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package main
import (
"bufio"
"context"
"fmt"
"golang.org/x/sync/errgroup"
"io"
"net"
"os"
"slices"
"strings"
"time"
"git.samanthony.xyz/hose/hosts"
"git.samanthony.xyz/hose/key"
"git.samanthony.xyz/hose/util"
)
const (
timeout = 1 * time.Minute
retryInterval = 500 * time.Millisecond
)
// handshake exchanges public keys with a remote host.
// The user is asked to verify the received key
// before it is saved in the known hosts file.
func handshake(rhost string) error {
util.Logf("initiating handshake with %s...", rhost)
errs := make(chan error, 2)
defer close(errs)
group, ctx := errgroup.WithContext(context.Background())
group.Go(func() error {
if err := handshakeSend(rhost); err != nil {
errs <- err
}
return nil
})
group.Go(func() error {
if err := handshakeRecv(rhost); err != nil {
errs <- err
}
return nil
})
go func() { group.Wait() }() // cancel the context.
select {
case err := <-errs:
return err
case <-ctx.Done():
return nil
}
}
// handshakeSend sends the local public key to a remote host.
func handshakeSend(rhost string) error {
util.Logf("loading public key...")
pubkey, err := key.LoadPublicKey()
if err != nil {
return err
}
raddr := net.JoinHostPort(rhost, port)
util.Logf("connecting to %s...", raddr)
conn, err := dialWithTimeout(network, raddr, timeout)
if err != nil {
return err
}
defer conn.Close()
util.Logf("connected to %s", raddr)
if _, err := conn.Write(pubkey[:]); err != nil {
return err
}
util.Logf("sent public key to %s", rhost)
return nil
}
func dialWithTimeout(network, address string, timeout time.Duration) (net.Conn, error) {
ctx, cancel := context.WithTimeout(context.Background(), timeout)
defer cancel()
for {
select {
case <-ctx.Done(): // timeout.
return nil, fmt.Errorf("dial %s %s: connection refused", network, address)
default:
}
conn, err := net.Dial(network, address)
if err == nil {
return conn, nil
}
time.Sleep(retryInterval)
}
}
// handshakeRecv receives the public key of a remote host.
// The user is asked to verify the key before it is saved to the known hosts file.
func handshakeRecv(rhost string) error {
// Listen for connection.
laddr := net.JoinHostPort("", port)
ln, err := net.Listen(network, laddr)
if err != nil {
return err
}
defer ln.Close()
util.Logf("listening on %s", laddr)
conn, err := ln.Accept()
if err != nil {
return err
}
defer conn.Close()
util.Logf("accepted connection from %s", conn.RemoteAddr())
// Receive public key from remote host.
var rpubkey [32]byte
_, err = io.ReadFull(conn, rpubkey[:])
if err != nil {
return err
}
util.Logf("received public key from %s", conn.RemoteAddr())
// Ask user to verify the key.
ok, err := verifyPublicKey(conn.RemoteAddr(), rpubkey)
if err != nil {
return err
}
if !ok {
// User rejected the key.
return fmt.Errorf("host key verification failed")
}
return hosts.Set(conn.RemoteAddr(), rpubkey)
}
// verifyPublicKey asks the user to verify the public key of a remote host.
// It returns true if the user accepts the key, or false if they don't, or a non-nil error.
func verifyPublicKey(addr net.Addr, pubkey [32]byte) (bool, error) {
host, _, err := net.SplitHostPort(addr.String())
if err != nil {
return false, err
}
// Ask host to verify the key.
util.Logf("Public key of host %q: %x\nIs this the correct key (yes/[no])?",
host, pubkey[:])
response, err := scan([]string{"yes", "no", ""})
if err != nil {
return false, err
}
switch response {
case "yes":
return true, nil
case "no":
return false, nil
case "":
return false, nil // default option
}
panic("unreachable")
}
// scan reads from stdin until the user enters one of the valid responses.
func scan(responses []string) (string, error) {
scanner := bufio.NewScanner(os.Stdin)
scanner.Scan()
if err := scanner.Err(); err != nil {
return "", err
}
response := strings.TrimSpace(scanner.Text())
for !slices.Contains(responses, response) {
util.Logf("Please enter one of %q", responses)
scanner.Scan()
if err := scanner.Err(); err != nil {
return "", err
}
response = strings.TrimSpace(scanner.Text())
}
return response, nil
}
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