HELP! Unable to lower MTU on pppoe connection
 

Hi

Getting a little desperate here.... :confused:

I have an adsl connection on a firewall running Debian Sarge that uses PPPoE.

Since this firewall is also a VPN gateway (and I have trouble with PMTU, DF etc.) I need to lower the MTU to 1412 for the pppoe connection.

According to all I have read this must be done via the /etc/ppp/peers/dsl-provider file, but any changes to the mtu setting in this is ignored.

I get an mtu of 1492 no matter what I try.

I have also tried setting the mtu option in /etc/ppp/options (and even the mru same place).

I have tried setting "mtu 1412" for the physical interface (eth2) in /etc/network/interfaces.

No matter what I do I get an mtu of 1492 on the ppp0 interface when viewed with "ifconfig -a".

I had the same setup privously running on a Slackware box, where lowering the mtu wasn't a problem. But the configuration files for pppd is a little different in Debian.

What am I doing wrong?

Here is the (slightly modified) output from /var/log/debug from when the connection is set up (I have enabled "debug" in /etc/ppp/options):

Oct 13 13:53:21 mail kernel: eth2: Identified 8139 chip type 'RTL-8100B/8139D'
Oct 13 13:53:22 mail pppd[364]: rcvd [LCP ConfReq id=0x2 <mru 1492> <auth pap> <magic 0xe63da026>]
Oct 13 13:53:22 mail pppd[364]: sent [LCP ConfAck id=0x2 <mru 1492> <auth pap> <magic 0xe63da026>]
Oct 13 13:53:22 mail pppd[364]: sent [LCP EchoReq id=0x0 magic=0x794b9ff8]
Oct 13 13:53:22 mail pppd[364]: sent [PAP AuthReq id=0x1 user="hidden" password=<hidden>]
Oct 13 13:53:22 mail pppd[364]: rcvd [LCP EchoRep id=0x0 magic=0xe63da026]
Oct 13 13:53:22 mail pppd[364]: rcvd [PAP AuthAck id=0x1 ""]
Oct 13 13:53:22 mail pppd[364]: sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Oct 13 13:53:22 mail pppd[364]: rcvd [IPCP ConfReq id=0x1 <addr 213.173.x.x>]
Oct 13 13:53:22 mail pppd[364]: sent [IPCP ConfAck id=0x1 <addr 213.173.x.x>]
Oct 13 13:53:22 mail pppd[364]: rcvd [IPCP ConfNak id=0x1 <addr 213.173.x.x> <ms-dns1 195.184.96.2> <ms-dns3 195.184.96.3>]
Oct 13 13:53:22 mail pppd[364]: sent [IPCP ConfReq id=0x2 <addr 213.173.x.x> <ms-dns1 195.184.96.2> <ms-dns3 195.184.96.3>]
Oct 13 13:53:22 mail pppd[364]: rcvd [IPCP ConfAck id=0x2 <addr 213.173.x.x> <ms-dns1 195.184.96.2> <ms-dns3 195.184.96.3>]
Oct 13 13:53:22 mail pppd[364]: Script /etc/ppp/ip-up started (pid 657)
Oct 13 13:53:22 mail pppd[364]: Script /etc/ppp/ip-up finished (pid 657), status = 0x0

Thanks,
Henning

You can configure the mtu on the interface which pppoe uses, e.g. eth0:

auto eth0
iface eth0 inet dhcp
mtu 1452

I have the same problem with pppoe, but setting the mtu on the hardware interface solves it for me.