[TUHS] Ethernet in /dev (was Re: Were all of you.. Hippies?)

Norman Wilson norman at oclsc.org
Fri Mar 31 01:55:25 AEST 2017


Josh Good:

  Which brings up a question I have: why didn't UNIX implement ethernet
  network interfaces as file names in the filesystem? Was that "novelty" a
  BDS development straying away from AT&T UNIX?

======

Remember that UNIX has long been a family of systems;
it's risky to make blanket statements!

The following is from memory; I haven't looked at this
stuff for a while and am a few kilometers from my
manuals as I type this.  I can dig out the complete
story later if anyone wants it; for now, this is just
the flavour and the existence proof.

Research UNIX, once it supported Ethernet at all, did
so using devices in /dev; e.g. /dev/qe0[0-7] were the
conventional names for the first DEQNA/DELQA device on a
MicroVAX.  There were eight subdevices per physical
device, each a channel that might receive datagrams
of a particular 16-bit type, programmed by an ioctl.

To set up what we now call an IP interface, one did
the following:

a.  Open an unused channel for the proper Ethernet
physical device.  (I think the devices were exclusive-
open to make this easier: open /dev/qe00, then qe01,
and so on until one succeeds.)

b.  Issue the ioctl to set the desired datagram type,
usually 0x800.

c.  Push the IP stream line discipline onto the open file.

d.  Issue an ioctl to inform that IP instance of its
address and netmask.

Now datagrams of the specified type arriving on that
device are fed to the IP subsystem, and the IP
subsystem uses the IP address and mask and possibly
other information to decide which datagrams to route
to that IP instance, which sends them out that physical
device.

I forget how ARP and Ethernet encapsulation fit in.
I know that they were someone more naive early on,
and that in the 10/e systems I can now admit I have
running at home I made things a bit smarter and less
brittle.  But that's the basic architecture.

So how does one actually make, say, a TCP connection?
Another layer of the same sort:

There are devices /dev/ipX, served by an IP device
driver that is part of the IP subsystem.  Originally
minor device X was hard-connected to IP protocol X;
I later changed that to be ioctl-configured as well.

To make TCP usable:

a.  Open /dev/ip6 (old school), or find an unused
/dev/ipX (again they are exclusive-open) and configure
it to accept protocol 6.

b.  Push the TCP stream line discipline onto the
open file.

c.  There are probably things one could then configure,
but I don't remember them offhand.

To make a TCP call, open an unused /dev/tcpNN device;
write something to it (or maybe it was an ioctl) with
the desired destination address; wait for an error or
success.  On success, writes to the file descriptor send
to the network, encapsulated as a TCP stream; reads
receive.

To receive a TCP call, open an unused /dev/tcpNN device,
and write something (or ioctl) to say `I want to listen
on this local port.'  Then read the file.  When a call
arrives, you will read a message saying who's calling,
and what /dev/tcpXX device you should open to accept the
call.

Notice the general scheme: for both TCP and IP (and there
was a primitive UDP at one point, but it has fallen out
of use on my systems), the protocol implementation
comprises:

1.  A line discipline: push it onto devices that will
transport your data.

2.  A device driver: use those devices to send and
receive calls.

The two are inextricably coupled inside the operating
system, of course.

There are all sorts of unfortunate details surrounding
communications; e.g. the TCP code knows it is talking
to IP, and constructs datagrams with partly-filled-in
IP headers.  (It is not clear one can do better than
that in practice, because the protocols themselves really
are linked, but I still think it's unfortunate.)

On the other hand, that the junctions between plumbing
are accessible makes some things very simple.  I wrote
a PPP implementation in user mode, with no kernel
changes: to plug itself into IP, it just pushed the
IP line discipline onto one end of a pipe, and read
and wrote datagrams on the other.  I later extended it
to PPPoE by having it open an Ethernet device set to
the proper datagram types (there is one for data and
another for connection setup).

On the other other hand, there are no permissions on
stream line disciplines, so an untrustworthy person
(if I allowed such on my systems) could push the IP
line discipline onto his own pipe and send whatever
datagrams he liked.  This is decidedly a flaw.

Those familiar with the original stream-system
implementation have already spotted a lesser flaw:
the file descriptor with IP pushed on (or TCP, or
whatever) must remain open; when it is closed,
everything shuts down.  In practice it is usually
useful to have a daemon listening to that file anyway;
that's a good way for the system to report errors or
confusion.  In practice, TCP incalls and outcalls
all went through a special daemon anyway, so that
programs didn't have to be full of TCP-specific crap;
that's what Dave Presotto's `connection server' is
all about.

Norman Wilson
Toronto ON


More information about the TUHS mailing list