Re: How to masquerade when using ROUTE

["Taylor, Grant" <gtaylor@riverviewtech.net>]

> I want to route all outgoing port-25-traffic produced by the clients
> over ppp1. The rest schould take the default route over ppp0.
> 
> I used the following command:
> 
> iptables -A PREROUTING -p tcp --dport 25 -t mangle -i eth0 -j ROUTE
> --oif ppp1

This seems reasonable enough.

> But the source addresses of the packets are wrong as "tcpdump -ni ppp1"
> shows:
> 
> 01:20:24.422756 IP 192.168.0.4.32825 > 160.45.10.13.25: S
> 2020082843:2020082843(0) win 5840 <mss 1460,sackOK,timestamp 266850[|tcp]>
> 
> 192.168.0.4 is the IP adress of the client which tried to connect to the
> mailserver 160.45.10.13.25.
> 
> Masquarding is done to 192.168.0.0/24:
> 
> # iptables -t nat -L POSTROUTING
> Chain POSTROUTING (policy ACCEPT)
> target     prot opt source               destination
> MASQUERADE  all  --  192.168.0.0/24       anywhere
> 
> My default route is set to ppp0.
> 
> So, how can I replace/masquerade the 192.168.0.4 by the official IP
> address of ppp1?

It looks like there might be a chance that traffic that goes out ppp0 and ppp1 are matching your one POSTROUTING rule.  Is there a reason that you are not specifying an interface the traffic is going out to match against?  I.e.

iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
iptables -t nat -A POSTROUTING -o ppp1 -j MASQUERADE

I'm wondering if there is not some confusion in the kernel / routing code as to which source IP to use when going out ppp1.



Grant. . . .