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resolvconf: use /run/systemd/resolve/resolv.conf if systemd-resolved manages DNS

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Signed-off-by: Tibor Vass <tibor@docker.com>
This commit is contained in:
Tibor Vass 2019-05-31 19:16:20 +00:00
parent 992e4b903a
commit c575631528
2 changed files with 40 additions and 6 deletions
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42
libnetwork/resolvconf/resolvconf.go
View file

@ -15,10 +15,44 @@ import (
)
const (
// DefaultResolvConf points to the default file used for dns configuration on a linux machine
DefaultResolvConf = "/etc/resolv.conf"
// defaultPath is the default path to the resolv.conf that contains information to resolve DNS. See Path().
defaultPath = "/etc/resolv.conf"
// alternatePath is a path different from defaultPath, that may be used to resolve DNS. See Path().
alternatePath = "/run/systemd/resolve/resolv.conf"
)
var (
detectSystemdResolvConfOnce sync.Once
pathAfterSystemdDetection = defaultPath
)
// Path returns the path to the resolv.conf file that libnetwork should use.
//
// When /etc/resolv.conf contains 127.0.0.53 as the only nameserver, then
// it is assumed systemd-resolved manages DNS. Because inside the container 127.0.0.53
// is not a valid DNS server, Path() returns /run/systemd/resolve/resolv.conf
// which is the resolv.conf that systemd-resolved generates and manages.
// Otherwise Path() returns /etc/resolv.conf.
//
// Errors are silenced as they will inevitably resurface at future open/read calls.
//
// More information at https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html#/etc/resolv.conf
func Path() string {
detectSystemdResolvConfOnce.Do(func() {
candidateResolvConf, err := ioutil.ReadFile(defaultPath)
if err != nil {
// silencing error as it will resurface at next calls trying to read defaultPath
return
}
ns := GetNameservers(candidateResolvConf, types.IP)
if len(ns) == 1 && ns[0] == "127.0.0.53" {
pathAfterSystemdDetection = alternatePath
logrus.Infof("detected 127.0.0.53 nameserver, assuming systemd-resolved, so using resolv.conf: %s", alternatePath)
}
})
return pathAfterSystemdDetection
}
var (
// Note: the default IPv4 & IPv6 resolvers are set to Google's Public DNS
defaultIPv4Dns = []string{"nameserver 8.8.8.8", "nameserver 8.8.4.4"}
@ -55,7 +89,7 @@ type File struct {
// Get returns the contents of /etc/resolv.conf and its hash
func Get() (*File, error) {
return GetSpecific(DefaultResolvConf)
return GetSpecific(Path())
}
// GetSpecific returns the contents of the user specified resolv.conf file and its hash
@ -78,7 +112,7 @@ func GetIfChanged() (*File, error) {
lastModified.Lock()
defer lastModified.Unlock()
resolv, err := ioutil.ReadFile("/etc/resolv.conf")
resolv, err := ioutil.ReadFile(Path())
if err != nil {
return nil, err
}

4
libnetwork/sandbox_dns_unix.go
View file

@ -213,8 +213,8 @@ func (sb *sandbox) setupDNS() error {
originResolvConfPath := sb.config.originResolvConfPath
if originResolvConfPath == "" {
// if not specified fallback to default /etc/resolv.conf
originResolvConfPath = resolvconf.DefaultResolvConf
// fallback if not specified
originResolvConfPath = resolvconf.Path()
}
currRC, err := resolvconf.GetSpecific(originResolvConfPath)
if err != nil {