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<hr><p align="left"><small>发信人: guru (好读书，不求甚解), 信区: UNP <br>
标 题: Using SOCK_PACKET to gain full access to Ethernet <br>
发信站: UNIX编程 (2001年08月15日09:48:20 星期三), 站内信件 <br>
<br>
Using the SOCK_PACKET mechanism in Linux <br>
To Gain Complete Control of an Ethernet Interface <br>
Daniel Senie <br>
Amaranth Networks, Inc. <br>
I have put together this web page in response to many queries from multiple peop <br>
le. Rather than continue to write individual responses, I have put together this <br>
page to explain what I was trying to do, and how I got it to work. <br>
<br>
<br>
First, some background. To simulate software that was intended to run on a diffe <br>
rent (and not yet built) platform, I needed a convenient way to exercise the cod <br>
e against live networks. I first tried using a Solaris system, using the DLPI dr <br>
iver. This <br>
allowed me to do most things, but failed when I needed to be able to set the sou <br>
rce Ethernet MAC address. The Solaris DLPI driver provides no way to override th <br>
e hardware on a per-packet basis. <br>
<br>
<br>
Next, I started looking at mechanisms in Linux. The mechanism that seemed to fit <br>
the best was SOCK_PACKET, which is used by tcpdump among other things. To Make <br>
this work for me, though, it was necessary to keep the Linux machine from doing <br>
anything on <br>
the interface, other than letting my programs at it. <br>
<br>
How To Do It <br>
This information and these instructions work for RedHat Linux 4.2 with a 2.0.30 <br>
kernel. I expect they'll work fine on a 2.0.32 kernel as well, and with other Li <br>
nux distributions. I have heard that a better mechanism for providing this facil <br>
ity is <br>
coming in a newer kernel. If or when I get more information on that, I'll see ab <br>
out adding another page on that. <br>
<br>
First, the interface needs to be told NOT to run ARP. Promiscuous mode should be <br>
enabled if you need to hear everything on the wire.: <br>
<br>
ifconfig eth1 -ARP PROMISC UP 10.1.1.1 <br>
<br>
Then tell the Linux stack it's not supposed to see any of the traffic to or from <br>
this port: <br>
<br>
<br>
<br>
ipfwadm -O -a deny -P all -S 0/0 -D 0/0 -W eth1 <br>
ipfwadm -I -a deny -P all -S 0/0 -D 0/0 -W eth1 <br>
<br>
In the program, you need to do several things. First, the socket call: <br>
<br>
s = socket(AF_INET, SOCK_PACKET, htons(0x0003)); <br>
<br>
to get the socket set up. <br>
<br>
Next I bind the specific Ethernet NIC I want: <br>
<br>
struct sockaddr myaddr; <br>
<br>
memset(&myaddr, '\0', sizeof(myaddr)); <br>
myaddr.sa_family = AF_INET; <br>
strcpy(myaddr.sa_data, "eth1"); /* or whatever device */ <br>
<br>
r = bind(s, &myaddr, sizeof(struct sockaddr)); <br>
<br>
and check the return code for any errors. <br>
<br>
Now, when you want to send or receive, this socket is bound to the proper device <br>
. One word of caution, though, ALWAYS check the received packets to be sure you <br>
got them on the right device. There's a race condition between making the socket <br>
call and <br>
the bind call where you'll get all packets from ALL interfaces... not what you w <br>
ant! <br>
<br>
So, to send a packet: <br>
<br>
struct sockaddr from; <br>
int fromlen; <br>
<br>
memset(&from, '\0', sizeof(from)); <br>
from.sa_family = AF_INET; <br>
strcpy(from.sa_data, "eth1"); /* or whatever device */ <br>
fromlen = sizeof(from); <br>
<br>
r = sendto(s, msg, msglen, 0, &from, fromlen); <br>
<br>
and check the return code. Note that msg is the pointer to the packet, starting <br>
with the MAC header. Be sure you put the proper source MAC address into your pac <br>
kets! Also, msglen is the length of the packet including the MAC header, but not <br>
including <br>
including <br>
the CRC (which I do not worry about, but the hardware does supply). <br>
<br>
Receive is pretty similar: <br>
<br>
struct sockaddr from; <br>
int fromlen; <br>
<br>
fromlen = sizeof(from); <br>
<br>
r = recvfrom(s, msg, 2048, 0, &from, &fromlen); <br>
if (r == -1) <br>
{ <br>
/* deal with error */ <br>
} <br>
if (strcmp(from.sa_data, "eth1") != 0) <br>
{ <br>
/* not from the interface we wanted, discard */ <br>
} <br>
<br>
if r == -1, you have an error. If r > 0, then r is the length of the received pa <br>
cket. The strcmp ensures the packet came from the right interface. <br>
<br>
<br>
If you want to receive for MAC addresses other than the one the board has in it, <br>
use promiscuous mode. To get the mac address from your program, there's an ioct <br>
l call SIOCGIFHWADDR. In the return from that call is also the hardware type, so <br>
you can <br>
ensure it's Ethernet. Another call, SIOCGIFMTU will tell you the MTU of the inte <br>
rface. <br>
<br>
Caveats <br>
Do not use this methodology on your primary Ethernet interface. Instead, install <br>
a second (and if needed, third) NIC card for use in this way. I've successfully <br>
used 5 NIC cards in one machine, 1 under the control of Linux, the rest bypasse <br>
d to my <br>
programs. <br>
Be VERY sure you set up the ipfwadm commands. Failure to do so will make a huge <br>
mess, likely causing networking problems for other hosts on your lan. <br>
<br>
-------------------------------------------------------------------------------- <br>
<br>
If you found this information helpful and useful, please let me know. If you req <br>
uire further information or assistance in this area, this can be arranged. For c <br>
onsultation beyond simple questions, Amaranth Networks, Inc. can provide advice, <br>
services <br>
and information for a fee. <br>
<br>
<br>
Copyright ? 1998, Amaranth Networks, Inc. <br>
All Rights Reserved <br>
-- <br>
Target Locked:Guru In Darkness. <br>
我只是一只静静卧着的狮子。。。 <br>
※ 来源:·UNIX编程 www.tiaozhan.com/unixbbs/·[FROM: 202.114.36.225] <br>
</small><hr>
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