centos opens only 505 ssh remote tunnels?

Asked Aug 15 '20 at 19:57

Active Aug 16 '20 at 06:51

Viewed 65 times

to evaluate how many ssh tunnels does centos 8 support, I write an app on windows that initiates ssh reverse tunnel to centos 8 through a loop

var session = ssh("centos", "user1", "password1");

for (int i = 60001; i <= 61000; i++) {
    session.start_remote_tunnel(i, "127.0.0.1", 50000);
}

after opening 505 tunnels the loop throws an exception (remote tunnel failed to start).

no error in the sshd log; UsePAM=yes; selinux is in permissive mode.

edited Aug 16 '20 at 06:51

asked Aug 15 '20 at 19:57

geek11

A well-known limit on the Windows side. – Gerard H. Pille Aug 15 '20 at 20:10
@GerardH.Pille how to overcome this limit? – geek11 Aug 15 '20 at 20:15
@GerardH.Pille What limit is that? I've never heard of it. – Michael Hampton Aug 15 '20 at 20:23
That's why they are called "ephemeral", those ports. The number of them may be quite limited, on certain Windows versions. I don't know the function ssh.start_remote_tunnel and the meaning of the "i" parameter, but you wouldn't be trying to use a port already in use? – Gerard H. Pille Aug 16 '20 at 08:10
the code opens single tcp connection with single ssh session with multiple port forwarding channels, so that has nothing to do with the ephemeral ports limit. – geek11 Aug 16 '20 at 10:21

0 Answers0