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// Package a2s implements a small portion of the r2 netchannel used by Northstar
// to probe servers.
package a2s
import (
"bytes"
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"encoding/binary"
"errors"
"fmt"
"net"
"net/netip"
"os"
"time"
)
const ProbeUID uint64 = 1000000001337
var ErrTimeout = errors.New("connection timed out")
func Probe(addr netip.AddrPort, timeout time.Duration) error {
conn, err := net.DialUDP("udp", nil, net.UDPAddrFromAddrPort(addr))
if err != nil {
return fmt.Errorf("connect to server: %w", err)
}
defer conn.Close()
t := time.Now().Add(timeout)
conn.SetWriteDeadline(t)
conn.SetReadDeadline(t)
pkt, err := r2cryptoEncrypt(r2encodeGetChallenge(ProbeUID))
if err != nil {
return fmt.Errorf("encrypt connection packet: %w", err)
}
if _, err := conn.Write(pkt); err != nil {
if errors.Is(err, os.ErrDeadlineExceeded) {
err = fmt.Errorf("%w: %v", ErrTimeout, err)
}
return fmt.Errorf("send connection packet: %w", err)
}
resp := make([]byte, 1500)
if n, err := conn.Read(resp); err != nil {
if errors.Is(err, os.ErrDeadlineExceeded) {
err = fmt.Errorf("%w: %v", ErrTimeout, err)
}
return fmt.Errorf("receive packet: %w", err)
} else {
resp = resp[:n]
}
dec, err := r2cryptoDecrypt(resp)
if err != nil {
return fmt.Errorf("failed to decrypt received packet: %w", err)
}
uid, _, err := r2decodeChallenge(dec)
if err != nil {
return fmt.Errorf("invalid challenge: %w", err)
}
if uid != ProbeUID {
return fmt.Errorf("invalid challenge")
}
return nil
}
const (
r2cryptoNonceSize = 12
r2cryptoTagSize = 16
r2cryptoKey = "X3V.bXCfe3EhN'wb"
r2cryptoAAD = "\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f\x10"
)
func r2cryptoEncrypt(b []byte) ([]byte, error) {
pkt := make([]byte, r2cryptoNonceSize+r2cryptoTagSize+len(b))
nonce := pkt[:r2cryptoNonceSize]
if _, err := rand.Read(nonce); err != nil {
return nil, err
}
c, err := aes.NewCipher([]byte(r2cryptoKey))
if err != nil {
panic(err)
}
a, err := cipher.NewGCMWithTagSize(c, r2cryptoTagSize)
if err != nil {
panic(err)
}
// Go appends the ciphertext, then the tag to the dest (nonce)
tmp := a.Seal(nil, nonce, b, []byte(r2cryptoAAD))
copy(pkt[r2cryptoNonceSize:], tmp[len(b):])
copy(pkt[r2cryptoNonceSize+r2cryptoTagSize:], tmp)
return pkt, nil
}
func r2cryptoDecrypt(b []byte) ([]byte, error) {
if len(b) < r2cryptoNonceSize+r2cryptoTagSize+1 {
return nil, fmt.Errorf("packet too small")
}
c, err := aes.NewCipher([]byte(r2cryptoKey))
if err != nil {
panic(err)
}
a, err := cipher.NewGCMWithTagSize(c, r2cryptoTagSize)
if err != nil {
panic(err)
}
tmp := make([]byte, len(b)-r2cryptoNonceSize)
copy(tmp, b[r2cryptoNonceSize+r2cryptoTagSize:])
copy(tmp[len(tmp)-r2cryptoTagSize:], b[r2cryptoNonceSize:])
if tmp, err = a.Open(tmp[:0], b[:r2cryptoNonceSize], tmp, []byte(r2cryptoAAD)); err != nil {
return nil, err
}
return tmp, nil
}
func r2encodeGetChallenge(uid uint64) []byte {
var b bytes.Buffer
binary.Write(&b, binary.LittleEndian, int32(-1))
binary.Write(&b, binary.LittleEndian, uint8(72))
binary.Write(&b, binary.LittleEndian, []byte("connect\x00"))
binary.Write(&b, binary.LittleEndian, uint64(uid))
binary.Write(&b, binary.LittleEndian, uint8(2))
return b.Bytes()
}
func r2decodeChallenge(b []byte) (uint64, int32, error) {
var pkt struct {
Seq int32
Type uint8
Challenge int32
UID uint64
}
if err := binary.Read(bytes.NewReader(b), binary.LittleEndian, &pkt); err != nil {
return 0, 0, err
}
if pkt.Seq != -1 {
return 0, 0, fmt.Errorf("not a connectionless packet")
}
if pkt.Type != 73 {
return 0, 0, fmt.Errorf("not a challenge response")
}
return pkt.UID, pkt.Challenge, nil
}
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