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pingdns.go
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pingdns.go
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package pingdns
import (
"context"
"errors"
"fmt"
"net"
"go.uber.org/zap"
"golang.org/x/net/icmp"
"golang.org/x/net/ipv4"
)
const (
icmpv4ChecksumLength = 16
pingMinimumDataLength = 64
pingLeadingDataLength = 16
defaultSnaplen = 1600
defaultNetwork = "ip4:icmp"
defaultAddress = "0.0.0.0"
)
type Server struct {
snaplen int
network, address string
logger *zap.Logger
}
func NewServer(logger *zap.Logger) Server {
return Server{
snaplen: defaultSnaplen, network: defaultNetwork, address: defaultAddress, logger: logger,
}
}
func (s Server) WithSnaplen(snaplen int) {
s.snaplen = snaplen
}
func (s Server) WithNetwork(network string) {
s.network = network
}
func (s Server) Address(address string) {
s.address = address
}
func (s Server) Serve() error {
conn, err := icmp.ListenPacket(s.network, s.address)
if err != nil {
return fmt.Errorf("doicmp: listening failed: %w", err)
}
defer conn.Close()
for {
request := make([]byte, s.snaplen)
n, addr, err := conn.ReadFrom(request)
if err != nil {
return fmt.Errorf("doicmp: reading packets failed: %w", err)
}
response, err := s.handleBytes(request[:n])
if err != nil {
s.logger.Warn("failed to handle request", zap.Error(err))
continue
}
if response == nil {
continue
}
if _, err := conn.WriteTo(response, addr); err != nil {
s.logger.Warn("failed to write response", zap.Error(err))
}
}
}
func (s Server) handleBytes(requestBytes []byte) (response []byte, err error) {
parsed, err := icmp.ParseMessage(ipv4.ICMPTypeEcho.Protocol(), requestBytes)
if err != nil {
return nil, fmt.Errorf("doicmp: parsing message failed: %w", err)
}
switch parsed.Type {
case ipv4.ICMPTypeEcho:
default:
return nil, nil
}
icmpEchoRequest, ok := parsed.Body.(*icmp.Echo)
if !ok {
return nil, errors.New("packet wasn't icmp echo?")
}
icmpMessageResponse, err := s.handleICMPEcho(icmpEchoRequest)
if err != nil {
return nil, err
}
return icmpMessageResponse.Marshal(nil)
}
func (s Server) handleICMPEcho(request *icmp.Echo) (response *icmp.Message, err error) {
if l := len(request.Data); l < icmpv4ChecksumLength {
return nil, fmt.Errorf("icmp echo data was too short? got %v bytes, expected at least %v",
l, icmpv4ChecksumLength)
}
name, err := s.extractNameFromPayload(request.Data[icmpv4ChecksumLength:])
if err != nil {
return nil, fmt.Errorf("failed to extract name from icmp payload: %w", err)
}
ipv4s, err := net.DefaultResolver.LookupIP(context.Background(), "ip4", name)
if err != nil {
return nil, fmt.Errorf("failed to resolve name: %w", err)
}
return &icmp.Message{
Type: ipv4.ICMPTypeEchoReply,
Code: 0,
Body: &icmp.Echo{
ID: request.ID,
Seq: request.Seq,
Data: prepareResponseData(ipv4s),
},
}, nil
}
func prepareResponseData(ipv4s []net.IP) []byte {
// When ping presents the "wrong data" message, it chops off `pingLeadingDataLength`
// of leading data. Pad that out with zeroes, so our IPs get shown:
responseData := make([]byte, pingLeadingDataLength)
responseData = append(responseData, flatten(ipv4sToByteSlices(ipv4s))...)
padding := make([]byte, pingMinimumDataLength-len(responseData))
responseData = append(responseData, padding...)
return responseData
}
func (s Server) extractNameFromPayload(payload []byte) (string, error) {
s.logger.Debug("extracting request name from payload", zap.String("payload", string(payload)))
start := findIndex(payload, '?')
if start == -1 {
return "", errors.New("failed to find ? delimiter in icmp echo request data")
}
end := findIndex(payload[start+1:], '?')
if end == -1 {
return "", errors.New("failed to find ? delimiter in icmp echo request data")
}
name := string(payload[start+1 : start+1+end])
s.logger.Debug("got request name from payload", zap.String("name", name))
return name, nil
}
func findIndex(bytes []byte, delim byte) int {
for i := range bytes {
if bytes[i] == delim {
return i
}
}
return -1
}
func flatten(slices [][]byte) []byte {
var res []byte
for _, slice := range slices {
res = append(res, slice...)
}
return res
}
func ipv4sToByteSlices(ips []net.IP) [][]byte {
var slices [][]byte
for _, ip := range ips {
if ip == nil || len(ip) != 16 {
continue
}
slices = append(slices, ip[12:])
}
return slices
}