Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.debian.user > #248845

Re: OpenSSH: cause of random kex_exchange_identification errors?

Show all headers | View raw

On Tue, 7 Jun 2022, Vincent Lefevre wrote:

> On 2022-02-05 18:39:27 -0300, Henrique de Moraes Holschuh wrote:
>> If it is sshd, ensure it is actually logging all you need, and carefully
>> study the logs.
>
> Nothing interesting in the logs, according to the admins of the server.
>
>> If nothing helps, packet-dump both sides (client and server) and find
>> out what sent the TCP RST, as that might give you clues for the "why".
>> A middlebox might be doing it...
>
> I eventually did a packet capture on the client side as I was able to
> reproduce the problem. When it occurs, I get the following sequence:
>
> Client ? Server: [SYN] Seq=0
> Server ? Client: [SYN, ACK] Seq=0
> Client ? Server: [ACK] Seq=1
> Server ? Client: [FIN, ACK] Seq=1
> Client ? Server: Client: Protocol (SSH-2.0-OpenSSH_9.0p1 Debian-1)
> Server ? Client: [RST] Seq=2
> Client ? Server: [FIN, ACK] Seq=33
> Server ? Client: [RST] Seq=2
>
> So the issue comes from the server, which sends [FIN, ACK] to terminate
> the connection. In OpenSSH's sshd.c, this could be due to
>
>                        if (unset_nonblock(*newsock) == -1 ||
>                            drop_connection(*newsock, startups) ||
>                            pipe(startup_p) == -1) {
>                                close(*newsock);
>                                continue;
>                        }
>
> At least 2 kinds of errors are not logged:
>
> * In unset_nonblock(), a "fcntl(fd, F_SETFL, val) == -1" condition.
>
> * the "pipe(startup_p) == -1" condition.
>
> I'm not sure about drop_connection(), which is related to MaxStartups.
>

I've not seen the start of this thread but is this occasional or always?
If occasional, how many concurrent connections do you have starting all
at once. The default ssh config has a super-annoying default that
randomly kills sessions if too many are handshaking at once.

It's the MaxStartups setting you allude to. I've been bitten by this
where cron jobs all start at the same time and ssh to the same host.

Back to linux.debian.user | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-02-02 15:50 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? Hans <hans.ullrich@loop.de> - 2022-02-02 16:20 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-02-02 16:30 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? Bijan Soleymani <bijan@psq.com> - 2022-02-02 17:50 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? David Wright <deblis@lionunicorn.co.uk> - 2022-02-02 18:00 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? gene heskett <gheskett@shentel.net> - 2022-02-02 20:30 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Greg Wooledge <greg@wooledge.org> - 2022-02-02 20:30 +0100
      Reusing ssh keys on a new installation, was Re: OpenSSH: cause of  random kex_exchange_identification errors? David Wright <deblis@lionunicorn.co.uk> - 2022-02-02 22:00 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-02-03 04:20 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? Henrique de Moraes Holschuh <hmh@debian.org> - 2022-02-05 22:40 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-02-08 11:20 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-07 18:00 +0200
      Re: OpenSSH: cause of random kex_exchange_identification errors? Tim Woodall <debianuser@woodall.me.uk> - 2022-06-07 18:20 +0200
        Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-14 13:00 +0200
          Re: OpenSSH: cause of random kex_exchange_identification errors? Tim Woodall <debianuser@woodall.me.uk> - 2022-06-14 20:20 +0200
            Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-15 03:50 +0200
              Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-15 15:20 +0200
                Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-15 16:00 +0200

csiph-web