Description

Users may receive the "rlogind[29289]: protocol error: expected 0 byte" log message while trying to log in to the Flexible PIC Controller (FPC) from the shell by using rlogin in QFX Series devices.

This article clarifies that the message is only a warning about the new rcmd protocol and that it can be safely ignored.

Symptoms

When users try to log in to the FPC by using rlogin for troubleshooting (or log collection) from the shell, they may see the following log: 

labroot@labbox> start shell user root 
Password:
root@labboxc:RE:0% 
root@labboxc:RE:0% 
root@labboxc:RE:0% 
root@labboxc:RE:0% rlogin -Ji fpc0 

root@fpc0:~# exit
logout
rlogin: connection closed
root@labbox:RE:0% cli
{MASTER}
labroot@labbox> show log messages | match rlogin 
Apr  8 12:48:51.000  labbox fpc0 fpc0 rlogind[29289]: Connect from 128.0.0.1:1013
Apr  8 12:48:51.000  labbox fpc0 fpc0 rlogind[29289]: protocol error: expected 0 byte
Apr  8 12:48:51.000  labbox fpc0 fpc0 rlogind[29290]: Connect from 128.0.0.1:751

Solution

vJunos rlogin sends "1:" first as per the new rcmd implementation, which gets rejected by the FPC daemon "rlogind" because it does not understand the new implementation. Then vJunos continues with the normal/legacy login by sending a "NULL" character followed by the username.

This is due to design wherein a protocol failure is triggered when the server does not understand the new protocol, so that there is an automatic fallback to the old one.

The above log is only a warning message that can be safely ignored. The log message indicates that the FPC daemon "rlogind" does not understand the new rcmd protocol, and that it expects the traditional legacy rcmd protocol. 

No other action is required for this message.