Re: Bash script to see if PPP link is up...
From: Lew Pitcher (lpitcher_at_sympatico.ca)
Date: 08/23/03
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Date: Fri, 22 Aug 2003 21:11:52 -0400
Alan Connor wrote:
> On Fri, 22 Aug 2003 03:53:41 +0000 (UTC), David Efflandt <efflandt@xnet.com> wrote:
>
>>
>>On Thu, 21 Aug 2003 21:28:57 GMT, Alan Connor <xxxxxx@xxxx.xxx> wrote:
>>
>>>On Thu, 21 Aug 2003 21:54:51 +0100, JRH <john.nntp@yoredale2003.uklinux.net> wrote:
>>
>>ifconfig may be able to tell if it is up, but can it tell if it is
>>connected or not? For example when you use demand pppd, the ppp0
>>interface appears in ifconfig, but it is not connected until there is
>>traffic routed to it. Likewise if an idle time is set for a demand
>>connection and it disconnects, ifconfig would still show it up. That is
>>why I use ip-up & ip-down scripts (which are reliable even for demand and
>>idle).
>>
>
>
> To repeat:
>
> If my ppp connection is not up, /sbin/ifconfig ppp0 exits with an error
> message and the exit code reads 1
>
> If the connection is up, then I get the full info and the exit code is 0.
The keys to this are the words "my" and "I", as in "my ppp connection" and "I get".
Your experience is not the be-all and end-all of Linux, and ifconfig is not a reliable
way to determine if pppd has established an internet connection.
[snip]
> It seems that some folks have the mistaken notion that all programs and
> utilities with the same name behave just the same, even on differently
> configured OSs.
Including you. You seem to think that ifconfig is a reliable indicator, and /we/
have experience that it is not.
[snip]
So, instead of debating who's right and who's wrong, lets /see/ what happens.
On /my/ system, I'll try /your/ suggestion and JRH's script, and we'll see if
either of you have a solution that will work for me.
I run Slackware Linux 9.0 (I've run Slackware Linux since it's 3.0 release),
with current levels of all relevant packages and the 2.4.21 kernel. I have
pppd running in "dial-on-demand" mode so that any off-lan activity from my
lan gets routed through the ppp interface to the internet.
/I/ say that your ifconfig trick doesn't reliably indicate when this config
is "on the internet" and when it is "off". /I/ say that JRH's script suffers
from the same unreliability, although his script gives a little better info
than technique does. /I/ say that the only reliable way /I've/ found to
determine the state of my internet connection is to use the features of
the /etc/ppp/ip-up and /etc/ppp/ip-down scripts.
So, here's my demonstration...
Linux Kernel 2.4.21 compiled for Pentium processor
root@merlin:~# uname -a
Linux merlin 2.4.21 #17 Sat Jun 21 21:34:23 EDT 2003 i586 unknown
Slackware Linux 9.0 installation (all packages updated)
root@merlin:~# cat /etc/slackware-version
Slackware 9.0.0
The ppp-2.4.1-i386-2 package from slackware.com
root@merlin:~# ls /var/adm/packages/ppp*
/var/adm/packages/ppp-2.4.1-i386-2
Verify that...
root@merlin:~# head -16 /var/adm/packages/ppp-2.4.1-i386-2
PACKAGE NAME: ppp-2.4.1-i386-2
COMPRESSED PACKAGE SIZE: 186 K
UNCOMPRESSED PACKAGE SIZE: 480 K
PACKAGE LOCATION: slackware/n/ppp-2.4.1-i386-2.tgz
PACKAGE DESCRIPTION:
ppp: ppp (Point-to-Point Protocol)
ppp:
ppp: The Point-to-Point Protocol (PPP) provides a method for transmitting
ppp: data over serial links. It's commonly used for connecting to the
ppp: Internet using a modem. This package includes the PPP daemon (pppd),
ppp: which negotiates with the peer to establish the link and sets up the
ppp: ppp network interface, and pppsetup, an easy-to-use utility for
ppp: setting up your PPP daemon.
ppp:
ppp:
ppp:
Even pppd agrees.
root@merlin:~# pppd -h
pppd version 2.4.1
Usage: pppd [ options ], where options are:
<device> Communicate over the named device
<speed> Set the baud rate to <speed>
<loc>:<rem> Set the local and/or remote interface IP
addresses. Either one may be omitted.
asyncmap <n> Set the desired async map to hex <n>
auth Require authentication from peer
connect <p> Invoke shell command <p> to set up the serial line
crtscts Use hardware RTS/CTS flow control
defaultroute Add default route through interface
file <f> Take options from file <f>
modem Use modem control lines
mru <n> Set MRU value to <n> for negotiation
See pppd(8) for more options.
