gensio - General Stream I/O

This is gensio (pronounced gen'-see-oh), a framework for giving a consistent view of various stream (and packet) I/O types. You create a gensio object (or a gensio), and you can use that gensio without having to know too much about what is going on underneath. You can stack gensio on top of another one to add protocol functionality. For instance, you can create a TCP gensio, stack SSL on top of that, and stack Telnet on top of that. It supports a number of network I/O and serial ports. It also supports sound interfaces. gensios that stack on other gensios are called filters.

You can do the same thing with receiving ports. You can set up a gensio accepter to accept connections in a stack. So in our previous example, you can setup TCP to listen on a specific port and automatically stack SSL and Telnet on top when the connection comes in, and you are not informed until everything is ready.

gensio works on Linux, BSDs, MacOS, and Windows. On Windows, it gives you a single-threaded capable (but also multi-thread capable) event-driven interface (with blocking interfaces available) to simplify programming with lots of I/Os. It goes a long way to making writing portable I/O driven code easy.

A very important feature of gensio is that it makes establishing encrypted and authenticated connections much easier than without it. Beyond basic key management, it's really no harder than TCP or anything else. It offers extended flexibility for controlling the authentication process if needed. It's really easy to use.

Note that the gensio(5) man page has more details on individual gensio types.

For instructions on building this from source, see the "BUILDING" document.

For info on how to use the library, see the "USING" document.

gensio tools

A couple of tools are available that use gensios, both as an example and for trying things out. These are:

gensiot

A tool for making basic gensio connections. You can create any arbitrary gensio setup you like. See gensiot(1) for details.

gtlsshd

An sshd-like daemon that uses certauth, ssl, and SCTP or TCP gensios for making connections. It uses standard PAM authentication and uses ptys. See gtlsshd(8) for details.

There is a item in FAQ.rst named "How to run gtlsshd on Windows", see that and the Building on Windows section in the BUILDING document for more details, as there are a few tricky things you have to handle.

gtlssh

An ssh-like program that can connect to gtlsshd. It can also be used with ser2net to make establishing encrypted and authenticated connections easier. See gtlssh(1) for details.

gtlssh-keygen

Used for handling keys for gtlssh and gtlsshd. See gtlssh-keygen(1) for details.

gmdns

Used to provide and query MDNS. See gmdns(1) for details.

gsound

Used to play or record sound. See gsound(1) for details.

Available gensios

The following gensios are available in the library:

sctp

Normal SCTP communication. Streams and out of bound data are supported. End of message demarcation is not supported because it doesn't currently work on Linux.

tcp

Normal TCP communication. Out of bound data is supported.

unix

Unix stream domain socket.

unixdgram

Unix datagram domain socket. This is sort-of connection oriented.

unixseq

Unix seqpacket domain socket. Probably only works on Linux.

udp

Sort-of connection oriented UDP.

stdio

Access to either the calling program's stdio, or the ability to run a program and connect to its stdin, stdout, and stderr. NOTE: Do not use this for file I/O. Use the file gensio.

file

Used for accessing files. Allows both input and output file, and streams the data to/from the files. No accepter available.

pty

Run a program in a PTY and use the gensio to communicate with its tty. No accepter available.

serialdev

Connect to a device. You can use "sdev" as shorthand for "serialdev". It can hook to termios type devices, like ptys and /dev/tty, more than just serial ports. No accepter available.

dev

Connects to devices (like serialdev does) but does not do any serial port processing. It also has a write-only option for talking to printer ports or other write-only devices. It also has a rdonly option for talking to read-only devices. No accepter available.

ipmisol

Connect to a remote over IPMI SOL. Full serial port capabilities are available. No accepter available, unfortunately.

dummy

An accepter that doesn't do anything except look like an accepter to the user. Useful in some situations where an accepter is expected but you don't need to do anything.

echo

A gensio that echos everything that is sent to it. Useful for testing. No accepter available.

telnet

A filter gensio that implements the telnet protocol. It can do full serial support with RFC2217.

ssl

Implement SSL/TLS as a gensio filter. It supports client authentication, too.

certauth

A user authentication protocol implemented as a gensio filter.

mux

A channel multiplexer. You can create channels on top of it using open_channel(). Channels work as normal gensio, so you can have a number of gensios running on top of a single gensio. It also has end-of-message demarcation and obviously full flow-control capability individually on each channel. If you just need a gensio with end-of-message demarcation, you can use this as without creating channels.

msgdelim

Converts an unreliable stream interface into an unreliable packet interface. This is primarily so a reliable packet interface like relpkt can run over a serial port. It does not support streaming of data, so it's not very useful by itself.

