>> IP_BIND_ADDRESS_NO_PORT did not fix your somewhat similar problem in your
>> Haproxy setup, because all the connections are to the same dst tuple <ip, port>
>> (i.e 127.0.0.1:ExtORPort).
>> The connect() system call is looking for a unique 5-tuple <protocol, srcip,
>> srcport, dstip, dstport>. In the Haproxy setup, the only free variable is
>> srcport <tcp, 127.0.0.1, srcport, 127.0.0.1, ExtORPort>, so toggling
>> IP_BIND_ADDRESS_NO_PORT makes no difference.

> No—that is what I thought too, at first, but experimentally it is not
> the case. Removing the IP_BIND_ADDRESS_NO_PORT option from Haproxy and
> *doing nothing else* is sufficient to resolve the problem. Haproxy ends
> up binding to the same address it would have bound to with
> IP_BIND_ADDRESS_NO_PORT, and there are the same number of 5-tuples to
> the same endpoints, but the EADDRNOTAVAIL errors stop. It is
> counterintuitive and unexpected, which why I took the trouble to write
> it up.

> As I wrote at #40201, there are divergent code paths for connect in the
> kernel when the port is already bound versus when it is not bound. It's
> not as simple as filling in blanks in a 5-tuple in otherwise identical
> code paths.

> Anyway, it is not true that all connections go to the same (IP, port).
> (There would be no need to use a load balancer if that were the case.)
> At the time, we were running 12 tor processes with 12 different
> ExtORPorts (each ExtORPort on a different IP address, even: 127.0.3.1,
> 127.0.3.2, etc.). We started to have EADDRNOTAVAIL problems at around
> 3000 connections per ExtORPort, which is far too few to have exhausted
> the 5-tuple space. Please check the discussion at #40201 again, because
> I documented this detail there.

> I urge you to run an experient yourself, if these observations are not
> what you expect. I was surprised, as well.