tcpdump equivalent for the serial port
Hello!
I'm interested if somebody has any ideea how to write a program that is able to sniff the data passing through a serial port the same way tcpdump is able to show the data passing through the ethernet cable, i.e. without disturbing the application that sends/receives the data.
I've seen few programs that basically work as tunnels opening a pseudo-console where the application connects and writing on the other side to the serial device. This approach is not very useful because I want to be able to start/stop my sniffer without interupting the communication and also the application can control different settings of the serial port which probably will not be forwarded by the tunnel.
Thanks,
mache
Hi Nicolae,
I think that what you want to do is not posible just with a software. You have to build a special cable to do that (somebody tell me if I'm wrong, or if knows another option)
In my job we have one of those cables because we needed to see what was going on between two applications that talk to each other via serial port. The bad thing is that you need a third computer that would be the sniffer.
-----Mensaje original-----
De: [email protected] [mailto:[email protected]]En nombre de Nicolae Mihalache
Enviado el: Viernes, 12 de Septiembre de 2003 10:37
Para: [email protected]
Asunto: tcpdump equivalent for the serial port
Hello!
I'm interested if somebody has any ideea how to write a program that is able to sniff the data passing through a serial port the same way tcpdump is able to show the data passing through the ethernet cable, i.e. without disturbing the application that sends/receives the data. I've seen few programs that basically work as tunnels opening a pseudo-console where the application connects and writing on the other side to the serial device. This approach is not very useful because I want to be able to start/stop my sniffer without interupting the communication and also the application can control different settings of the serial port which probably will not be forwarded by the tunnel.
Thanks,
mache
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Quoted 2 lines · Mariano Moreyra
Hi Nicolae,
Hi Mariano!
Quoted 7 lines
I think that what you want to do is not posible just with a software. You have to build a special cable to do that (somebody tell me if I'm wrong, or if knows another option)
In my job we have one of those cables because we needed to see what was going on between two applications that talk to each other via serial port. The bad thing is that you need a third computer that would be the sniffer.
The problem is that opening the cable and connecting it to a third port may introduce some variations in the signals going over the serial cable. This is something that I want to avoid as much as possible. The other end of the serial cable is not another computer but a device that is itself the subject of development and test so it can not be assumed as working nor it can be developed with a non-nominal serial cable connected to it. Even worse, my serial link is not RS232 but RS422 which has a higher speed and it's more difficult to have break-out capability without interferences. This is why I'm looking for a software solution, i.e. something that communicates with the serial driver and gets all the messages that pass through it.
Nicolae
I'm using that cable between a PC and a Fiscal Printer, but it's a RS232 link...
We didn't have any problem with signal variations. But I understand that with an RS422 link you could have more problems.
Wish I could help you, but I don't know how to write a software to do this kind of sniffing.
-----Mensaje original-----
De: Nicolae Mihalache [mailto:[email protected]]
Enviado el: Viernes, 12 de Septiembre de 2003 11:02
Para: [email protected]; [email protected]
Asunto: Re: tcpdump equivalent for the serial port
Quoted 2 lines · Mariano Moreyra
Hi Nicolae,
Hi Mariano!
Quoted 2 lines
I think that what you want to do is not posible just with a software. You have to build a special cable to do that (somebody tell me if I'm
wrong,
Quoted 5 lines
or if knows another option)
In my job we have one of those cables because we needed to see what was going on between two applications that talk to each other via serial port. The bad thing is that you need a third computer that would be the sniffer.
The problem is that opening the cable and connecting it to a third port may introduce some variations in the signals going over the serial cable. This is something that I want to avoid as much as possible. The other end of the serial cable is not another computer but a device that is itself the subject of development and test so it can not be assumed as working nor it can be developed with a non-nominal serial cable connected to it. Even worse, my serial link is not RS232 but RS422 which has a higher speed and it's more difficult to have break-out capability without interferences.
This is why I'm looking for a software solution, i.e. something that communicates with the serial driver and gets all the messages that pass through it.
Nicolae
On Fri, 2003-09-12 15:37:24 +0200, Nicolae Mihalache <[email protected]> wrote in message <[email protected]>:
Quoted 12 lines
Hello!
I'm interested if somebody has any ideea how to write a program that is able to sniff the data passing through a serial port the same way tcpdump is able to show the data passing through the ethernet cable, i.e. without disturbing the application that sends/receives the data. I've seen few programs that basically work as tunnels opening a pseudo-console where the application connects and writing on the other side to the serial device. This approach is not very useful because I want to be able to start/stop my sniffer without interupting the communication and also the application can control different settings of the serial port which probably will not be forwarded by the tunnel.
