serial console access

[beavers]

I've got a PC Engines APU2 and a regular desktop with a serial port, and a null modem cable between them. I'm trying to get connectivity in both directions via cu(1), but it only ever works one time. After the first connection attempt (from either side), subsequent connection attempts elicit:

# cu -s 115200
cu: open("/dev/cua00"): Device busy

On the APU2, I've got things enabled in /etc/boot.conf:

stty com0 115200
set tty com0

And on both machines, I have this in /etc/ttys:

tty00   "/usr/libexec/getty std.115200" vt220    on secure

Is there something I'm missing?

[victorvas]

Try with default settings:
9600 baud
8 bits
no parity
1 stop bit

[beavers]

Originally Posted by victorvas

Try with default settings:
9600 baud
8 bits
no parity
1 stop bit

No change, I'm afraid. Or do you mean I should change the settings in /etc/ttys?

[TronDD]

Pretty sure you can't listen and call on the same tty. ttyU00 and cuaU00 are the same port, just different in activation signal. See tty(4)

Once getty is listening for incoming connections (or maybe once it answers, which might be why it works once), you can't use cu to dial out on the same port.

[J65nko]

You could check /var/spool/lock for a lock file. A lock file there could cause the issue that you see.

I always add myself to the group dialer. IIRC that solved the lock file issue.
Some time ago cu(1) was rewritten:

HISTORY

The cu command appeared in 4.2BSD. This version was written for OpenBSD 5.4 by Nicholas Marriott.

The previous versions <5.4 included a FILE section in the man page:
From https://man.openbsd.org/OpenBSD-5.3/cu.1

FILES

/var/log/aculog
    line access log
/var/spool/lock/LCK..*
    lock file to avoid conflicts with uucp

I am not sure whether these two files are not used anymore, or if it is a simple man page omission/regression ;-)

[jggimi]

I have a small fleet of Alix platforms, and use a USB-attached serial port on a laptop when connecting to them via a null modem cable. I provision the speed to 9600 baud on all of them through their BIOS -- as their default speeds vary by BIOS version. I added this in /etc/remote so I can $ cu usb when connecting:

usb|For outbound ucom(4) to all 9600 baud systems:\
        :dv=/dev/cuaU0:br#9600:tc=unixhost:

[beavers]

From cu(1)

~^D or ~. Drop the connection and exit. Only the connection is dropped - the login session is not terminated.

Wondering if the bolded could be my issue? When the issue occurs, I'm noticing this on each box:

On the APU2:

$ ps aux | grep cu
root     41193  0.0  0.1   368  1396 00  S+     12:15PM    0:00.03 login -p -- d64 (desktop.domain
root     18478  0.0  0.1   272  1200 00  S<+p   12:23PM    0:00.01 passwd -v login=yes -s login -- d64 (desktop.domain default (login_passwd)

...and on the desktop:

$ ps aux | grep cu
root     82990  0.0  0.0   360  1424 cu  S+     12:15PM    0:00.00 login -p -- D
root     94152  0.0  0.0   268  1240 cu  S<+p   12:23PM    0:00.00 passwd -v login=yes -s login -- D/amd64 (apu.domain) (tt default (login_passwd)

Eventually those disappear and I'm able to connect again. Should I be disconnecting my sessions with something other than ~^D?