github.com/tonistiigi/docker@v0.10.1-0.20240229224939-974013b0dc6a/libnetwork/internal/resolvconf/resolvconf_path.go (about) 1 //go:build go1.21 2 3 package resolvconf 4 5 import ( 6 "context" 7 "net/netip" 8 "sync" 9 10 "github.com/containerd/log" 11 ) 12 13 const ( 14 // defaultPath is the default path to the resolv.conf that contains information to resolve DNS. See Path(). 15 defaultPath = "/etc/resolv.conf" 16 // alternatePath is a path different from defaultPath, that may be used to resolve DNS. See Path(). 17 alternatePath = "/run/systemd/resolve/resolv.conf" 18 ) 19 20 // For Path to detect systemd (only needed for legacy networking). 21 var ( 22 detectSystemdResolvConfOnce sync.Once 23 pathAfterSystemdDetection = defaultPath 24 ) 25 26 // Path returns the path to the resolv.conf file that libnetwork should use. 27 // 28 // When /etc/resolv.conf contains 127.0.0.53 as the only nameserver, then 29 // it is assumed systemd-resolved manages DNS. Because inside the container 127.0.0.53 30 // is not a valid DNS server, Path() returns /run/systemd/resolve/resolv.conf 31 // which is the resolv.conf that systemd-resolved generates and manages. 32 // Otherwise Path() returns /etc/resolv.conf. 33 // 34 // Errors are silenced as they will inevitably resurface at future open/read calls. 35 // 36 // More information at https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html#/etc/resolv.conf 37 // 38 // TODO(robmry) - alternatePath is only needed for legacy networking ... 39 // 40 // Host networking can use the host's resolv.conf as-is, and with an internal 41 // resolver it's also possible to use nameservers on the host's loopback 42 // interface. Once legacy networking is removed, this can always return 43 // defaultPath. 44 func Path() string { 45 detectSystemdResolvConfOnce.Do(func() { 46 rc, err := Load(defaultPath) 47 if err != nil { 48 // silencing error as it will resurface at next calls trying to read defaultPath 49 return 50 } 51 ns := rc.nameServers 52 if len(ns) == 1 && ns[0] == netip.MustParseAddr("127.0.0.53") { 53 pathAfterSystemdDetection = alternatePath 54 log.G(context.TODO()).Infof("detected 127.0.0.53 nameserver, assuming systemd-resolved, so using resolv.conf: %s", alternatePath) 55 } 56 }) 57 return pathAfterSystemdDetection 58 }