Right now, ppp dial-on-demand is down, so no ppp0 shows up in ifconfig
root@merlin:~# /sbin/ifconfig
eth0 Link encap:Ethernet HWaddr 00:C0:F0:37:C2:80
inet addr:192.168.11.1 Bcast:192.168.11.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:7306 errors:38 dropped:0 overruns:0 frame:38
TX packets:57475 errors:0 dropped:0 overruns:0 carrier:0
collisions:847 txqueuelen:100
RX bytes:758130 (740.3 Kb) TX bytes:6155617 (5.8 Mb)
Interrupt:10 Base address:0x6000
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:6329 errors:0 dropped:0 overruns:0 frame:0
TX packets:6329 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:3116710 (2.9 Mb) TX bytes:3116710 (2.9 Mb)
We try your trick of checking the returncode from ifconfig,
and find that ifconfig returns 1 when no PPP daemon is running
root@merlin:~# /sbin/ifconfig ppp0
ppp0: error fetching interface information: Device not found
root@merlin:~# echo $?
1
Now, I start up the ppp0 dial-on-demand daemon (by hand, although
I usually use the equivalent ppp-go script).
root@merlin:~# rm -f /var/LCK* /var/run/ppp*.pid
root@merlin:~# /usr/sbin/pppd file "/etc/ppp/options.demand" &
[1] 3333
Now, ifconfig shows that ppp0 is up, connecting local IP address 10.64.64.64
to remote IP address 10.10.10.10, and ifconfig returns a zero returncode.
root@merlin:~# /sbin/ifconfig ppp0
ppp0 Link encap:Point-to-Point Protocol
inet addr:10.64.64.64 P-t-P:10.10.10.10 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
root@merlin:~# echo $?
0
However,
1) My wife is using our one and only phone line at the moment, so
ppp0 *is not* connected to the internet. For that matter, chat
hasn't even picked up the phone, let alone tried to dial out.
2) Since ppp0 is not connected to the internet, neither the remote IP
address nor the local IP address reported are valid (they are both
placeholder addresses which will be discarded once I /do/ connect
to the internet.
So, let's see how JRH's script works (posted in message <3f45318b@212.67.96.135> )
I cut'n'paste his script directly from his post into the terminal window and
execute it. I get...
root@merlin:~# EXTIF="ppp0" # external interface, may be empty
root@merlin:~# IFCONFIG=/sbin/ifconfig
root@merlin:~# AWK=/bin/awk
root@merlin:~#
root@merlin:~# # get the external IP address - it returns empty string if it is not up
root@merlin:~# if [ -n "$EXTIF" ]; then
> EXTIP="`$IFCONFIG $EXTIF 2>/dev/null | $AWK \
> /$EXTIF/'{next}//{split($0,a,":");split(a[2],a," ");print a[1];exit}'`"
> fi
root@merlin:~# echo " External IP: $EXTIP"
External IP: 10.64.64.64
Again, this is wrong. I'm *not* connected to the internet, and, while the IP
address reported is the placeholder address assigned by pppd, it is *not* the
IP address used by my dial-on-demand pppd.
So, neither your technique nor JRH's technique properly reports the state of
my system's internet connection.
Now, the technique that /I/ use is to have the /etc/ppp/ip-up script create
a sentinal file when pppd establishes it's IP connection to the internet, and
have the /etc/ppp/ip-down script delete the sentinal file. This way, external
processes can check for the existance of the sentinal and determine indirectly
the state of the internet connection.
Take a look at the manpage for pppd...
root@merlin:~# man pppd
PPPD(8) PPPD(8)
NAME
pppd - Point to Point Protocol daemon
SYNOPSIS
pppd [ tty_name ] [ speed ] [ options ]
DESCRIPTION
The Point-to-Point Protocol (PPP) provides a method for transmit-
ting datagrams over serial point-to-point links. PPP is composed
of three parts: a method for encapsulating datagrams over serial
links, an extensible Link Control Protocol (LCP), and a family of
Network Control Protocols (NCP) for establishing and configuring
different network-layer protocols.
[snip]
SCRIPTS
Pppd invokes scripts at various stages in its processing which
can be used to perform site-specific ancillary processing. These
scripts are usually shell scripts, but could be executable code
files instead. Pppd does not wait for the scripts to finish.
The scripts are executed as root (with the real and effective
user-id set to 0), so that they can do things such as update
routing tables or run privileged daemons. Be careful that the
contents of these scripts do not compromise your system's secu-
rity. Pppd runs the scripts with standard input, output and
error redirected to /dev/null, and with an environment that is
empty except for some environment variables that give information
about the link. The environment variables that pppd sets are:
[snip]
Pppd invokes the following scripts, if they exist. It is not an
error if they don't exist.
[snip]
/etc/ppp/ip-up
A program or script which is executed when the link is
available for sending and receiving IP packets (that is,
IPCP has come up). It is executed with the parameters
interface-name tty-device speed local-IP-address remote-
IP-address ipparam
/etc/ppp/ip-down
A program or script which is executed when the link is no
longer available for sending and receiving IP packets.
This script can be used for undoing the effects of the
/etc/ppp/ip-up script. It is invoked in the same manner
and with the same parameters as the ip-up script.
