On my machine (linux 2.6.7 but could upgrade to current), I sometimes have
two ppp interfaces,
ppp0 is a pppoE using ADSL
ppp1 is a pt-to-pt traditional pppd over the serial port
These are both "on the internet", i.e. both have genuine external internet
addresses, let's say
ppp0 66.66.66.66
ppp1 129.129.129.129
(relevant part of) My route -n looks something like this
64.64.64.64 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
165.165.165.165 0.0.0.0 255.255.255.255 UH 0 0 0 ppp1
0.0.0.0 64.64.64.64 0.0.0.0 UG 0 0 0 ppp0
where the first two are the remote ends of the pt-to-pt's.
I would like all tcp/udp traffic that has a source port of 25 (SMTP) to go
out on ppp1, regardless of destination address (which could be any mail
exchanger anywhere). (Reason explained below) Is there any way I can do
this without alterning my default route?
I thought maybe I could use mangle POSTROUTING -p tcp --sport 25 -j ROUTE
--oif=ppp1
BUT then I read this in the netfilter extensions howto:
--oif ifname
Send the packet out using `ifname' network interface. The destination
host must be on the same link or the interface must be a tunnel. Otherwise,
arp resolution cannot be performed and the packet is dropped.
If I understand this, then my idea won't work as the destination address is
many links distant. (Didn't actually try it as it appears I don't actually
even have the libipt_ROUTE.so).
Would iproute2 help here? Or what? Code it myself? (Would try
if feasible). Hoping it's easier than that.
John
Reason I want to do this: very simple - my ADSL ISP blocks SMTP traffic
as they want to be the sole e-mail server. Occasionally I need to
receive large e-mails directly to my machine (into sendmail) and have a
dial-up ISP that permits this, but I don't want any other traffic to go over
this dial-up interface. I arrange for the sending email MX to send
to my ppp1 address, but (my) sendmail then likes to establish a reverse
connection back to the external MX, and of course that goes out via the
default gw on ppp0 - and dies at the hands of the ADSL ISP. Currently I
can do this the "hard" way by setting an OUTPUT log rule on --sport 25 so
that I can see who it is going to and then selectively adding that MX's
address into my route over dev ppp1. Would like to automate it.
