getting the device of the ppp connection

getting the device of the ppp connection

[Mark Ryden]

Hello,
  Is there a way from user space to know which device is the one which
the ppp connection uses ?

What I mean is this: I have eth0 - eth3 on a machine, and a ppp
connection. I want to know by a
short program which device is the one which the ppp connection is
using (I know that usually this can
be deduced by running "ps" for the corresponding ppp daemon, and
inspecting the parameters; for example, with pppoe, I have "-I eth1"
parameter (-I stands for interface). But I want a more general
way which can be used in cases when we cannot get this info).

I tried to get info by an ioctl on ("dev/ppp") thus:

  ...
  ...
  int fd = open("/dev/ppp", O_RDWR);
  int ifunit;
  int res;
  if (fd>0) {
      res = ioctl(fd,PPPIOCATTACH,&ifunit);
		
      if (res=0)
	printf("ifunit=%d\n",ifunit);
  }

and got ifunit=0 when using eth0 ; but I ****also**** got  ifunit=1
when using eth1.
So - is there a way to get the interface from the unit number ? or any other
way to get this info ?

Rgs,
Mark

Re: getting the device of the ppp connection

[Charlie Brady]

On Fri, 13 Nov 2009, Mark Ryden wrote:

> What I mean is this: I have eth0 - eth3 on a machine, and a ppp
> connection. I want to know by a
> short program which device is the one which the ppp connection is
> using (I know that usually this can
> be deduced by running "ps" for the corresponding ppp daemon, and
> inspecting the parameters; for example, with pppoe, I have "-I eth1"
> parameter (-I stands for interface). But I want a more general
> way which can be used in cases when we cannot get this info).

I would do this via an environment variable that you set via your start 
script:

PPPOE_DEVICE=eth1 pppoe .... -I eth1

Have a look at daemontools or runit for mechanisms to give you good 
reliable control of the execution environment of long-running processes.

---
Charlie

Re: getting the device of the ppp connection

[walter harms]

Charlie Brady schrieb:
> 
> On Fri, 13 Nov 2009, Mark Ryden wrote:
> 
>> What I mean is this: I have eth0 - eth3 on a machine, and a ppp
>> connection. I want to know by a
>> short program which device is the one which the ppp connection is
>> using (I know that usually this can
>> be deduced by running "ps" for the corresponding ppp daemon, and
>> inspecting the parameters; for example, with pppoe, I have "-I eth1"
>> parameter (-I stands for interface). But I want a more general
>> way which can be used in cases when we cannot get this info).
> 
> I would do this via an environment variable that you set via your start
> script:
> 
> PPPOE_DEVICE=eth1 pppoe .... -I eth1
> 
> Have a look at daemontools or runit for mechanisms to give you good
> reliable control of the execution environment of long-running processes.
> 
> ---

you can scan the output of ifconfig -a,
but normaly pppd setups a ppp0.

re,
 wh

Re: getting the device of the ppp connection

[James Carlson]

walter harms wrote:
> 
> Charlie Brady schrieb:
>> Have a look at daemontools or runit for mechanisms to give you good
>> reliable control of the execution environment of long-running processes.
>>
>> ---
> 
> you can scan the output of ifconfig -a,
> but normaly pppd setups a ppp0.

I think he's actually asking about the device _underneath_ PPP that's in
use, not the "ppp0" network interface that's visible to IP.

For a serial link, that underlying interface is something like "tty0"
(perhaps including information about the number dialed, if it's a modem
or TA link), and for a PPPoE session, that's something like "eth0" plus
access server name and server Ethernet address.  There are many other
cases, including tunneled connections (L2TP and the like) and more
exotic media such as PPPoA and Frame Relay where the underlying
mechanism may have multiple forms of addressing.  In fact, it may have
multiple layers of information to dig through.  (SNMP uses ifstack for
this, I think.)

This is information that's really known only to the dialer involved, and
not to pppd, which 'sees' only a channel for PPP frames.  You can hack
around the issue by setting an environment variable to communicate the
information needed, but there's no native support in the current design
for recording this medium-dependent information.

There probably should be something added, and I'm sure something could
be hacked together for a few special cases, but it's likely a
complicated problem to solve _well_.

