /app
COPY --from=builder /app /app
COPY run.sh run.sh
RUN chmod +x run.sh
ENV GODEBUG="netdns=go+2"
ENV CGO_ENABLED=0
ENV GOOS=linux
CMD /app/run.sh

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go package net: built with netgo build tag; using Go's DNS resolver
go package net: hostLookupOrder(localhost) = dns,files
Get "http://localhost:8080": dial tcp 127.0.0.1:8080: connect: connection refused

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// hostLookupOrder specifies the order of LookupHost lookup strategies.
// It is basically a simplified representation of nsswitch.conf.
// "files" means /etc/hosts.
type hostLookupOrder int
const (
	// hostLookupCgo means defer to cgo.
	hostLookupCgo      hostLookupOrder = iota
	hostLookupFilesDNS                 // files first
	hostLookupDNSFiles                 // dns first
	hostLookupFiles                    // only files
	hostLookupDNS                      // only DNS
)

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func (c *conf) hostLookupOrder(r *Resolver, hostname string) (ret hostLookupOrder) {
	// ... 省略
	nss := c.nss
	srcs := nss.sources["hosts"]
	// If /etc/nsswitch.conf doesn't exist or doesn't specify any
	// sources for "hosts", assume Go's DNS will work fine.
	if os.IsNotExist(nss.err) || (nss.err == nil && len(srcs) == 0) {
		if c.goos == "solaris" {
			// illumos defaults to "nis [NOTFOUND=return] files"
			return fallbackOrder
		}
		if c.goos == "linux" {
			// glibc says the default is "dns [!UNAVAIL=return] files"
			// https://www.gnu.org/software/libc/manual/html_node/Notes-on-NSS-Configuration-File.html.
			return hostLookupDNSFiles
		}
		return hostLookupFilesDNS
	}
	if nss.err != nil {
		// We failed to parse or open nsswitch.conf, so
		// conservatively assume we should use cgo if it's
		// available.
		return fallbackOrder
	}
}