gsmtcp

A golang library for TCP communication over an embedded GSM device. It was primarily developed with the SIM800 series of devices interfacing with Raspberry Pi, and is still very much a work in progress.

Usage example

When working with a Raspberry Pi, for example, the peripheral drivers must first be initialised. Utilising periph.io:

pState, err := host.Init()
if err != nil {
    log.Error(err)
    os.Exit(1)
}
for _, d := range pState.Failed {
    log.Warn("failed to load", d.String(), ":", d.Err.Error())
}

The GSM module can then be created and initialised:

g, err := gsm.NewGsmModule("/dev/ttyS0")
if err != nil {
    log.Error(err)
    return
}

err = g.Init()
if err != nil {
    log.Error(err)
    return
}
defer func() {
    err := g.Shutdown()
    if err != nil {
        log.Error(err)
    }
}()

A TCP connection can now be created:

conn, err := gsm.NewConnection(g, "<IPv4>:<PORT>")
if err != nil {
    log.Error("could not establish a new connection", err)
    os.Exit(1)
}
defer func() {
    err := conn.Close()
    if err != nil {
        log.Error(err.Error())
    }
    time.Sleep(1 * time.Second)
}()

Establishing a TLS connection

A secure connection can be established by utilising golang's standard libraries:

tlsConfig := &tls.Config{
    Certificates: []tls.Certificate{*cert},
    VerifyPeerCertificate: func(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
        for _, chain := range verifiedChains {
            for _, c := range chain {
                // peer certificate verification
            }
        }
        return nil
    },
    InsecureSkipVerify: true,
    Rand:               rand.Reader,
    MinVersion:         tls.VersionTLS12,
}

tlsConn := tls.Client(conn, tlsConfig)
defer func() {
    err := tlsConn.Close()
    if err != nil {
        log.Error(err)
    }
}()

If the TLS TCP connection will be used as-is to send raw data, the handshake can be established as follows:

// perform the handshake
err = tlsConn.Handshake()
if err != nil {
    log.Error(err)
}

The certificate (*cert) can be generated with the help of Talisman.

JSON-RPC

var reply greeter.HelloReply
err = gsmtcp.DoRequest("https://some-server", tlsConn, 
	"Greeter.SayHello",greeter.HelloRequest{Name: "Homer"}, &reply)