relpkt

Converts an unreliable packet interface to a reliable packet interface (that also supports streaming). Made for running over msgdelim. It will run over UDP, but it's not ideal for that because it doesn't do all the internet-friendly flow control and such that SCTP and TCP do.

trace

A transparent gensio that allows the data read and/or written to be sent to a file, either as raw data or as human-readable hex data. It can also be used to block data flowing in one or both directions.

perf

A gensio that can send/receive data on top of a stack of gensios and measure the throughput on the channel. The received data from perf is information about the channel throughput.

conacc

A gensio accepter that takes a gensio stack string as a parameter. This lets you use a gensio as an accepter. When conacc is started, it opens the gensio, and when the gensio opens it reports a new child for the accepter. When the child closes it attempts to open the child again and go through the process again (unless accepts have been disabled on conacc).

Why would you want to use this? Say in ser2net you wanted to connect one serial port to another. You could have a connection like:

connection: &con0
  accepter: conacc,serialdev,/dev/ttyS1,115200
  connector: serialdev,/dev/ttyS2,115200

And it would connect /dev/ttyS1 to /dev/ttyS2. Without conacc, you could not use serialdev as an accepter. It would also let you use gtlsshd on a serial port if you wanted encrypted authenticated logins over a serial port. If you ran gtlsshd with the following:

gtlsshd --notcp --oneshot --nodaemon --other_acc
   'conacc,relpkt(mode=server),msgdelim,serialdev,/dev/ttyUSB1,115200n81'

You could connect with:

gtlssh --transport 'relpkt,msgdelim,serialdev,/dev/ttyUSB2,115200n81' USB2

This creates a reliable packet transport over a serial port. The mode=server is required to make relpkt run as the server, since it would normally run as a client since it is not being started as an accepter. The ssl gensio (which runs over the transport) requires reliable communication, so it won't run directly over a serial port.

xlt

This gensio allows character translations to be done on data flowing through this filter. It's primarily to convert carraige returns and line feeds.

mdns

This gensio uses mDNS to lookup a service (protocol type, network type, port, address) and then connect to that service. If you have a program like ser2net that advertise mDNS service, you don't have to worry about finding port numbers and such, it's all handled for you.

keepopen

This gensio presents an always open connection to the upper layer and keeps the lower layer connection open. If it closes, it re-opens it.

script

This gensio executes an external program with the external program's stdio connected to the child of this gensio. Once the external program terminates, this gensio will report that it is open and pass all the data through. This can be used to run scripts to set things up on a connection before hooking to the parent gensio.

sound

A gensio that provides access to sound devices and files. It's a little complicated, read the docs in gensio.5

afskmdm

Yes, it looks like a jumble of letters.

A filter gensio that sits on top of the sound gensio and does an Audio Frequency Shift Keying modem, like is used on AX.25 amateur radio.

kiss

An amateur radio protocol for talking to TNCs. This is used by AX25 in many cases.

ax25

An amateur radio protocol for packet radio. To fully use this you would need to write code, since it uses channels and oob data for unnumbered information, but you can do basic things with just gensiot if all you need is one communication channel. For instance, if you wanted to chat with someone over the radio, and the kiss port is on 8001 on both machines, on the accepting machine you can run:

gensiot -i 'stdio(self)' -a \
    'ax25(laddr=AE5KM-1),kiss,conacc,tcp,localhost,8001'

which will hook to the TNC and wait for a connection on address AE5KM-1. Then you could run:

gensiot -i 'stdio(self)' \
    'ax25(laddr=AE5KM-2,addr="0,AE5KM-1,AE5KM-2"),kiss,tcp,localhost,8001'

on the other machine. This will connect to the other machine over TNC 0 with the given address. Then anything you type in one will appear on the other, a line at a time. Type "Ctrl-D" to exit. The 'stdio(self)' part turns off raw mode, so it's a line at a time and you get local echo. Otherwise every character you types would send a packet and you couldn't see what you were typing.

To hook to the N5COR-11 AX.25 BBS system, you would do:

gensiot -i 'xlt(nlcr),stdio(self)' \
  'ax25(laddr=AE5KM-2,addr="0,N5COR-11,AE5KM-2"),kiss,tcp,localhost,8001'

Most BBS systems use CR, not NL, for the new line, so the xlt gensio is used to translate incoming these characters.

