[tor-relays] inet_csk_bind_conflict

[David Fifield]

On Fri, Dec 09, 2022 at 09:47:07AM +0000, Alexander Færøy wrote:
> On 2022/12/01 20:35, Christopher Sheats wrote:
> > Does anyone have experience troubleshooting and/or fixing this problem?
> 
> Like I wrote in [1], I think it would be interesting to hear if the
> patch from pseudonymisaTor in ticket #26646[2] would be of any help in
> the given situation. The patch allows an exit operator to specify a
> range of IP addresses for binding purposes for outbound connections. I
> would think this could split the load wasted on trying to resolve port
> conflicts in the kernel amongst the set of IP's you have available for
> outbound connections.

This sounds similar to a problem we faced with the main Snowflake
bridge. After usage passed a certain threshold, we started getting
constant EADDRNOTAVAIL, not on the outgoing connections to middle nodes,
but on the many localhost TCP connections used by the pluggable
transports model.

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40198
https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40201

Long story short, the only mitigation that worked for us was to bind
sockets to an address (with port number unspecified, and with
IP_BIND_ADDRESS_NO_PORT *unset*) before connecting them, and use
different 127.0.0.0/8 addresses or ranges of addresses in different
segments of the communication chain.

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/merge_requests/120
https://gitlab.torproject.org/dcf/extor-static-cookie/-/commit/a5c7a038a71aec1ff78d1b15888f1c75b66639cd

IP_BIND_ADDRESS_NO_PORT was mentioned in another part of the thread
(https://lists.torproject.org/pipermail/tor-relays/2022-December/020895.html).
For us, this bind option *did not help* and in fact we had to apply a
workaround for Haproxy, which has IP_BIND_ADDRESS_NO_PORT hardcoded.
*Why* that should be the case is a mystery to me, as is why it is true
that bind-before-connect avoids EADDRNOTAVAIL even when the address
manually bound to is the very same address the kernel would have
automatically assigned. I even spent some time reading the Linux 5.10
source code trying to make sense of it. In the source code I found, or
at least think I found, code paths for the behvior I observed; but the
behavior seems to go against how bind and IP_BIND_ADDRESS_NO_PORT are
documented to work.

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40201#note_2839472

> Although my understanding of what Linux is doing is very imperfect, my
> understanding is that both of these questions have the same answer:
> port number assignment in `connect` when called on a socket not yet
> bound to a port works differently than in `bind` when called with a
> port number of 0. In case (1), the socket is not bound to a port
> because you haven't even called `bind`. In case (2), the socket is not
> bound to a port because haproxy sets the `IP_BIND_ADDRESS_NO_PORT`
> sockopt before calling `bind`. When you call `bind` *without*
> `IP_BIND_ADDRESS_NO_PORT`, it causes the port number to be bound
> before calling `connect`, which avoids the code path in `connect` that
> results in `EADDRNOTAVAIL`.
>
> I am confused by these results, which are contrary to my understanding
> of what `IP_BIND_ADDRESS_NO_PORT` is supposed to do, which is
> precisely to avoid the problem of source address port exhaustion by
> deferring the port number assignment until the time of `connect`, when
> additional information about the destination address is available. But
> it's demonstrable that binding to a source port before calling
> `connect` avoids `EADDRNOTAVAIL` errors in our use cases, whatever the
> cause may be.