How to fix request failed on channel 0

Question

When I want to connect to my server like this

ssh -a username@my-server.de -p 22

it gives me two error messages:

PTY allocation request failed on channel 0
shell request failed on channel 0

When I use the parameter -T the first error message goes away. But how to fix the second one? I can't connect. To other servers I can connect without any problems.

I'm on MAC OS 10.9.

The parameter -v shows me this debug output:

OpenSSH_6.2p2, OSSLShim 0.9.8r 8 Dec 2011
debug1: Reading configuration data /etc/ssh_config
debug1: /etc/ssh_config line 20: Applying options for *
debug1: Connecting to xxx.your-server.de [188.40.3.15] port 22.
debug1: Connection established.
debug1: identity file /Users/xxx/.ssh/id_rsa type -1
debug1: identity file /Users/xxx/.ssh/id_rsa-cert type -1
debug1: identity file /Users/xxx/.ssh/id_dsa type -1
debug1: identity file /Users/xxx/.ssh/id_dsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.2
debug1: Remote protocol version 2.0, remote software version mod_sftp/0.9.8
debug1: no match: mod_sftp/0.9.8
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA 55:f5:ca:ca:01:45:0f:7b:71:0a:1f:ba:9e:25:17:fb
debug1: Host 'xxx.your-server.de' is known and matches the RSA host key.
debug1: Found key in /Users/xxx/.ssh/known_hosts:1
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Trying private key: /Users/xxx/.ssh/id_rsa
debug1: Trying private key: /Users/xxx/.ssh/id_dsa
debug1: Next authentication method: password

After I enter the password, I get this:

debug1: Authentication succeeded (password).
Authenticated to xxx.your-server.de ([xxx.xxx.3.15]:22).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: Sending environment.
debug1: Sending env LANG = de_DE.UTF-8
shell request failed on channel 0

The client is requesting a shell session with a TTY and the server is rejecting it. The server doesn't provide the client with a detailed reason why the request was rejected. You need to troubleshoot this on the server rather than the client. — Kenster, Nov 19 '14 at 19:25
Thank you for your answer. You're right, the server does not provide this request. I don't know ho to find out where the problem is. I read some other posts before but they didn't match exactly my problem. Can anybody give me some hints, please? — user1895268, Nov 20 '14 at 08:14
Try `ssh [-neccessary options] /bin/bash -i`. This might give a more informative error message from the server side (e.g. "permission denied" or "segmentation fault") — Hans Lub, Dec 07 '14 at 10:29
The `debug1: Remote protocol version 2.0, remote software version mod_sftp/0.9.8` line indicates that the remote server is proftpd+mod_sftp, and the mod_sftp module does not implement/support shell requests, only SFTP/SCP. Thus the `shell request failed` error. — Castaglia, Feb 21 '16 at 18:48
when the server is jenkins it only supports a limited command set, not a shell. So it responds also with "shell request failed on channel 0" when given an accepted command like "help" it works fine. — U.V., Dec 16 '20 at 13:18

vharron · Answer 1 · 2015-01-23T19:03:43.953

26

PTY allocation request failed on channel 0

There is a limit of 256 pseudo terminals on a system. Maybe you have an application that is leaking pseudo terminals. Use

lsof /dev/pts/*

to see what processes have open pseudo-terminals

shell request failed on channel 0

I was getting this error (without PTY allocation error). It turns out that one of my applications (QtCreator 3.0.?) was leaking Zombie processes. Other users were able to log in so I might have been hitting my per user process quota (if there is such a thing). I've updated to QtCreator 3.3. So far so good.

edited Jan 23 '15 at 19:03

answered Jan 23 '15 at 18:55

vharron

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How to check that on windows, my remote machine is linux however I connect to this remote machine using ssh on a Windows computer using GitBash. How to do this command on Windows – Hana90 May 09 '18 at 07:20
1