My /etc/ppp/ip-up script creates (or replaces) the /var/run/ppp0.ip file
when ppp0 comes up. The file contains the local IP address assigned by
my ISP, in textual form. This way, I can test for the existance of the
file to determine whether or not my internet connection is up, and /if/
it is up, I can determine the IP address without resorting to the
ifconfig/grep/awk awkwardness.
root@merlin:~# cat /etc/ppp/ip-up
#!/bin/sh
#
# ip-up interface-name tty-device speed local-IP remote-IP ipparm
# $1 $2 $3 $4 $5 $6
#
# (NB: ipparm is string from ipparm parameter in pppd options)
#
# This file /etc/ppp/ip-up is run by pppd when there's a
# successful ppp connection.
#
# The environment is cleared before executing this script
# so the path must be reset.
#
PATH=/usr/bin:/usr/sbin:/usr/local/bin:/sbin:/bin
export PATH
#
SCRIPT=`/usr/bin/basename $0`
#
echo $4 >/var/run/$1.ip
My /etc/ppp/ip-down script deletes the /var/run/ppp0.ip file, so that when I loose
my internet connection, the sentinal file disappears. Again, if an external process
looks for the file, and /does not/ find it, it can assume that /etc/ppp/ip-down
deleted it, and infer that the internet connection has been dropped.
root@merlin:~# cat /etc/ppp/ip-down
#!/bin/sh
#
# This script is run by pppd after the PPP connection is ended.
#
# ip-down interface-name tty-device speed local-IP remote-IP ipparm
# $1 $2 $3 $4 $5 $6
#
# (NB: ipparm is string from ipparm parameter in pppd options)
#
PATH=/usr/sbin:/sbin:/usr/bin:/usr/local/bin:/bin
export PATH
#
SCRIPT=`/usr/bin/basename $0`
#
rm /var/run/$1.ip
Finally, here's an example of a script that determines if my internet
connection is live or not.
root@merlin:~# cat is_ppp_up
#!/bin/bash
if [ -f /var/run/ppp0.ip ]
then
echo "Yes, PPP0 is UP with IP address " `cat /var/run/ppp0.ip`
exit 0
else
echo "No, PPP0 is DOWN"
exit 1
fi
When I run it while I'm "offline", I get
root@merlin:~# ./is_ppp_up
No, PPP0 is DOWN
However, if I do something that requires an internet connection
(like try to browse http://slashdot.net/ under Mozilla), I get
root@merlin:~# ./is_ppp_up
Yes, PPP0 is UP with IP address 64.228.63.114
If I try the ifconfig returncode trick, I get the same results as when my internet
connection is down.
root@merlin:~# /sbin/ifconfig ppp0
ppp0 Link encap:Point-to-Point Protocol
inet addr:64.228.63.114 P-t-P:64.228.63.241 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:100 errors:1 dropped:0 overruns:0 frame:0
TX packets:105 errors:0 dropped:10 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:49724 (48.5 Kb) TX bytes:9794 (9.5 Kb)
root@merlin:~# echo $?
0
If I try JRH's script, I get marginally different results from when my internet
connection was down. At least this time JRH's script properly reports my IP address.
root@merlin:~# EXTIF="ppp0" # external interface, may be empty
root@merlin:~# IFCONFIG=/sbin/ifconfig
root@merlin:~# AWK=/bin/awk
root@merlin:~#
root@merlin:~# # get the external IP address - it returns empty string if it is not up
root@merlin:~# if [ -n "$EXTIF" ]; then
> EXTIP="`$IFCONFIG $EXTIF 2>/dev/null | $AWK \
> /$EXTIF/'{next}//{split($0,a,":");split(a[2],a," ");print a[1];exit}'`"
> fi
root@merlin:~# echo " External IP: $EXTIP"
External IP: 64.228.63.114
And if I try it again after my internet connection comes down ("idle time out"),
I get /the same/ results. It appears that JRH's script thinks that my internet
connection is still up.
root@merlin:~# ./is_ppp_up
No, PPP0 is DOWN
root@merlin:~# EXTIF="ppp0" # external interface, may be empty
root@merlin:~# IFCONFIG=/sbin/ifconfig
root@merlin:~# AWK=/bin/awk
root@merlin:~#
root@merlin:~# # get the external IP address - it returns empty string if it is not up
root@merlin:~# if [ -n "$EXTIF" ]; then
> EXTIP="`$IFCONFIG $EXTIF 2>/dev/null | $AWK \
> /$EXTIF/'{next}//{split($0,a,":");split(a[2],a," ");print a[1];exit}'`"
> fi
root@merlin:~# echo " External IP: $EXTIP"
External IP: 64.228.63.114
root@merlin:~#
In all, ifconfig is not reliable in determining whether pppd has an internet
connection or not. The only reliable way so far seems to be to get the info
directly from pppd through one of it's exits (like /etc/ppp/ip-up).
-- Lew Pitcher Master Codewright and JOAT-in-training Registered Linux User #112576 (http://counter.li.org/) Slackware - Because I know what I'm doing.
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