ipam/host-local: add ResolvConf argument for DNS configuration

This adds the option `resolvConf` to the host-local IPAM configuration.
If specified, the plugin will try to parse the file as a resolv.conf(5)
type file and return it in the DNS response.

plugins/ipam/host-local/dns.go

@@ -0,0 +1,64 @@
+// Copyright 2016 CNI authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"bufio"
+	"os"
+	"strings"
+
+	"github.com/containernetworking/cni/pkg/types"
+)
+
+// parseResolvConf parses an existing resolv.conf in to a DNS struct
+func parseResolvConf(filename string) (*types.DNS, error) {
+	fp, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+
+	dns := types.DNS{}
+	scanner := bufio.NewScanner(fp)
+	for scanner.Scan() {
+		line := scanner.Text()
+		line = strings.TrimSpace(line)
+
+		// Skip comments, empty lines
+		if len(line) == 0 || line[0] == '#' || line[0] == ';' {
+			continue
+		}
+
+		fields := strings.Fields(line)
+		if len(fields) < 2 {
+			continue
+		}
+		switch fields[0] {
+		case "nameserver":
+			dns.Nameservers = append(dns.Nameservers, fields[1])
+		case "domain":
+			dns.Domain = fields[1]
+		case "search":
+			dns.Search = append(dns.Search, fields[1:]...)
+		case "options":
+			dns.Options = append(dns.Options, fields[1:]...)
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	return &dns, nil
+}