alibaba/kt-connect

//go:build !windows

package dns

import (
    "bufio"
    "fmt"
    "github.com/alibaba/kt-connect/pkg/kt/util"
    "github.com/rs/zerolog/log"
    "os"
    "regexp"
    "strings"
)

// listen address of systemd-resolved
const resolvedAddr = "127.0.0.53"
const resolvedConf = "/run/systemd/resolve/resolv.conf"

// GetLocalDomains get domain search postfixes
func GetLocalDomains() string {
    f, err := os.Open(util.ResolvConf)
    if err != nil {
        return ""
    }
    defer f.Close()

    var localDomains []string
    scanner := bufio.NewScanner(f)
    for scanner.Scan() {
        line := scanner.Text()
        if strings.HasPrefix(line, util.FieldDomain) {
            localDomains = append(localDomains, strings.TrimSpace(strings.TrimPrefix(line, util.FieldDomain)))
        } else if strings.HasPrefix(line, util.FieldSearch) {
            for _, s := range strings.Split(strings.TrimPrefix(line, util.FieldSearch), " ") {
                if s != "" {
                    localDomains = append(localDomains, s)
                }
            }
        }
    }
    return strings.Join(localDomains, ",")
}

// GetNameServer get primary dns server
func GetNameServer() string {
    ns := fetchNameServerInConf(util.ResolvConf)
    if util.IsLinux() && ns == resolvedAddr {
        log.Debug().Msgf("Using systemd-resolved")
        return fetchNameServerInConf(resolvedConf)
    }
    return ns
}

func fetchNameServerInConf(resolvConf string) string {
    f, err := os.Open(resolvConf)
    if err != nil {
        return ""
    }
    defer f.Close()

    scanner := bufio.NewScanner(f)
    pattern, _ := regexp.Compile(fmt.Sprintf("^%s[ \t]+" + util.IpAddrPattern, util.FieldNameserver))
    for scanner.Scan() {
        line := scanner.Text()
        if pattern.MatchString(line) {
            return strings.TrimSpace(strings.TrimPrefix(line, util.FieldNameserver))
        }
    }
    return ""
}