add resolvconf

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)

networkfs/resolvconf/resolvconf.go

@@ -0,0 +1,87 @@
+package resolvconf
+
+import (
+	"bytes"
+	"io/ioutil"
+	"regexp"
+	"strings"
+)
+
+func Get() ([]byte, error) {
+	resolv, err := ioutil.ReadFile("/etc/resolv.conf")
+	if err != nil {
+		return nil, err
+	}
+	return resolv, nil
+}
+
+// getLines parses input into lines and strips away comments.
+func getLines(input []byte, commentMarker []byte) [][]byte {
+	lines := bytes.Split(input, []byte("\n"))
+	var output [][]byte
+	for _, currentLine := range lines {
+		var commentIndex = bytes.Index(currentLine, commentMarker)
+		if commentIndex == -1 {
+			output = append(output, currentLine)
+		} else {
+			output = append(output, currentLine[:commentIndex])
+		}
+	}
+	return output
+}
+
+// GetNameservers returns nameservers (if any) listed in /etc/resolv.conf
+func GetNameservers(resolvConf []byte) []string {
+	nameservers := []string{}
+	re := regexp.MustCompile(`^\s*nameserver\s*(([0-9]+\.){3}([0-9]+))\s*$`)
+	for _, line := range getLines(resolvConf, []byte("#")) {
+		var ns = re.FindSubmatch(line)
+		if len(ns) > 0 {
+			nameservers = append(nameservers, string(ns[1]))
+		}
+	}
+	return nameservers
+}
+
+// GetNameserversAsCIDR returns nameservers (if any) listed in
+// /etc/resolv.conf as CIDR blocks (e.g., "1.2.3.4/32")
+// This function's output is intended for net.ParseCIDR
+func GetNameserversAsCIDR(resolvConf []byte) []string {
+	nameservers := []string{}
+	for _, nameserver := range GetNameservers(resolvConf) {
+		nameservers = append(nameservers, nameserver+"/32")
+	}
+	return nameservers
+}
+
+// GetSearchDomains returns search domains (if any) listed in /etc/resolv.conf
+// If more than one search line is encountered, only the contents of the last
+// one is returned.
+func GetSearchDomains(resolvConf []byte) []string {
+	re := regexp.MustCompile(`^\s*search\s*(([^\s]+\s*)*)$`)
+	domains := []string{}
+	for _, line := range getLines(resolvConf, []byte("#")) {
+		match := re.FindSubmatch(line)
+		if match == nil {
+			continue
+		}
+		domains = strings.Fields(string(match[1]))
+	}
+	return domains
+}
+
+func Build(path string, dns, dnsSearch []string) error {
+	content := bytes.NewBuffer(nil)
+	for _, dns := range dns {
+		if _, err := content.WriteString("nameserver " + dns + "\n"); err != nil {
+			return err
+		}
+	}
+	if len(dnsSearch) > 0 {
+		if _, err := content.WriteString("search " + strings.Join(dnsSearch, " ") + "\n"); err != nil {
+			return err
+		}
+	}
+
+	return ioutil.WriteFile(path, content.Bytes(), 0644)
+}