Incomplete SB string

[jiez]

I'm using ser2net on one raspberry pi board, then use microcom to connect to it. I got an error "Incomplete SB string". I enabled debug and got this

jie@P5510:~/sources/microcom$ ./microcom -dd -t 192.168.1.34:2001
connected to 192.168.1.34 (port 2001)
-> WILL COM_PORT_CONTROL
-> DO BINARY_TRANSMISSION
-> WILL BINARY_TRANSMISSION
-> IAC SB COM_PORT_CONTROL SET_BAUDRATE_CS 0x1c200 IAC SE
-> IAC SB COM_PORT_CONTROL SET_CONTROL_CS 1 IAC SE
Escape character: Ctrl-\
Type the escape character to get to the prompt.

ser2net port tcp,2001 device serialdev, /dev/ttyUSB0, 115200n81,local [115200N81,CLOCAL,HANGUP_WHEN_DONE] (Debian GNU/Linux)

WILL COM_PORT_CONTROL
                     DO #255 -> WONT
                                    SB COM_PORT_CONTROL ??? 194 IAC SE
                                                                      Incomplete SB string
                  Success
                         SB COM_PORT_CONTROL ??? 255 240

I tried using socat instead of ser2net, I got the same error. But if I use telnet instead of microcom, telnet works fine.

[ukleinek]

Which version of microcom are you using? I guess you would have said if your binary bases on top of #57?!

Can you provide a a network dump, that would make it easier to understand/reproduce the issue.

[jiez]

Which version of microcom are you using? I guess you would have said if your binary bases on top of #57?!

I tried both the current main head 8ee78d9 and #57 . I saw the same issue on both.

Can you provide a a network dump, that would make it easier to understand/reproduce the issue.

I will try to get one.

[jluebbe]

@jiez mentioned this on labgrid-project/labgrid#1804 as well.

[jonrebm]

Hello @jiez, thank you very much for you report. This is the type of issue for which I rewrote the telnet command handling in #57.

In the process, I also removed the "Incomplete SB string" message from the code, which is why I suspect that something went wrong when you tried to test that branch.

Could you give it another try? The build issues you had with that branch should now be resolved, and I moved the pr out of draft stage.

For your convenience, I pushed this to main at https://github.com/jonrebm/microcom.git too.

If it resolves your issues I'd happy to merge it very soon.

[jiez]

I did some debugging. telnet doesn't send com port control commands but microcom does. socat receives these commands and sends them out to ttyUSB0 device. Then socat receives similar commands like echoed back from ttyUSB0 and sends back them to microcom. Because these echoed back commands are invalid, we get the errors.

My question is: should socat forward these com port control commands to the ttyUSB0 device?

[jonrebm]

From my understanding, yes, it should.
And it should not be any issue for microcom telnet to receive unsupported com port commands (regardless of whether they are RFC 2217 of nonstandard commands), as long as they are properly embedded in telnet subnegotiations. In fact, although microcom labels its telnet "RFC 2217" it does not act upon most of the received commands anyway and just negotiates them as unsupported.

The issue you are seeing, "Incomplete SB string", is an implementation issue in the old microcom telnet implementation, also see #47, where it fails to process a subnegotiation to the end. This is why I rewrote the telnet command handling.

Please test https://github.com/jonrebm/microcom.git (#57), as the telnet implementation on main is obsolete.

If that still causes issues, we can try to resolve them.

[ukleinek]

To clearify: socat should translate the com port commands to whatever they correspond to for the serial device. They should not be sent literally to the device.

[jiez]

To clearify: socat should translate the com port commands to whatever they correspond to for the serial device. They should not be sent literally to the device.

That's also what I understand. These com port control commands are for controlling the modem connected to the access server (using the terms from RFC2217). They should not be sent through the connection to the remote side.

But ser2net has the same behavior as socat. I am a little worried that my understanding is wrong.

[jiez]

Please test https://github.com/jonrebm/microcom.git (#57), as the telnet implementation on main is obsolete.

I tried it. I only added an "-t" option. microcom exit with code 1 before trying connecting to socat.

[jiez]

To clearify: socat should translate the com port commands to whatever they correspond to for the serial device. They should not be sent literally to the device.

That's also what I understand. These com port control commands are for controlling the modem connected to the access server (using the terms from RFC2217). They should not be sent through the connection to the remote side.

But ser2net has the same behavior as socat. I am a little worried that my understanding is wrong.

I need to enable support for rfc2217 in ser2net! When it's enabled, microcom (the upstream main branch) works as expected. It seems socat does not support rfc2217.