# Pppd (简体中文)

Summary help replacing me
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 netcfg. 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
    wall $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

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
    /sbin/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/sbin/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.

Category:

- Networking (简体中文)