[CentOS] Problem with ssh disconnecting

[H]

On 02/12/2018 06:34 PM, Stephen John Smoogen wrote:
> On Mon, Feb 12, 2018 at 6:25 PM H <agents at meddatainc.com> wrote:
>
>> Running CentOS 7 on workstation and having a problem with ssh disconnects.
>> My ssh_config contains:
>>
>> Host *
>> TCPKeepAlive yes
>> ServerAliveInterval 30
>> ServerAliveCountMax 300
>>
>> and sshd_config on the server contains:
>>
>> TCPKeepAlive yes
>> ClientAliveInterval 60
>> ClientAliveCountMax 300
>>
>> Have I missed any setting needed to prevent these random disconnects? I
>> don't think there is anything wrong with the network card, the driver, or
>> the cable, since if I am on a VPN connection via another server, the VPN
>> and any ssh connection stay up indefinitely.
>>
>> Thanks.
>
>
> There are usually 2 different reasons for this:
> 1. The VPN is UDP and times out/drops keeps alives so that they no longer
> function properly. [The UDP connection will make it look like you have a
> new SSH connection which of course the system will drop because that would
> allow for security problems.]
>
> 2. A firewall in the chain of things (system you are on, the system you are
> going to, or somewhere in between) has session flushing issues. If you have
> the firewall set up to only accept NEW port 22 connections and then just
> looks to see if the ESTABLISHED, RELATED tables are accepted elsewhere then
> if the session somehow ages out or is flushed due to usage, the ssh
> connection can get dropped.
>
> The solution to one is to see if a TCP VPN fixes the problem. The second
> one is to either make the iptables kernel tables larger or to have all port
> 22 accepted even if it is not ESTABLISHED.
>
> These aren’t the only ways the problem you see can occur but they are some
> of the most common I have run into.
>
>
>
>>
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Not sure if I am reading your reply correctly but I should clarify that I have problems when running naked ssh to the server, when I run ssh to the same server but over the VPN connection (that goes via third server) everything is flawless.

I should also explain that:

- I am on a workstation (located in the US), ssh-ing into server 1 (located in the US).

- From server 1 I use scp to transfer large files from server 2 (located in Europe) to server 1 (in the US).

The above randomly disconnects.

However, when:

- I use a VPN connection to server 3 (also located in Europe).

- From the same workstation as above, do exactly as above, connections are rock-solid.