moby/libnetwork/resolvconf/resolvconf.go

// Package resolvconf provides utility code to query and update DNS configuration in /etc/resolv.conf
package resolvconf

import (
	"bytes"
	"fmt"
	"os"
	"strings"

	"github.com/docker/docker/libnetwork/internal/resolvconf"
	"github.com/opencontainers/go-digest"
)

// constants for the IP address type
const (
	IP = iota // IPv4 and IPv6
	IPv4
	IPv6
)

// File contains the resolv.conf content and its hash
type File struct {
	Content []byte
	Hash    []byte
}

func Path() string {
	return resolvconf.Path()
}

// Get returns the contents of /etc/resolv.conf and its hash
func Get() (*File, error) {
	return GetSpecific(Path())
}

// GetSpecific returns the contents of the user specified resolv.conf file and its hash
func GetSpecific(path string) (*File, error) {
	resolv, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	hash := digest.FromBytes(resolv)
	return &File{Content: resolv, Hash: []byte(hash)}, nil
}

// FilterResolvDNS cleans up the config in resolvConf.  It has two main jobs:
//  1. It looks for localhost (127.*|::1) entries in the provided
//     resolv.conf, removing local nameserver entries, and, if the resulting
//     cleaned config has no defined nameservers left, adds default DNS entries
//  2. Given the caller provides the enable/disable state of IPv6, the filter
//     code will remove all IPv6 nameservers if it is not enabled for containers
func FilterResolvDNS(resolvConf []byte, ipv6Enabled bool) (*File, error) {
	rc, err := resolvconf.Parse(bytes.NewBuffer(resolvConf), "")
	if err != nil {
		return nil, err
	}
	rc.TransformForLegacyNw(ipv6Enabled)
	content, err := rc.Generate(false)
	if err != nil {
		return nil, err
	}
	hash := digest.FromBytes(content)
	return &File{Content: content, Hash: []byte(hash)}, nil
}

// GetNameservers returns nameservers (if any) listed in /etc/resolv.conf
func GetNameservers(resolvConf []byte, kind int) []string {
	rc, err := resolvconf.Parse(bytes.NewBuffer(resolvConf), "")
	if err != nil {
		return nil
	}
	nsAddrs := rc.NameServers()
	var nameservers []string
	for _, addr := range nsAddrs {
		if kind == IP {
			nameservers = append(nameservers, addr.String())
		} else if kind == IPv4 && addr.Is4() {
			nameservers = append(nameservers, addr.String())
		} else if kind == IPv6 && addr.Is6() {
			nameservers = append(nameservers, addr.String())
		}
	}
	return nameservers
}

// GetNameserversAsCIDR returns nameservers (if any) listed in
// /etc/resolv.conf as CIDR blocks (e.g., "1.2.3.4/32")
// This function's output is intended for net.ParseCIDR
func GetNameserversAsCIDR(resolvConf []byte) []string {
	rc, err := resolvconf.Parse(bytes.NewBuffer(resolvConf), "")
	if err != nil {
		return nil
	}
	nsAddrs := rc.NameServers()
	nameservers := make([]string, 0, len(nsAddrs))
	for _, addr := range nsAddrs {
		str := fmt.Sprintf("%s/%d", addr.WithZone("").String(), addr.BitLen())
		nameservers = append(nameservers, str)
	}
	return nameservers
}

// GetSearchDomains returns search domains (if any) listed in /etc/resolv.conf
// If more than one search line is encountered, only the contents of the last
// one is returned.
func GetSearchDomains(resolvConf []byte) []string {
	rc, err := resolvconf.Parse(bytes.NewBuffer(resolvConf), "")
	if err != nil {
		return nil
	}
	return rc.Search()
}

// GetOptions returns options (if any) listed in /etc/resolv.conf
// If more than one options line is encountered, only the contents of the last
// one is returned.
func GetOptions(resolvConf []byte) []string {
	rc, err := resolvconf.Parse(bytes.NewBuffer(resolvConf), "")
	if err != nil {
		return nil
	}
	return rc.Options()
}

// Build generates and writes a configuration file to path containing a nameserver
// entry for every element in nameservers, a "search" entry for every element in
// dnsSearch, and an "options" entry for every element in dnsOptions. It returns
// a File containing the generated content and its (sha256) hash.
//
// Note that the resolv.conf file is written, but the hash file is not.
func Build(path string, nameservers, dnsSearch, dnsOptions []string) (*File, error) {
	content := bytes.NewBuffer(nil)
	if len(dnsSearch) > 0 {
		if searchString := strings.Join(dnsSearch, " "); strings.Trim(searchString, " ") != "." {
			if _, err := content.WriteString("search " + searchString + "\n"); err != nil {
				return nil, err
			}
		}
	}
	for _, dns := range nameservers {
		if _, err := content.WriteString("nameserver " + dns + "\n"); err != nil {
			return nil, err
		}
	}
	if len(dnsOptions) > 0 {
		if optsString := strings.Join(dnsOptions, " "); strings.Trim(optsString, " ") != "" {
			if _, err := content.WriteString("options " + optsString + "\n"); err != nil {
				return nil, err
			}
		}
	}

	if err := os.WriteFile(path, content.Bytes(), 0o644); err != nil {
		return nil, err
	}

	hash := digest.FromBytes(content.Bytes())
	return &File{Content: content.Bytes(), Hash: []byte(hash)}, nil
}