TCP Forwarding hangs when TCP service is unresponsive, even when TCP client exits

[Corey Hickey]

When a TCP client does not receive a response from a service, the client
can opt to time out and exit. If the connection is passed through an SSH
tunnel, however, certain circumstances can make the SSH tunnel hang
indefinitely. This affects both remote port forwarding (-R) and local
(-L).

This report is for current openssh-portable git running on Linux. For
released versions, at least OpenSSH 8.9p1 is affected as well, though I
did not test other versions.

--- Remote Forwarding ---
To reproduce, first start up a TCP service which will accept connections
but fail to respond thereafter. One way to do this is by stopping netcat
shortly after startup (options are for the OpenBSD version of netcat).

$ nc -k -l 127.0.0.1 9999 > /dev/null & sleep 1 ; kill -STOP %%

Next use a different terminal to start an SSH service. Debug options are
not required but are helpful for diagnosis. This example uses port 2222
in order to avoid conflict with the system SSH service. The service can
be run on the same local host or on a remote host.

$ sudo /usr/sbin/sshd -d -d -f /dev/null -o Port=2222 -o \
   HostKey=/etc/ssh/ssh_host_ed25519_key

Next use a different terminal to start an SSH client which runs a TCP
client over a forwarded connection. This example uses wget, but any TCP
client that can be configured to time out should behave the same.

$ ssh localhost -p 2222 -R 8888:127.0.0.1:9999 -v -v wget \
   --timeout=1 --tries=1 http://127.0.0.1:8888


The observed results are that the TCP client (wget) exits, but the SSH
client hangs until either manually killed or the TCP service (netcat) is
resumed.

In detail, the sequence of events is as follows:
1. The SSH client connects to the server; the client and server set up
channels as usual, including one for the port-forwarding. The SSH client
starts a TCP client on the SSH server.
2. The TCP client connects to the SSH server's listening socket, and the
SSH client connects to the TCP service's listening socket. The 3-way
handshakes complete, but when the TCP client sends data to the service,
the service never responds.
3. The TCP client times out, closes its socket for the connection to the
SSH server, and exits. The SSH server sends the client an EOF on the
forwarded channel, but does not close its own socket for the connection
to the now-exited TCP client; this socket remains in CLOSE_WAIT.
4. The SSH client receives the EOF and drains the channel output, but
continues to wait for data on the channel input. The SSH server won't
close the channel until the client does, and the client won't close the
channel until it receives data (or an error) from the channel.

--- Local Forwarding ---
The situation for local forwarding is similar, but requires different
steps to reproduce. First start a TCP service and an SSH service as
described above. Then use a new terminal to start an SSH client:

$ ssh localhost -p 2222 -L 8888:127.0.0.1:9999 -v -v

Use a new terminal to run a TCP client over the forwarded connection.

$ wget --timeout=1 --tries=1 http://127.0.0.1:8888

Lastly, exit from the SSH client's interactive shell.

The observed results are then the same hang as for remote forwarding.


I will send a patch shortly that fixes the issue for me, though I do not
know if my fix is correct.

Thanks,
Corey