Re: How can I create and use a Tap device
From: Robert Watson (rwatson_at_freebsd.org)
Date: 11/18/04
- Previous message: Elton Machado: "Re: Help ! Proxy arp at PPTP (FIXED)"
- In reply to: Elton Machado: "How can I create and use a Tap device"
- Next in thread: Julian Elischer: "Re: How can I create and use a Tap device"
- Reply: Julian Elischer: "Re: How can I create and use a Tap device"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Date: Thu, 18 Nov 2004 16:48:28 +0000 (GMT) To: Elton Machado <elton.machado@norteglobal.com>
On Thu, 18 Nov 2004, Elton Machado wrote:
> I need a virtual ethernet device, can I use tap for that?
>
> How can I create it?
Really quite easy, and fairly well documented in the man page. Basically,
you need to:
- Load the module or compile it in.
- Open /dev/tapX where X is the interface number. When the device is
opened, the network interface will be instantiated.
- Read ethernet frames -- you'll get one per read, make sure to provide
enough buffer room for a full frame at the MTU of choice.
- Write ethernet frames -- one per write.
Typically input from a tap device will look something like this:
char packet[MAXPACKET];
struct ether_header *h;
ssize_t len, recvlen;
len = MAXPACKET;
recvlen = read(tap_fd, packet, len);
if (recvlen == -1) {
perror("read");
return (-1);
}
if (len < sizeof(struct ether_header)) {
fprintf(stderr, "short frame read");
return (-1);
}
eh = (struct ether_header *)(packet);
...
And a write will look something like this:
sendlen = write(tap_fd, packet, recvlen);
if (sendlen == -1) {
perror("write");
return (-1);
}
if (sendlen != recvlen) {
fprintd(stderr, "short frame write");
return (-1);
}
Make sure to properly initialize the ethernet frame header with the
protocol type, source/destination ethernet addresses, and so on.
A couple of performance caveats:
- Every packet delivery requires going to user space, so possibly a
context switch and certainly a system call.
- Every packet is copied to user space, and/or from user space, so you get
a lot of memory copying.
For prototyping or light-weight stuff, tap is a great tool, but to improve
performance you want to run network code in the kernel, especially if
there are other applications running (and/or processing packets), which
will increase the number of context switches. The cost as it stands isn't
bad -- I regularly use tap-derived tunnel software for remote network
access without a hitch. There were recently some posts made with patches
to optimize the allocation of kernel memory for packets sent using a tap
device, which are in the mailing list archives (not sure if they were
merged yet).
Robert N M Watson FreeBSD Core Team, TrustedBSD Projects
robert@fledge.watson.org Principal Research Scientist, McAfee Research
_______________________________________________
freebsd-net@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-net
To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org"
- Previous message: Elton Machado: "Re: Help ! Proxy arp at PPTP (FIXED)"
- In reply to: Elton Machado: "How can I create and use a Tap device"
- Next in thread: Julian Elischer: "Re: How can I create and use a Tap device"
- Reply: Julian Elischer: "Re: How can I create and use a Tap device"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Relevant Pages
|
|
