[SSHD-800] Numerous SSH_MSG_CHANNEL_FAILURE messages sent for SSH_MSG_CHANNEL_DATA on unknown channel

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.7.0
• Fix Version/s: 2.0.0
• Assignee: Lyor Goldstein
• Reporter: Fulvio Cavarretta
• Created: 01/Feb/18 17:15
• Updated: 01/Feb/18 18:01
• Resolved: 01/Feb/18 18:01

Description

Hi in case the SSHD server receives a message for a not existing channel, it sends back a SSH_MSG_CHANNEL_FAILURE.

This behaviour is not described in RFC4254, which, I agree, is unclear about what should be do in this case.

Since the current SSHD implementation send back a SSH_MSG_CHANNEL_FAILURE message (with the local channel id), in our environment, it put the receipient in an inconsistent state. 

In out enviroment happened that the receipient has sent a SSH_MSG_CHANNEL_DATA while SSHD had already closed the channel.

I fully agree that this should not happen (I will open another issue about this) but the SSH_MSG_CHANNEL_FAILURE further complicates things.

 

The problem can be overcome modifying line 78 of org.apache.sshd.common.session.helpers.DefaultUnknownChannelReferenceHandler

 

letting

case SshConstants.SSH_MSG_CHANNEL_EXTENDED_DATA:
// Not sure if entirely compliant with RFC4254, but try to stem the flood	
wantReply = false;
break;

default: // do nothing

 

instead of

 

case SshConstants.SSH_MSG_CHANNEL_EXTENDED_DATA:
// Not sure if entirely compliant with RFC4254, but try to stem the flood	
wantReply = true;
break;

default: // do nothing