net: Go DNS resolver does not read /etc/hosts #22846
Description
What version of Go are you using (go version)?
go version go1.9.2 linux/amd64
What did you do?
The following script creates a Docker container with a cgo-disabled Go binary that resolves a host defined in the /etc/hosts file.
#!/bin/bash
set -e
set -x
cat > Dockerfile <<EOF
FROM alpine:3.6
COPY internal.sh /tmp/internal.sh
COPY go_dns /tmp/go_dns
CMD /tmp/internal.sh
EOF
cat > go_dns.go <<EOF
package main
import (
"net"
"fmt"
)
func main() {
fmt.Println(net.LookupHost("example.com"))
}
EOF
cat > internal.sh <<EOF
set -e
set -x
export GODEBUG=netdns=10
cat /etc/resolv.conf
cat /etc/hosts
/tmp/go_dns
nslookup example.com
EOF
chmod +x internal.sh
go version
GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -o go_dns go_dns.go
docker build --no-cache -t go_dns .
docker run --rm \
--add-host example.com:192.168.2.50 \
-i \
-t \
go_dnsWhat did you expect to see?
Go's DNS resolver resolve the address that is contained in /etc/hosts.
Output:
+ cat
+ cat
+ cat
+ chmod +x internal.sh
+ go version
go version go1.9.2 linux/amd64
+ GOOS=linux
+ GOARCH=amd64
+ CGO_ENABLED=0
+ go build -o go_dns go_dns.go
+ docker build --no-cache -t go_dns .
Sending build context to Docker daemon 2.691 MB
Step 1 : FROM alpine:3.6
---> 053cde6e8953
Step 2 : COPY internal.sh /tmp/internal.sh
---> 58c4e4eabbd7
Removing intermediate container e915818d62ae
Step 3 : COPY go_dns /tmp/go_dns
---> bcf50d520610
Removing intermediate container 85918214a98c
Step 4 : CMD /tmp/internal.sh
---> Running in cf4216686f20
---> c75e5b4ccec8
Removing intermediate container cf4216686f20
Successfully built c75e5b4ccec8
+ docker run --rm --add-host example.com:192.168.2.50 -i -t go_dns
+ export GODEBUG=netdns=10
+ cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 8.8.8.8
nameserver 8.8.4.4
#nameserver 127.0.1.1
+ cat /etc/hosts
127.0.0.1 localhost
::1 localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
192.168.2.50 example.com
172.17.0.2 b44f6648150b
+ /tmp/go_dns
go package net: built with netgo build tag; using Go's DNS resolver
go package net: hostLookupOrder(example.com) = dns,files
[93.184.216.34 2606:2800:220:1:248:1893:25c8:1946] <nil>
+ nslookup example.com
nslookup: can't resolve '(null)': Name does not resolve
Name: example.com
Address 1: 192.168.2.50 example.com
What did you see instead?
An external DNS lookup was performed.
(This may be the same issue that was happening here #14170)