SSH Connection Timeout

Question

I am trying to make SSH connections using golang.org/x/crypto/ssh and I am kinda surprised that I can't seem to find out how to timeout the NewSession function (I actually don't seen any way to timeout anything). When I try to connect to a server that is having issues, this just hangs for a very long time. I have written something to use select with a time.After but it just feels like a hack. Something I haven't tried yet is to keep the underlying net.Conn in my struct and just keep doing Conn.SetDeadline() calls. Haven't tried this yet because I don't know if the crypto/ssh library overrides this or anything like that.

Anyone have a good way to timeout dead servers with this library? Or does anyone know of a better library?

Answer 1

One way to handle this transparently with the ssh package, is to create a connection with an idle timeout via a custom net.Conn which sets deadlines for you. However, this will cause the background Reads on a connection to timeout, so we need to use ssh keepalives to keep the connection open. Depending on your use case, simply using ssh keepalives as an alert for a dead connection may suffice.

// Conn wraps a net.Conn, and sets a deadline for every read
// and write operation.
type Conn struct {
    net.Conn
    ReadTimeout  time.Duration
    WriteTimeout time.Duration
}

func (c *Conn) Read(b []byte) (int, error) {
    err := c.Conn.SetReadDeadline(time.Now().Add(c.ReadTimeout))
    if err != nil {
        return 0, err
    }
    return c.Conn.Read(b)
}

func (c *Conn) Write(b []byte) (int, error) {
    err := c.Conn.SetWriteDeadline(time.Now().Add(c.WriteTimeout))
    if err != nil {
        return 0, err
    }
    return c.Conn.Write(b)
}

You can then use net.DialTimeout or a net.Dialer to get the connection, wrap it in your Conn with timeouts, and pass it into ssh.NewClientConn.

func SSHDialTimeout(network, addr string, config *ssh.ClientConfig, timeout time.Duration) (*ssh.Client, error) {
    conn, err := net.DialTimeout(network, addr, timeout)
    if err != nil {
        return nil, err
    }

    timeoutConn := &Conn{conn, timeout, timeout}
    c, chans, reqs, err := ssh.NewClientConn(timeoutConn, addr, config)
    if err != nil {
        return nil, err
    }
    client := ssh.NewClient(c, chans, reqs)

    // this sends keepalive packets every 2 seconds
    // there's no useful response from these, so we can just abort if there's an error
    go func() {
        t := time.NewTicker(2 * time.Second)
        defer t.Stop()
        for range t.C {
            _, _, err := client.Conn.SendRequest("[email protected]", true, nil)
            if err != nil {
                return
            }
        }
    }()
    return client, nil
}

Answer 2

Set the timeout on the ssh.ClientConfig.

cfg := ssh.ClientConfig{
    User: "root",
    Auth: []ssh.AuthMethod{
        ssh.PublicKeys(signer),
    },
    HostKeyCallback: ssh.FixedHostKey(hostKey),
    Timeout:         15 * time.Second, // max time to establish connection
}

ssh.Dial("tcp", ip+":22", &cfg)

When you call ssh.Dial, the timeout will be passed to net.DialTimeout.