"incomplete startup packet" on SGI

I have a working 8.1 server running on Linux and I can connect to it
from other Linux clients.  I built postgresql 8.1 on an SGI (using
--without-readline but otherwise stock) and it compiled OK and installed
fine.  But when I try to connect to the Linux server I get "could not
send startup packet: transport endpoint is not connected" on the client
end and "incomplete startup packet" on the server end.  Connectivity
between the two machines is working.

I could find basically no useful references to the former and the only
references to the latter were portscans and the like.

Browsing the source, I see a couple places that message could come
from.  One relates to SSL, which the output from configure says is
turned off on both client and server.  The other is just a generic comm
error--but would could cause a partial failure like that?