openssh failing with "client_loop: send disconnect: Broken pipe", but works in debug mode

laolux

1/22/23, 2:42 AM

I am trying to run openssh on my openwrt router. To test it, I start openssh via /usr/sbin/sshd -D. When I try to connect from my computer I get asked for the password and then (if the password is correct) the connection drops and client_loop: send disconnect: Broken pipe is shown in my terminal (bash, if someone is curious).

Now I tried to debug the problem, but when I run openssh in debug mode via /usr/sbin/sshd -D -d, then connecting just works and no error is encountered!

What could be the issue and how to fix it?

EDIT: As suggested by Michael Hampton, I had a log at the log using logread.

Sun Sep 26 13:31:40 2021 auth.info sshd[14488]: Accepted password for root from 192.168.2.50 port 48292 ssh2
Sun Sep 26 13:31:40 2021 auth.crit sshd[14488]: fatal: privsep_preauth: preauth child terminated by signal 12

Any ideas of how to fix that?

EDIT2: Information about the server, as suggested by Dennis Nolte:
OS: openwrt, built from master a couple of days ago
architecture: MIPS
CPU: MediaTek mt7621
kernel version: 5.10.x
openssl version: 1.1.1l
openssh version: 8.7p1
selinux enabled, but logs do not show any denials
PAM is available, but nothing changes if I select UsePAM=yes or no in sshd_config

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Michael Hampton

1/22/23, 1:57 PM

Check the logs on the ssh server.

laolux

1/26/23, 1:38 PM

Thanks, I had a look at the log (see edit), but I do not know how to fix that.

Michael Hampton

1/26/23, 3:03 PM

That's quite an odd one. Does that occur every time, or is the error different each time?

laolux

1/27/23, 11:57 AM

Yes, it is actually extremely reliable. Only the port number changes, obviously.

Dennis Nolte

1/27/23, 9:51 PM

please have a look at this debian bugreport and possible the openssh bugreport linked there, might help you -> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=946242 seems this openbsh ssh-server bug might be related as well https://github.com/openssl/openssl/issues/9984

Dennis Nolte

1/27/23, 9:52 PM

additionally it often helps tou post what Operating system you are using on the server, which Version of the Program (OpenSSH possibly) you have installed on the server, and additional security constraints you have like selinux or apparmor settings you or the initial installation of the server have set.

laolux

1/28/23, 12:54 AM

Thanks, I read the github issue before, but that should well be fixed in my setup. I agree, adding system info is useful. Anyways, what is really bothering me is that running openssh with the debug option works just fine.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: openssh failing with "client_loop: send disconnect: Broken pipe", but works in debug mode

TH: opensh ล้มเหลวด้วย "client_loop: ส่งการตัดการเชื่อมต่อ: ท่อแตก" แต่ทำงานในโหมดดีบัก

RO: openssh eșuează cu „client_loop: send disconnect: Broken pipe”, dar funcționează în modul de depanare

RU: Сбой openssh с «client_loop: отключение отправки: сломанный канал», но работает в режиме отладки

VI: openssh không thành công với "client_loop: gửi ngắt kết nối: Đường ống bị hỏng", nhưng hoạt động ở chế độ gỡ lỗi

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