This is caused by the method modifying the server addresses in place. Instead the method should create a copy of the slice and modify that instead.

hi.
in $GOPATH,I RAN "go get -u -v github.com/bogdanovich/dns_resolver" AS FOLLOWS:

yudeMacBook-Air:gopath brite$ go get -u -v github.com/bogdanovich/dns_resolver
github.com/bogdanovich/dns_resolver (download)

cd .; git clone https://github.com/bogdanovich/dns_resolver /Users/yuming/go1.10.2/go/gopath/src/github.com/bogdanovich/dns_resolver

Cloning into '/Users/yuming/go1.10.2/go/gopath/src/github.com/bogdanovich/dns_resolver'...
fatal: unable to access 'https://github.com/bogdanovich/dns_resolver/': Failed to connect to github.com port 443: Operation timed out
package github.com/bogdanovich/dns_resolver: exit status 128
yudeMacBook-Air:gopath brite$
yudeMacBook-Air:gopath brite$ go get -u -v github.com/bogdanovich/dns_resolver
github.com/bogdanovich/dns_resolver (download)
github.com/miekg/dns (download)
Fetching https://golang.org/x/crypto/ed25519?go-get=1
Parsing meta tags from https://golang.org/x/crypto/ed25519?go-get=1 (status code 200)
get "golang.org/x/crypto/ed25519": found meta tag get.metaImport{Prefix:"golang.org/x/crypto", VCS:"git", RepoRoot:"https://go.googlesource.com/crypto"} at https://golang.org/x/crypto/ed25519?go-get=1
get "golang.org/x/crypto/ed25519": verifying non-authoritative meta tag
Fetching https://golang.org/x/crypto?go-get=1
Parsing meta tags from https://golang.org/x/crypto?go-get=1 (status code 200)
golang.org/x/crypto (download)
Fetching https://golang.org/x/crypto/ed25519/internal/edwards25519?go-get=1
Parsing meta tags from https://golang.org/x/crypto/ed25519/internal/edwards25519?go-get=1 (status code 200)
get "golang.org/x/crypto/ed25519/internal/edwards25519": found meta tag get.metaImport{Prefix:"golang.org/x/crypto", VCS:"git", RepoRoot:"https://go.googlesource.com/crypto"} at https://golang.org/x/crypto/ed25519/internal/edwards25519?go-get=1
get "golang.org/x/crypto/ed25519/internal/edwards25519": verifying non-authoritative meta tag
Fetching https://golang.org/x/net/ipv4?go-get=1
Parsing meta tags from https://golang.org/x/net/ipv4?go-get=1 (status code 200)
get "golang.org/x/net/ipv4": found meta tag get.metaImport{Prefix:"golang.org/x/net", VCS:"git", RepoRoot:"https://go.googlesource.com/net"} at https://golang.org/x/net/ipv4?go-get=1
get "golang.org/x/net/ipv4": verifying non-authoritative meta tag
Fetching https://golang.org/x/net?go-get=1
Parsing meta tags from https://golang.org/x/net?go-get=1 (status code 200)
golang.org/x/net (download)
Fetching https://golang.org/x/net/bpf?go-get=1
Parsing meta tags from https://golang.org/x/net/bpf?go-get=1 (status code 200)
get "golang.org/x/net/bpf": found meta tag get.metaImport{Prefix:"golang.org/x/net", VCS:"git", RepoRoot:"https://go.googlesource.com/net"} at https://golang.org/x/net/bpf?go-get=1
get "golang.org/x/net/bpf": verifying non-authoritative meta tag
Fetching https://golang.org/x/net/internal/iana?go-get=1
Parsing meta tags from https://golang.org/x/net/internal/iana?go-get=1 (status code 200)
get "golang.org/x/net/internal/iana": found meta tag get.metaImport{Prefix:"golang.org/x/net", VCS:"git", RepoRoot:"https://go.googlesource.com/net"} at https://golang.org/x/net/internal/iana?go-get=1
get "golang.org/x/net/internal/iana": verifying non-authoritative meta tag
Fetching https://golang.org/x/net/internal/socket?go-get=1
Parsing meta tags from https://golang.org/x/net/internal/socket?go-get=1 (status code 200)
get "golang.org/x/net/internal/socket": found meta tag get.metaImport{Prefix:"golang.org/x/net", VCS:"git", RepoRoot:"https://go.googlesource.com/net"} at https://golang.org/x/net/internal/socket?go-get=1
get "golang.org/x/net/internal/socket": verifying non-authoritative meta tag
Fetching https://golang.org/x/sys/unix?go-get=1
Parsing meta tags from https://golang.org/x/sys/unix?go-get=1 (status code 200)
get "golang.org/x/sys/unix": found meta tag get.metaImport{Prefix:"golang.org/x/sys", VCS:"git", RepoRoot:"https://go.googlesource.com/sys"} at https://golang.org/x/sys/unix?go-get=1
get "golang.org/x/sys/unix": verifying non-authoritative meta tag
Fetching https://golang.org/x/sys?go-get=1
Parsing meta tags from https://golang.org/x/sys?go-get=1 (status code 200)
golang.org/x/sys (download)
Fetching https://golang.org/x/net/ipv6?go-get=1
Parsing meta tags from https://golang.org/x/net/ipv6?go-get=1 (status code 200)
get "golang.org/x/net/ipv6": found meta tag get.metaImport{Prefix:"golang.org/x/net", VCS:"git", RepoRoot:"https://go.googlesource.com/net"} at https://golang.org/x/net/ipv6?go-get=1
get "golang.org/x/net/ipv6": verifying non-authoritative meta tag
github.com/bogdanovich/dns_resolver
yudeMacBook-Air:gopath brite$ which dns_resolver
yudeMacBook-Air:gopath brite$

thank