Hi all!
Here is my network

linux1:
eth0=192.168.1.1
ppp0=192.168.5.2
gw=192.168.1.2 ( thru ADSL)
compA=192.168.1.6
compB=192.168.1.15


.........................user1,user2
comp A---------|----------------|................................|
......................| linux1----------|------------adsl--------|
comp B---------|----------------|...............................|internet
.............................|......................|--------|
............................. \------ppp0-----|linux2 |---------|
.....................................................|--------|............|

Better network diagram is here:
http://desima.objectis.net/network-diag/


Question: how to allow compB and user2 to use ppp0?

if I change default route to ppp0 then it is posiible to set compA use adsl and compB to use ppp0, if I change default route back to eth0(192.168.1.2) then it is not possible for compB( or user2) use ppp0 for getting acces to internet
got error MASQUERADE: Route sent us somewhere else.

If I route thru other comp on 192.168.1.217 ( which has ppp to internet)
ip route change default via 192.168.1.217 table T2
everithing works!

config:
ip rule ls:
0: from all lookup local
32750: from all fwmark 0x1 lookup T2
32751: from 192.168.5.2 lookup T2
32752: from 192.168.5.1 lookup T2
32756: from 192.168.1.6 lookup T1
32757: from 192.168.1.15 lookup T2
32766: from all lookup main
32767: from all lookup default

ip route table T2
192.168.5.0 dev ppp0 scope link src 192.168.5.2
192.168.1.0/24 dev eth0 scope link
127.0.0.0/8 dev lo scope link
default via 192.168.5.1 dev ppp0

ip route ls
192.168.1.0 dev eth0 scope link src 192.168.1.1
192.168.5.1 dev ppp0 proto kernel scope link src 192.168.5.2
203.97.61.42 via 192.168.1.2 dev eth0
192.168.1.0/24 dev eth0 scope link
127.0.0.0/8 dev lo scope link
default via 192.168.1.2 dev eth0

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

What I am doing wrong?

for CompB sorted using
iptables -t nat -A POSTROUTING -j SNAT --to 192.168.5.2 --src 192.168.1.15

or better
iptables -t nat -A POSTROUTING -o ppp0 -j SNAT --to-source 192.168.5.2

but stil no luck for user2

Have a look at this, Section 4..
http://tldp.org/HOWTO/Adv-Routing-HOWTO

You need to set some routing tables to match.
Look carefully how replies come back...

Yes it is the first place when I've looked.

When user send a packet it is going thru ppp0 but replies go from linux2 to 192.168.1.1 not thru ppp0. (so they never arrived)

If I change it with SNAT then they come back at ppp0 (192.168.5.2) but not reaching the user

Can you make a clearer diagram pls..

I can't see what is connected to where..

Better network diagram is here:

http://desima.objectis.net/network-diag/