-- 
James Carlson         42.703N 71.076W         <carlsonj@workingcode.com>

Re: getting the device of the ppp connection

[Charlie Brady]

On Fri, 13 Nov 2009, James Carlson wrote:

> This is information that's really known only to the dialer involved, and
> not to pppd, which 'sees' only a channel for PPP frames.  You can hack
> around the issue by setting an environment variable to communicate the
> information needed, but there's no native support in the current design
> for recording this medium-dependent information.
>
> There probably should be something added,

I don't agree. The caller of pppd can provide the information in the 
environment. Adding stuff to pppd would just be bloat, and more code 
generally means more bugs.

KISS.

> and I'm sure something could be hacked together for a few special cases, 
> but it's likely a complicated problem to solve _well_.

Indeed.

Re: getting the device of the ppp connection

[James Carlson]

Charlie Brady wrote:
> 
> On Fri, 13 Nov 2009, James Carlson wrote:
> 
>> This is information that's really known only to the dialer involved, and
>> not to pppd, which 'sees' only a channel for PPP frames.  You can hack
>> around the issue by setting an environment variable to communicate the
>> information needed, but there's no native support in the current design
>> for recording this medium-dependent information.
>>
>> There probably should be something added,
> 
> I don't agree. The caller of pppd can provide the information in the
> environment. Adding stuff to pppd would just be bloat, and more code
> generally means more bugs.
> 
> KISS.

I didn't say it would necessarily need to be something added to pppd.
How to arrange for storing and retrieving that information sounds like a
detailed design decision to me, and might not necessarily involve pppd.
 The high level issue is that it should be possible for the user (or the
user's application) to know just the "ppp0" identity and end up knowing
where that interface actually goes.

I think stuffing it into the environment is just a hack.  It sort of
works, but there's no standard way to encode the information in a string
format, so how it's done is going to end up varying by vendor or even by
system administrator whim.  That's not a great answer for folks trying
to build third party applications.  (Such as, say, an SNMP agent
supporting interface stacking ...)

For what it's worth, I agree that pppd probably shouldn't care much
about this.

-- 
James Carlson         42.703N 71.076W         <carlsonj@workingcode.com>

Re: getting the device of the ppp connection

[Paul Mackerras]

Mark Ryden writes:

>   Is there a way from user space to know which device is the one which
> the ppp connection uses ?

Yes, that information is in the tdb database (/var/run/pppd2.tdb).
As an example, here is the list of records in /var/run/pppd2.tdb for
my gateway machine (IP addresses obscured):

DEVICE=eth0: pppd2480
IPREMOTE=xx.xx.xx.xx: pppd2480
IFNAME=ppp0: pppd2480
pppd2480:  ORIG_UID=0;PPPD_PID$83;PPPLOGNAME=root;DEVICE=eth0;LINKNAME=transact;IPLOCAL=yy.yy.yy.yy;IPREMOTE=xx.xx.xx.xx;IFNAME=ppp0
LINKNAME=transact: pppd2480
PPPD_PID$83: pppd2480

That is for one connection.

Below is the source for the little "tdbdump" program that I used to
print the above.  It needs to be linked with pppd/tdb.o and
pppd/spinlock.o.

It wouldn't be hard to write a program to use the lookup routines
in pppd/tdb.c and extract the underlying device for a given ppp
interface.

Paul.

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/time.h>
#include "tdb.h"

static int print_rec(TDB_CONTEXT *tdb, TDB_DATA key, TDB_DATA dbuf, void *p)
{
	printf("%*.*s: %*.*s\n", 
	       (int)key.dsize, (int)key.dsize, key.dptr, 
	       (int)dbuf.dsize, (int)dbuf.dsize, dbuf.dptr);
	return 0;
}

main(int ac, char **av)
{
	TDB_CONTEXT *tdb;

	if (ac != 2) {
		fprintf(stderr, "Usage: %s database\n", av[0]);
		exit(1);
	}

	tdb = tdb_open(av[1], 0, 0, O_RDONLY, 0600);
	if (tdb = 0) {
		perror(av[1]);
		exit(1);
	}

	tdb_traverse(tdb, print_rec, NULL);

	exit(0);
}