I understand incoming virtual hosts with aliased IPs to an eth device
but is there some way to force outgoing packets to appear to come from
some other device/IP as to what the kernel defaults to ? I can apreciate
this is tantamount to spoofing and "not encouraged" but I now have yet
another legit need for this.

a) I set up small ISPs where both the client dial-in PPP connection and
the main outgoing IPPP (ISDN) link are both connected to the same box with
no LAN effectively in use (ie; the eth0 card could be removed right now).
Here's the routing table with two dialed-in users....

host# route
Kernel IP routing table
Destination     Gateway   Genmask         Flags Metric Ref    Use Iface
gympie-023.blah *         255.255.255.255 UH    0      0      377 ppp5
gympie-008.blah *         255.255.255.255 UH    0      0       92 ppp13
123.45.67.0     *         255.255.255.0   U     0      0     1994 eth0
139.130.0.0     *         255.255.0.0     U     0      0      691 ippp0
127.0.0.0       *         255.0.0.0       U     0      0     1968 lo
default         *         0.0.0.0         U     0      0   171026 ippp0

The eth0 card is ifconfig'd to (say) 123.45.67.1 and the upstream supplier
provides two IPs for the main ISDN connection. For most services, the box
appears from the outside world as the name of the eth0 interface which is
what is wanted. For some services, like IRC, when a dial-in client connects
elsewhere they appear to originate from the assigned ippp0 139.130.*.* IP
rather than the 123.45.67.* eth0 range. If the host was conected to a seperate
router via a LAN where outgoing packets flowed naturally via the eth0
interface then this effect would not be apparent nor a problem. Things work
so it's not a disaster but it makes SSH, tunneling and VPNs rather awkward.

b) Another server _is_ on a LAN with multiple outgoing links, some cheap(er)
and others more expensive. I want this box mainly on the expensive link
so it's web service is snappy to the outside world, but, incoming ftp
transfers are also forced via this same link so I've disabled all mirroring
(yuk, no CPAN). If I could force some eth0 bound packets to appear to eminate
from a different network then I could take advantage of the cheaper/slower
links available for things like large ftp mirrors.

So, is there some "legit" way to do the reverse of virtual IP aliasing and
demand which interface/IP a packet appears to be source'd from ?

--markc
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