Of course, this being gensio, you can put any workable gensio underneath ax25 that you would like. So if you want to play around or test without a radio, you could do ax25 over UDP multicast. Here's the accepter side:

gensiot -i 'stdio(self)' -a \
'ax25(laddr=AE5KM-1),conacc,'\
'udp(mcast="ipv4,224.0.0.20",laddr="ipv4,1234",nocon),'\
'ipv4,224.0.0.20,1234'

and here's the connector side:

gensiot -i 'stdio(self)' \
'ax25(laddr=AE5KM-2,addr="0,AE5KM-1,AE5KM-2"),'\
'udp(mcast="ipv4,224.0.0.20",laddr="ipv4,1234",nocon),'\
'ipv4,224.0.0.20,1234'

kiss is not required because UDP is already a packet-oriented media. Or you can use the greflector program to create a simulated radio situation. On the machine "radiopi2", run:

greflector kiss,tcp,1234

which will create a program that will reflect all received input to all other connections. Then on the accepter side:

gensiot -i 'stdio(self)' -a \
'ax25(laddr=AE5KM-1),kiss,conacc,tcp,radiopi2,1234'

and the connecting side:

gensiot -i 'stdio(self)' \
'ax25(laddr=AE5KM-2,addr="0,AE5KM-1,AE5KM-2"),kiss,tcp,radiopi2,1234'

The test code uses the reflector for some testing, since it's so convenient to use.

ratelimit

Limit the data throughput for a gensio stack.

cm108gpio

Allow a GPIO on a CMedia CM108 or equivalent sound device to be controlled. Used with afskmdm for keying a transmitter.

These are all documented in detail in gensio(5). Unless otherwise stated, these all are available as accepters or connecting gensios.

Creating Your Own Gensios

You can create your own gensios and register them with the library and stack them along with the other gensios.

The easiest way to do this is to steal code from a gensio that does kind of what you want, then modify it to create your own gensio. There is, unfortunately, no good documentation on how to do this.

The include file include/gensio/gensio_class.h has the interface between the main gensio library and the gensio. The gensio calls all come through a single function with numbers to identify the function being requested. You have to map all these to the actual operations. This is somewhat painful, but it makes forwards and backwards compatibility much easier.

Creating your own gensio this way is fairly complex. The state machine for something like this can be surprisingly complex. Cleanup is the hardest part. You have to make sure you are out of all callbacks and no timers might be called back in a race condition at shutdown. Only the simplest gensios (echo, dummy), strange gensios (conadd, keepopen, stdio), and gensios that have channels (mux, ax25) directly implement the interface. Everything else uses include/gensio/gensio_base.h. gensio_base provides the basic state machine for a gensio. It has a filter portion (which is optional) and a low-level (ll) portion, which is not.

The filter interface has data run through it for the processing. This is used for things like ssl, certauth, ratelimit, etc. Filter gensios would use this. These all use gensio_ll_gensio (for stacking a gensio on top of another gensio) for the ll.

Terminal gensios each have their own ll and generally no filter. For lls based on a file descriptor (fd), gensio_ll_fd is used. There is also an ll for IPMI serial-over-lan (ipmisol) and for sound. Most of the terminal gensios (tcp, udp, sctp, serial port, pty) use the fd ll, obviously.

Once you have a gensio, you can compile it as a module and stick it in $(moduleinstalldir)/<version>. Then the gensio will just pick it up and use it. You can also link it in with your application and do the init function from your application.

mDNS support

The mdns gensio has already been discussed, but the gensio library provides an easy to use mDNS interface. The include file for it is in gensio_mdns.h, and you can use the gensio_mdns(3) man page to get more information on it.

To make an mdns connection using gensiot, say you have ser2net set up with mdns enabled like:

connection: &my-port
  accepter: telnet(rfc2217),tcp,3001
  connector: serialdev,/dev/ttyUSB1,115200N81
  options:
    mdns: true

then you can connection to it with gensiot:

gensiot 'mdns,my-port'

gensiot will find the server, port, and whether telnet and rfc2217 are enabled and make the connection.

In addition, there is an gmdns tool that lets you do queries and advertising, and gtlssh can do mDNS queries to find services. If you have secure authenticated logins for ser2net, and you enable mdns on ser2net, like:

connection: &access-console
  accepter: telnet(rfc2217),mux,certauth(),ssl,tcp,3001
  connector: serialdev,/dev/ttyUSBaccess,115200N81
  options:
    mdns: true

it makes the setup very convenient, as you can just do:

gtlssh -m access-console

That's right, you can just directly use the connection name, no need to know the host, whether telnet or rfc2217 is enabled, or what the port is. You still have to set up the keys and such on the ser2net server, of course, per those instructions.