From: Philipp Stanner > Sent: 10 February 2024 17:09 > > Am Samstag, dem 10.02.2024 um 17:03 +0000 schrieb David Laight: > > From: o.evistel@xxxxxxx > > > Sent: 09 February 2024 10:06 > > > > > > I am using linux-sctp as transport for SIGTRAN M3UA on RHEL 8.4 > > > with > > > multihoming (sctp_bindx(), sctp_connectx() API functions). > > > I would like to know, after association setup, if it is possible to > > > instruct SCTP to use a specific local address from the list of > > > bound > > > addresses to reach the peer. > > > > Unlikely in the extreme. > > > > If there are 'n' bound local addresses and 'm' remote addresses > > (IIRC from the INIT_ACK - but they come from the far end) then > > Linux only verifies a route to each local address and picks an > > appropriate local address for each one. > > So it only sends heartbeats to 'm' addresses, not on 'n * m' > > address pairs. > > > > So if anything of this nature did exist it would limit the > > remote addresses used, not the local ones. > > I've been told once that using the socket option SO_BINDTODEVICE should > serve the trick, i.e., it should provoke Linux into choosing the picked > device's IP addr as the source IP addr. > > I've never tested / verified that, though. > I'm also not sure if it would have other negative consequences, such as > limiting the reachability through non-bound devices. But won't that defeat the entire point of SCTP multihoming? You need two interfaces in different subnets that use entirely separate IP networks to connect to the two addresses the remote system gives you. (There are really only ever two addresses for each system.) I don't know what the standards people were smoking, but the default 'send all my IP addresses to the far end' is so broken. Requiring the application to know which addresses are 'local' (typically 192.178.x.x and 10.x.x.x) and pretty much must never be sent to a remote system (where they can easily mean something else) is just wrong, it ought to be a property of the protocol stack. Never mind that the fact that API calls like bindx() were originally implemented using socket options being embedded in the standard. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
