# Flow control in FreeBSD



## m69 (Jul 19, 2012)

I want to use flow control in FreeBSD for an interface. What I should to do for enabel or disable it in FreeBSD.
Thanks


----------



## michaelrmgreen (Jul 19, 2012)

Serial flow control, either software (XON/XOFF) or hardware (CTS/RTS or DSR/DTR) is controlled through getty(8). Have a look at the man page for getty and peruse the related files /etc/ttys and /etc/gettytab. There's also a useful Handbook page here: http://www.freebsd.org/doc/handbook/serial.html

I struggled for years trying to get good flow control to high speed terminals and it never worked to my satisfaction. In the end I accepted a lower data rate (9600 Baud) and no flow control.

[Edit] Further information: If you don't use flow control you can use simpler serial cables but you need to change std.9600 to 3wire.9600 in /etc/ttys.


----------



## m69 (Jul 25, 2012)

Thanks but it wasn't my aim. I want use flow control for a network interface in FreeBSD because I must do a test and I can't because flow control is enable. Is it enable as default? 
How I can disable it?


----------



## quintessence (Jul 26, 2012)

Hello,

As I see in changes by default it should be disabled in FreeBSD-9, and if you enable it you should see it in the ifconfig(8) output for example as:


```
100baseTX-FDX-flow
```
, where -flow indicates that flow control is enabled on your NIC.

From /usr/src/sys/net/if_media.h:



> { IFM_100_TX | IFM_FDX | IFM_FLOW,<---->"100baseTX-FDX-flow" },


----------



## phoenix (Jul 27, 2012)

ifconfig(8), especially the parts about *-m* (to see what mediaopts the driver supports) and *mediaopts*._


----------

