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Dear Brian,

Thank you for your advice on the NATs.

I am now trying to select by which interface a message is emitted on a
multihomed host.

The best way I can think of is to specify the source address but
modifying that in the ip header does not determine by which interface
the packet leaves.

I tried to setsockopt SOL_IP IP_PKTINFO which should allow me to specify
the interface, src address and destination address.

In lksctp where sendmsg and recvmsg are already used, the IP_PKTINFO
behaviour is undetermined. Regardless of whether I specify the interface
index of eth0 or eth1, all the traffic is leaving through eth1. For the
moment, I do not specify the src address ip_spec_dst and destination
address ipi_addr in the in_pktinfo structure. The man ip (7) pages are
quite confusing between src and dest addresses for this structure.

eth0 and eth1 are valid interfaces and both of their addresses are part
of the association.

Eventually, I would like to choose the src address in Randall's
reference implementation, i.e. on the raw sockets created.

Would you kindly advise if there are any fundamental problems with this
approach? i.e. does the packet follow the routing tables or cache no
matter what??

Is it really impossible to specify the source interface by which a
packet in the SCTP association is routed using IPv4, since it is
"destination-based"? How can one choose the src address in user space?

Thank you very much for your comments.

Regards,
Se-Hsieng

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• [SCTP-Discussion] Source Address Selection on multihomed host, Eng Se-Hsieng, 05/09/2003

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