The OP said they are on MacOS. Is this solution intended to work on MacOS? When I tried it (MacOS 10.15.5), I got: `lsof: status error on /dev/pts/*: No such file or directory`. – Jeremy Sep 10 '20 at 04:56

score 25 · Answer 2 · answered May 09 '17 at 12:49

25

unmount and mount /dev/pts worked for me

umount /dev/pts

mount devpts /dev/pts -t devpts

Reference: http://www.iitk.ac.in/LDP/LDP/lfs/5.0/html/chapter06/proc.html

answered May 09 '17 at 12:49

rajagopalx

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When able to login via ssh, "umount" will not work because the device is in use. – Steven the Easily Amused Aug 09 '17 at 17:36
1

I was playing around with namespaces and I wanted to ssh into a chroot jail running in a separate namespace. I was getting the same error, but `mount devpts /dev/pts -t devpts` did the trick for me. – Phoenix87 Jan 27 '18 at 16:40
Add that this is *on the server*, not in the client – somenxavier Nov 20 '19 at 11:50
It works to me. – Qian Chen Jan 01 '21 at 15:53

score 8 · Answer 3 · answered Jan 01 '18 at 14:37

I had the exact same error trying to connect via ssh to my server. As I can see you're using a server provided by Hetzner connecting to it on port 22:

debug1: Connecting to xxx.your-server.de [188.40.3.15] port 22.

The offical wiki/documention from Hetzner says:

Protocol for encrypted remote diagnostics for servers/computers(consoles). The SSH port to be used is 222.

So you have to connect via port 222:

ssh -p 222 username@my-server.de

score 6 · Answer 4 · answered Feb 07 '17 at 11:48

I solved a similar problem with one of our users who was used only for ssh port forwarding so he don't need to have access to PTY and it was prohibited in .ssh/authorized_keys file:

no-pty ssh-rsa AAA...nUB9 someuser

So when you tried to log in to this user, only message

PTY allocation request failed on channel 0

was returned. So check your user's authorized_keys file.

score 4 · Answer 5 · edited Mar 24 '18 at 19:20

4

Just add these lines to your /etc/mtab and /etc/fstab, and reboot the system.

none    /dev/pts    devpts    defaults    0    0

edited Mar 24 '18 at 19:20

Rudie

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answered May 12 '15 at 12:41

Mansur Ali

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1

This worked. On a live CD, you can also do: $ mount -t devpts none /target/dev/pts; chroot /target – Michael Galaxy Dec 18 '15 at 15:27
if I use Git bash to connect to aws server, and got this problem. should I modify these files from the main Git bash directory at Program Files? – Apr 27 '16 at 10:53
Useful tip: you can run a command without logging in and allocating a pty, e.g. `ssh root@example.com '/sbin/reboot'` – Robert Calhoun Jun 24 '16 at 21:09
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But what does this code do. You should provide an explanation – Tristan Jul 02 '16 at 15:55
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We did this operation (change fstab & mtab and reboot) and it worked... for a while. Apparently it was the reboot that resolved the issue, and now our suspicion is that the remote machine is running out of resources. – Steven the Easily Amused Aug 09 '17 at 17:35
[I don't think you should be editing /etc/mtab see this post](https://askubuntu.com/a/754275/323009). – PatS Sep 18 '18 at 22:42

score 4 · Answer 6 · answered Aug 02 '18 at 03:59

4

Try this:

vi /etc/security/limits.d/20-nproc.conf
*          soft    nproc     4096   # change to 65535 
root       soft    nproc     unlimited

answered Aug 02 '18 at 03:59

xmduhan

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score 4 · Answer 7 · answered Mar 17 '19 at 09:54

just found out, what was the problem in my case (provider strato): I had the same problem with output "shell request failed on channel 0" in the end.

I have to use the master password with the web-domain name as login. (In German www.wunschname.de, where wunschname is your web-address.)

A ssh login with sftp-user names and the corresponding passwords is without success. (Although scp and sftp works with these sftp users!)

score 2 · Answer 8 · answered Jul 12 '18 at 18:42

2

It's an old question, but if someone gets here like me...

