Ssh – Slow SSH Connection

networkingssh

I'm connecting to a Debian 5 server using PuTTY on Windows 7 from home. I have ADSL internet connection and a D-Link ADSL modem.

I've got a strange problem since 2 days ago. SSH connections became extremely slow, sometimes PuTTY fails to connect and sometimes it just hangs then times out in the middle of a session. My internet connection seems to be fine since my bandwidth is stable, i have no packet loss to anywhere including the server I'm talking about and there is no latency. Everything works just fine except for SSH.

I thought it might be something wrong with PuTTY so i tried 2 other SSH clients but i got the same strange slowness effect. I don't have access to any other server to see if SSH has the same problem or not. My friend connects to the mentioned server from somewhere else but he doesn't have this problem so i assume the server is not the cause.

I haven't tried to use another computer inside my home's NAT to see if it has the same problem or not. It makes no sense to me since everything was fine before 2 days ago. Could it be caused by the ISP?

Does anyone have an idea about how to fix the problem or how to find the cause?

Thanks

Best Answer

Which ISP are you using? That info might be useful as other people on the same ISP may be able to confirm the behaviour (as they have the same problem) or report all is well for them (indicating that the ISP doesn't have a general problem with SSH traffic).

One possibility if it is the ISP, is that they've changed their traffic management structure and it is mistaking your encrypted SSH connection as an encrypted P2P connection. If you can turn off the web server at the other end temporarily without causing to much trouble, try running SSH on port 80 to see if it runs better that way (if it does then something somewhere is probably doing port based traffic shaping that is affecting SSH). You could also try a bulk transfer via SSH (i.e. a download via SCP/SFTP) running through a HTTP based tunnel like this one to see what difference it makes (it will need to be a bulk transfer, as otherwise any performance issues will be hidden by the extra latency added by the tunnel).

Related Solutions

Ssh – Why do consoles sometimes hang forever when SSH connection breaks

There is a "secret" keyboard shortcut to force an exit :~) From the frozen session, hit these keys in order: Enter~. The tilde (only after a newline) is recognized as an escape sequence by the ssh client, and the period tells the client to terminate it's business without further ado.

The long-hang behavior on communication issues is not a bug, the SSH session is hanging out hoping the other side will come back. If the network breaks, sometimes even days later you can get an SSH session back. Of course you can specifically tell it to give up and die with the sequence above. There are also various things you can do such as setting keep-alive timeouts in your client so that if it doesn't have an active link for a certain amount of time it shuts off on it's own, but the default behavior is to stay as connected as possible!

Edit: Another useful application of this interrupt key is to get the attention of the local ssh client and background it to get back to your local shell for a minute —say to get something from your history— then forground it to keep working remotely. Enter~ Ctrl+Z to send the ssh client to the background job queue of your local shell, then fg as normal to get it back.

Edit: When dealing with nested SSH sessions, you can add multiple tilde characters to only break out of one of the SSH sessions in the chain, but retain the others. For example, if you're nested in 3 levels, (i.e. you ssh from local->Machine1->Machine2->Machine3), Enter~. will get you back to your local session, Enter~~. will leave you in Machine1, and Enter~~~. will leave you in Machine2. This works for other escape sequences as well, such as moving the ssh session to background temporarily. The above works for any level of nesting, by just adding more tilde's.

Finally, you can use Enter~? to print a help menu of available escape commands.

TL;DR - the supported escape commands are Supported escape sequences:

 ~.   - terminate connection (and any multiplexed sessions)
 ~B   - send a BREAK to the remote system
 ~C   - open a command line
 ~R   - request rekey
 ~V/v - decrease/increase verbosity (LogLevel)
 ~^Z  - suspend ssh
 ~#   - list forwarded connections
 ~&   - background ssh (when waiting for connections to terminate)
 ~?   - this message
 ~~   - send the escape character by typing it twice
(Note that escapes are only recognized immediately after newline.)

Windows – Is the password compromised because I forgot to hit Enter after ssh username

In short: yes.

# ssh 192.168.1.1 -l "myuser mypassword"
^C
# egrep "mypassword" /var/log/auth.log
Oct 19 14:33:58 host sshd[19787]: Invalid user myuser mypassword from 192.168.111.78
Oct 19 14:33:58 host sshd[19787]: Failed none for invalid user myuser mypassword from 192.168.111.78 port 53030 ssh2

Best Answer

Related Solutions

Ssh – Why do consoles sometimes hang forever when SSH connection breaks

Windows – Is the password compromised because I forgot to hit Enter after ssh username

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