pppd

Summary
This article explains how to set up a point-to-point connections using pppd and the kernel PPPoE driver.

ppp (Paul's PPP Package) is an open source package which implements the point-to-point protocol (PPP) on Linux and Solaris systems. It is implemented as single pppd daemon and acts as backend for xl2tpd, pptpd and netctl. 3G, L2TP and PPPoE connections are internally based on PPP protocol and therefore can be managed by ppp.

Installation

Install ppp, available in the official repositories.

Make sure that your kernel is compiled with PPPoE support (present in default kernel):

$ zgrep CONFIG_PPPOE /proc/config.gz

CONFIG_PPPOE=m

Configuration

PPPoE

Create the connection configuration file:

/etc/ppp/peers/your_provider

plugin rp-pppoe.so
# rp_pppoe_ac 'your ac name'
# rp_pppoe_service 'your service name'
  
# network interface
eth0
# login name
name "someloginname"
usepeerdns
persist
# Uncomment this if you want to enable dial on demand
#demand
#idle 180
defaultroute
hide-password
noauth

If you want usepeerdns to work, you have to edit your /etc/ppp/ip-up and add a command that copies /etc/ppp/resolv.conf to /etc/resolv.conf.

Edit /etc/ppp/pap-secrets:

Put a line like this in /etc/ppp/pap-secrets or /etc/ppp/chap-secrets as required by the authentication method used by your ISP. It's OK to write these two files at the same time, pppd will automatically use the appropriate one.

someloginname * yourpassword

You can now start the link using the command

# pppd call your_provider

Alternatively, you can use this

# pon your_provider

To see whether your pppoe connection is started correctly, check /var/log/errors.log first and then check /var/log/everything.log. On a successful connection, you should see something like the following in the everything.log:

# tail /var/log/everything.log 

Aug  9 00:18:08 localhost pppd[2268]: Using interface ppp0
Aug  9 00:18:08 localhost pppd[2268]: Connect: ppp0 <--> eth0
Aug  9 00:18:11 localhost pppd[2268]: CHAP authentication succeeded
Aug  9 00:18:11 localhost pppd[2268]: CHAP authentication succeeded
Aug  9 00:18:11 localhost pppd[2268]: peer from calling number 00:06:29:AF:4F:E0 authorized
Aug  9 00:18:11 localhost pppd[2268]: Cannot determine ethernet address for proxy ARP
Aug  9 00:18:11 localhost pppd[2268]: local  IP address 10.6.2.137
Aug  9 00:18:11 localhost pppd[2268]: remote IP address 10.6.1.1
Aug  9 00:18:11 localhost pppd[2268]: primary   DNS address 10.6.1.1
Aug  9 00:18:11 localhost pppd[2268]: secondary DNS address 210.21.196.6

By default the configuration in /etc/ppp/peers/provider is treated as the default, so if you want to make "your_provider" the default, you can create a link like this

# ln -s /etc/ppp/peers/your_provider /etc/ppp/peers/provider

Now you can start the link by simply running

# pon

To close a connection, use this

# poff your_provider

Starting pppd with Arch

• Configure the ppp_generic module to load on boot. See Kernel Modules#Loading for more information.
• Configure to autostart on boot the service ppp@your_provider.service, where your_provider is your configuration file. See Daemons for more information.

Tips and tricks

Do an auto redial

If pppd is running, you can force a connection reset by sending the SIGHUP signal to the process

# export PPPD_PID=$(pidof pppd)
# kill -s HUP $PPPD_PID 

And you have redialed the connection.

Make sure you have persist option enabled in your /etc/ppp/peers/provider tab.

ISP auto-disconnect after 24h

Note: If you aren't running your computer always on (running 24/7) then you can skip this step.

If you use a flat-rate always-on connection on a computer, some providers restart your connection after 24h. That makes sure that the IP is rotated every 24h. To compensate, you can use an dynamic DNS service in combination with inadyn (available on AUR) to compensate for the rotating IP address. But to avoid disconnects when you don't need it, you might try to restart the connection using a cron job at a time of day you know no one will be using the connection (ex. 4 AM).

As root, do the following:

Create a bash script similar to this and give it a name (ex pppd_redial.sh):

#!/bin/bash

message="Restarting the PPP connection @:" $(date)
pppd_id=$(pidof pppd)

kill -s HUP $pppd_id
echo $message

Give it execute permissions and put it on a path visible to root.

Then create a cron job using crontab -e. Check that your EDITOR env variable is set if the command fails. So add anywhere in the file,

0 4 * * * /bin/bash /root/pppd_redial.sh

Confirm that crond is up and running. If it isn't just enable it with,

# systemctl enable cronie.service
# systemctl start cronie.service

Save and exit. Your PPPoE connection will now restart every day at 4AM.

Troubleshooting

Default route

If you have a preconfigured default route before the pppd is started, the default route is kept, so take a look in /var/log/errors.log and if you have something like:

pppd[nnnn]: not replacing existing default route via xx.xx.xx.xx

and xx.xx.xx.xx is not the correct route for you

• Create a new script /etc/ppp/ip-pre-up

$ chmod +x /etc/ppp/ip-pre-up

with this content:

#!/bin/sh
/usr/bin/route del default

• Restart your pppd service.

Masquerading seems to be working fine but some sites don't work.

The MTU under pppoe is 1492 bytes. Most sites use an MTU of 1500. So your connection sends an ICMP 3:4 (fragmentation needed) packet, asking for a smaller MTU, but some sites have their firewall blocking that.

Using PMTU clamping can solve that:

iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

Now, for some reason, just trying to save the resulting iptables configuration with iptables-save and restoring it later, does not work. It has to be executed after the other iptables configuration had been loaded. So, here is a systemd unit to solve it:

pmtu-clamping.service

[Unit]
Description=PMTU clamping for pppoe
Requires=iptables.service
After=iptables.service

[Service]
Type=oneshot
ExecStart=/usr/bin/iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

[Install]
WantedBy=multi-user.target

pppd cannot load kernel module ppp_generic

Symptom: When starting PPTP Client, the pppd process cannot locate the appropriate module.

 Couldn't open the /dev/ppp device: No such device or address
 Please load the ppp_generic kernel module.

Solution: Edit the /etc/modprobe.d/modules.conf file and change

 alias char-major-108 ppp 

to

 alias char-major-108 ppp_generic

If there is no alias included add

 alias char-major-108 ppp_generic

and reboot.