This might be result of a wrong date in the server. If you are working with an embedded system this might be the cause... So check your date:

$ date

answered Jul 12 '18 at 18:42

Ian Koerich Maciel

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randhir singh · Answer 9 · 2018-01-16T08:45:11.377

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rebooting the instance from AWS console worked for me. There was a service that was leaking file connections that lsof helped find.

edited Jan 16 '18 at 08:45

answered Jan 15 '18 at 06:09

randhir singh

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score 1 · Answer 10 · answered Jul 31 '18 at 10:19

1

shell request failed on channel 0

mean you don't have shell or remote commands access, fix your user permission on server to have shell access or if you just want tunneling use -N and -T options

answered Jul 31 '18 at 10:19

ewwink

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would be awesome an example .. :D – David Noreña Nov 04 '18 at 15:47

score 1 · Answer 11 · answered Jun 07 '19 at 07:32

1

I also faced the same issue. Just restarting my servers solved the issue.

answered Jun 07 '19 at 07:32

Vivek Garg

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score 1 · Answer 12 · answered Apr 17 '20 at 00:33

This is what which helped me from the various answers provided.

Try logging in as root, that will get you in most of the times
Try logging in as a different user, it successful, it means that the problem is with a specific account & it implies that there are some process(es) already started by the problematic account which are consuming resources preventing login(most likely no of processes)
Increase the limit in /etc/security/limits.d/20-nproc.conf as mentioned by xmduhan above
Try to ssh again, it should work

score 1 · Answer 13 · answered Sep 26 '20 at 16:42

1

try with option -NT

ssh -NT ...

answered Sep 26 '20 at 16:42

Shahid

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Why? Can you add an explanation for long term value, and helpfulness to future visitors who may run into similar issues? – SherylHohman Sep 27 '20 at 00:43
this worked for me in testing with github to check if my RSA key worked – Michael Paccione May 19 '21 at 17:21

score 0 · Answer 14 · answered Feb 15 '19 at 14:58

0

remounting /dev/pts works for me. you can do this remotely via ssh if you run ssh like this against the affected machine. ssh doesn't request a tty when running commands like this and therefore this will allow you to remount /dev/pts remotely

ssh user@host -- 'mount -o remount,rw /dev/pts'

answered Feb 15 '19 at 14:58

Slavek Jurkowski

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Gives me back `mount: only root can use "--options" option`. Well, I don't have `root` previlege. – Boooooooooms Mar 04 '19 at 12:28

score 0 · Answer 15 · answered May 08 '19 at 02:36

I occasionally see this when spinning up a VM. Our automation system starts applying updates, so depending on timing can hit an update to critical packages.

Upshot - this might happen if ssh or other related packages are being updated on the destination machine.

score 0 · Answer 16 · answered May 22 '19 at 20:48

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I encountered this error while using my git bash. I was able to solve this by re-installing git for windows. More details in this answer.

answered May 22 '19 at 20:48

user3885927

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score 0 · Answer 17 · answered Sep 26 '19 at 18:03

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Should a person find themselves reading this QA while they are trying to ssh into a NetGear ReadyNAS device, be sure that the "rsync only" checkbox is unchecked in the dialog box for the ssh service in the admin interface.

answered Sep 26 '19 at 18:03

Chris K

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score 0 · Answer 18 · answered Oct 28 '19 at 13:52

0

This was happening when I was trying to use sudo on ssh -t git@github.com after adding my local user's public key to github

Just a head's up to the google happy people like me

answered Oct 28 '19 at 13:52

Jonathan dos Santos

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score 0 · Answer 19 · answered Mar 27 '20 at 12:03

0

Just rebooting a AWS instance works for me to clear the error "shell request failed on channel 0"

answered Mar 27 '20 at 12:03

Kiruthika kanagarajan

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score 0 · Answer 20 · answered Nov 30 '20 at 06:40

0

in my case, the SFTP server will reject your SSH connection.

answered Nov 30 '20 at 06:40

Theo Sun

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score 0 · Answer 21 · answered Feb 24 '21 at 14:27

As you already found the -T flag that create a PTY, I will just respond to the second part:

shell request failed on channel 0

You should pass a command:

ssh username@my-server.de -p 22 help

After reading back the manual here: https://www.jenkins.io/doc/book/managing/cli/, I find it not really clear that it would not work without a command. But as stated by @U.V., the ssh interface is not a console interface, rather a connection utility. So you need to pass a command...

If someone from the "jenkins" team pass accross this post, it would be great that if we pass no command, the help would show up :-)

How to fix request failed on channel 0

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