Your approach won't work, but I can hint you to another. If you don't need the *wire states*, but only the data your application receives (or transmits), then this is easy. Simply use strace or a preload lib to intercept open, close, read and write.
That way, you don't monitor the serial port itself, but the application using it. For me, that has always been enough:-)
MfG, JBG
Signature · 4 lines
Jan-Benedict Glaw [email protected] . +49-172-7608481 "Eine Freie Meinung in einem Freien Kopf | Gegen Zensur | Gegen Krieg fuer einen Freien Staat voll Freier Bürger" | im Internet! | im Irak! ret = do_actions((curr | FREE_SPEECH) & ~(IRAQ_WAR_2 | DRM | TCPA));
Quoted 9 lines · Jan-Benedict Glaw
Your approach won't work, but I can hint you to another. If you don't need the *wire states*, but only the data your application receives (or transmits), then this is easy. Simply use strace or a preload lib to intercept open, close, read and write.
That way, you don't monitor the serial port itself, but the application using it. For me, that has always been enough:-)
This seems a good ideea. Probably I will not use exactly strace but develop something using the same library it uses.
I will still keep the cable break-out solution as an option but definitely I wanted to be able to monitor the data without interfering with the electic signaling on the cable.
Thanks a lot!
nicolae
On Fri, 2003-09-12 16:23:32 +0200, Nicolae Mihalache <[email protected]> wrote in message <[email protected]>:
Quoted 7 lines
Quoted 4 lines · Jan-Benedict Glaw
That way, you don't monitor the serial port itself, but the application
using it. For me, that has always been enough:-)This seems a good ideea. Probably I will not use exactly strace but develop something using the same library it uses.
There isn't really a library involved. strace is basically a hugh wrapper around the ptrace() system call.
Quoted 3 lines
I will still keep the cable break-out solution as an option but definitely I wanted to be able to monitor the data without interfering with the electic signaling on the cable.
Then you most probably want to go the preload lib way. That is, you build a small lib containing open(), close(), read() and write() which you call the original program with:
$ LD_PRELOAD=/path/to/your/lib.so ./serial_app
The lib needs to call the libc routines, of course. You can do this by dlopen()ing libc (RTLD_NEXT) and get the original function (pointer) from it (by calling dlsym()).
This way, you get access to all parameters (you're able to get the open()ed file name, compare it to /dev/ttyS.*) and save the resultin fd from the libc open() call.
On upcoming read()s and write()s, you simply always compare the supplied fd with the initial fd() from the open() call to a serial device. It's really that easy:)
MfG, JBG
Signature · 4 lines
Jan-Benedict Glaw [email protected] . +49-172-7608481 "Eine Freie Meinung in einem Freien Kopf | Gegen Zensur | Gegen Krieg fuer einen Freien Staat voll Freier Bürger" | im Internet! | im Irak! ret = do_actions((curr | FREE_SPEECH) & ~(IRAQ_WAR_2 | DRM | TCPA));
Quoted 18 lines · Jan-Benedict Glaw
On Fri, 2003-09-12 16:23:32 +0200, Nicolae Mihalache <[email protected]> wrote in message <[email protected]>:
Quoted 11 lines
Quoted 5 lines · Jan-Benedict Glaw
That way, you don't monitor the serial port itself, but the application using it. For me, that has always been enough:-)
This seems a good ideea. Probably I will not use exactly strace but develop something using the same library it uses.
Hello again,
Now that I'm thinking better, this approach is not similar with tcpdump because if I monitor the application that reads the data, I won't see it until it is actualy read. The idea of my sniffer is to debug the device that runs on the other side so it would be interesting to display something as soon as it arrives rather than waiting till the application reads the data. I will try to see if there is some support provided in the kernel.
Thanks,
nicolae
I would probably try to see if I could alter the kernel serial port driver, there may even be an API in the driver for what you want. That's where I would start.
Peace,
Jamie
Quoted 15 lines · Nicolae Mihalache
Hello!
I'm interested if somebody has any ideea how to write a program that is able to sniff the data passing through a serial port the same way tcpdump is able to show the data passing through the ethernet cable, i.e. without disturbing the application that sends/receives the data.
I've seen few programs that basically work as tunnels opening a pseudo-console where the application connects and writing on the other side to the serial device. This approach is not very useful because I want to be able to start/stop my sniffer without interupting the communication and also the application can control different settings of the serial port which probably will not be forwarded by the tunnel.Thanks,
mache-
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