Start, Inspect and Stop PPP Connections with pon and poff

pon dials a named PPP peer by handing it straight to pppd, and it will not tell you a thing if it works. You will bring up one configured peer, check what actually happened, and stop the right connection with poff. The examples follow the installed Debian PPP scripts from package ppp 2.4.9-1+1.1ubuntu4. Allow about fifteen minutes, plus whatever your modem, chat script or provider needs.

You need ppp installed, a peer file under /etc/ppp/peers/, and permission to use it. Do not put passwords in commands or paste secrets into a support transcript.

1. Check the available peer and command

• Confirm the binaries exist. Run command -v pon poff plog and expect all three to resolve.
• Check the peer file is there. Replace PEER_NAME with the basename of the peer file you were given, such as provider or home-vpn, and list it with ls -l /etc/ppp/peers/PEER_NAME.
• Read the usage line. pon --help prints Usage: pon [OPTIONS] [provider] [arguments].

$ command -v pon poff plog
/usr/bin/pon
/usr/bin/poff
/usr/bin/plog
$ ls -l /etc/ppp/peers/PEER_NAME
$ pon --help
Usage: pon [OPTIONS] [provider] [arguments]

The peer name is not an arbitrary label: pon PEER_NAME selects /etc/ppp/peers/PEER_NAME directly. That file can hold a chat script, credentials and device settings, so inspect it through your normal privileged review process rather than editing a working file on a whim.

2. Start one named PPP peer

Starting a connection changes network state and may trigger calls or charges. Confirm the peer name before you run anything:

$ pon PEER_NAME

• Expect silence. pon normally produces no success message; it just passes the peer name to pppd.
• Watch extra arguments. Anything after the peer name is passed on as additional pppd arguments, so do not copy options from a different provider's instructions without checking that provider's peer configuration.
• Know the fallback. With no argument, pon first runs /etc/ppp/ppp_on_boot if that file exists and is executable, otherwise it falls back to /etc/ppp/peers/provider. That default can catch you out on a host with several peers, so name the peer explicitly.

Verify the process and inspect the latest log lines:

$ pgrep -a pppd
$ plog

plog shows the tail of /var/log/ppp.log when that file is non-empty. If it is absent or empty, it filters the tail of /var/log/syslog for pppd and chat messages instead. It needs root or membership of the adm group. For more lines, pass an option tail understands, for example plog -n 50.

3. Diagnose a failed start without repeating it blindly

If pon names a missing peer, it reports that /etc/ppp/peers/PEER_NAME does not exist. Check the exact spelling and list the directory with the access needed to do so:

$ ls -l /etc/ppp/peers/
$ plog -n 50

If a connection process exists but the link never became usable, read the chat and PPP messages before retrying. A second pon can create another simultaneous connection: the manual explicitly permits multiple connections, so check first rather than assume:

$ pgrep -a pppd
$ ps -o pid,args -C pppd

Do not treat an empty plog result as proof the start failed. Logging may be going to the system log, access may be restricted, or the process may have exited quickly. Use the exit status straight after the command you are testing:

$ pon PEER_NAME
$ printf 'pon exit status: %s\n' "$?"

A zero status means the start script accepted the request. It does not prove authentication, carrier detection or IP configuration succeeded; that needs the process and log checks above.

4. Stop exactly one connection

Stopping PPP is service-disrupting, so know what you are about to hit before you hit it.

• One connection running. poff with no argument stops or signals it.
• More than one connection running. An argument-free poff refuses to choose and exits with status 1, so name the peer: poff PEER_NAME.
• Wrong peer name. The script reports it could not find a matching pppd call PEER_NAME process, a useful safeguard: stop and check the name rather than sending signals to unrelated processes.

$ poff
$ printf 'poff exit status: %s\n' "$?"
$ pgrep -a pppd || true

$ poff PEER_NAME

5. Use the signalling options deliberately

poff accepts one option at a time:

• -r asks pppd to drop the line and redial.
• -d toggles its debug state.
• -c asks it to renegotiate compression.

$ poff -r PEER_NAME
$ poff -d PEER_NAME
$ poff -c PEER_NAME

These signal a live connection, so use them only when the peer's operational procedure calls for that particular action. For a normal shutdown, plain poff PEER_NAME is enough. poff -h prints help, and the installed script reports its own version with poff -v:

$ poff -v
/usr/bin/poff: 1.8