ssh port forwarding freezes after 10 simultaneous connections

Question

I am using port forwarding to utilize proxy server which is on remote machine.

Remote machine hostname is remotemachine. Proxy is listening on 8118 port. I am translating this port to local port 5223.

ssh -vvv -N remotemachine -L 5223:localhost:8118

Everything is fine if I don't use more than 10 simultaneous connections, when ssh just freezes. It no longer send data trough port forwarding.

I.e. ab -c 10 -t 5 -X 'localhost:5223' 'http://www.google.ru/' works, but ab -c 20 -t 5 -X 'localhost:5223' 'http://www.google.ru/' causes ssh to freeze

MaxSessions 30 parameter in /etc/ssh/sshd_config didn't helped at all.

What can I do to remove this 10 connection limit?

Nothing interesting in -vvv output =(

Solution 1

FYI, I had a very similar issue, and bumping up MaxSessions to 100 fixed it. I'm the only one using SSH on this server, so that high limit isn't an issue for me.

Solution 2

On the server, have a look at MaxStartups in sshd_config.

From the man page:

MaxStartups

     Specifies the maximum number of concurrent unauthenticated con-
     nections to the sshd daemon.  Additional connections will be
     dropped until authentication succeeds or the LoginGraceTime
     expires for a connection.  The default is 10.

     Alternatively, random early drop can be enabled by specifying the
     three colon separated values ``start:rate:full'' (e.g.,
     "10:30:60").  sshd will refuse connection attempts with a proba-
     bility of ``rate/100'' (30%) if there are currently ``start''
     (10) unauthenticated connections.  The probability increases lin-
     early and all connection attempts are refused if the number of
     unauthenticated connections reaches ``full'' (60).

Solution 3

Are you sure you have setup the local forwarding correctly? Should the destination of your port forwarding be remotemachine instead of localhost?

ssh -vvv -N remotemachine -L 5223:remotemachine:8188

Comments

[Marko Kevac]

I am sure. It could be anything that remote machine understands. localhost or remotemachine or any other host name.

[Marko Kevac]

I have opened two tunnels. One from 5223, other from 5224 port. Than I executed two ab programs with 11 concurent connections each. Everything is fine. 12 and more don't work - ssh tunnel freezes.

[riwi]

@Marko - Maybe, it's an ab issue rather than a ssh issue?

[Marko Kevac]

Nope. Chrome freezes because tunneled proxy don't work. It's something with ssh.

[riwi]

Could the issue be with your proxy server? What have you done to prove that your proxy server is not causing the "freeze" that you're experiencing.

[YwH]

MaxStartups looks like it only applies to unauthenticated connections, which doesn't seem to be the limit he is running into (his all appear authenticated).