Need to send messages on PC without configured IP using MAC address in C language as a reply on PC's broadcast message. I suppose I should work with ethernet-header and use some (linux) kernel's structures and subroutines, however I couldn't find good links and samples about my question.
Example of sending a raw Ethernet frame in Linux:
https://gist.github.com/austinmarton/1922600
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I want to send a packet to another machine but with a different MAC address in the ethernet header. For this I am using raw sockets in C and creating my own ethernet header so that I can set the source MAC as desired. The trouble is that I am not sure what destination MAC address to fill in the header. I know the IP of the destination machine but not the MAC. Even if I knew, I don't want to hardcode this MAC.
One option I see is that my machine would know the MAC in its ARP table and I could read it and get the MAC address and fill it in. But doing this before sending each packet is overhead. I could read it once and keep using but what if some day the destination machine gets replaced. The admin would assign the same IP address to the new machine but the MAC address would be different. Probably when the new machine boots up, it would send a Gratuitous ARP and my machine would update its ARP table. But my program wouldn't know this. My program would keep sending packets with old MAC.
I feel I am missing something very basic. Thoughts?
What you are seeing here is the same problem that everyone faces, who tries to implement a network stack.
You have several options:
If the packet is a reply packet, then just use the MAC address of the from address
You can maintain an own arp-table and send a arp request if the entry in your table is missing or outdated. Parse arp replies and update your table accordingly. Packets to be sent without a valid arp table entry have to be queued. This is the most elegant, but also a very demanding option.
You can simply send the packet to the MAC of your local router. It will forward the packet to the correct host specified in the IP header. If your local router employs protocols like vrrp, hsrp or gblp, then the MAC address is always the same and doesn't change, even if your router is replaced.
You can read the MAC address from /proc/net/arp or with ioctl(SIOCGARP, ...);, but the entry for the IP you are looking for might not be present, if your host hasn't tried to communicate with it recently. You could, of course, send a dummy packet with sendto(); to that host before reading the arp table.
If you describe in more detail what you are planning to do, the suggestions might get better.
You can set the MAC address for the device with SIOCSIFHWADDR ioctl request. The manual for netdevice is a good starting point for this.
I am trying to find out mac address of a machine in a switched environment after sending it a raw packet. I am trying to implement traceroute command . I want to know when i receive a ICMP time exceeded message how can I extract the mac address of that machine . I am a new to network programming so am confused what socket call will help me to extract the mac address.
Thanks .
No, you can not extract MAC address from ICMP reply.
You can only determine MAC addresses of linked machines next to you. In ICMP(tracert) you can just find out the IP address of target or middle machine.
If you want to detect MAC addresses, you should use ARP protcols where it's applicable in local networks not Internet.
ICMP protocol starts after IPv4 header[1] and MAC addresses is related to physical/link layer. In low level layers the MAC addresses will transparent from top level layers such as network(IP) or Transmission,...
To determining MAC addresses, you should use Raw sockets or PCAP SDKs to access lower layers of network programming. (I say again, these are not useful over Internet)
Like Masoud M said, you can only get the MAC address of machines that are on your local network. That said, you can parse the output the arp command to find the MAC address given the IP address of a machine one your local network.
In general, on internet, you don't even know the media a host is using for transmitting packets. Let's suppose a remote host is conected over a serial rs-232-C link with PPP protocol. It doesn't have a mac address. This also happens for example if the host uses a token ring interface or frame relay link. This makes determining the remote mac addresses of hosts a local issue completely. Normally, when you get a packet from a remote site over ethernet, the source mac addres you get in the packet is the one of the last router that links you to the internet, not the one of the original host that sent the IP packet. In the RFC on IP over avian carriers (rfc1149, rfc2549 and rfc6214) the media used for transmission doesn't allow to use mac addresses (the link address, if somewhat feasible on a pidgeon could be, would be its name)
If you want to read about traceroute on ethernet network of switches, perhaps you had to have a look at the IEEE802.1ag, that has an specification to do tracerouting over switches (tracelink service) but I think is far over the scope of this answer.
Please don't criticise the solution. It's not my design and yes, it sucks.
On a computer running Linux, and using C, we need to verify that a TCP connection made to a process is from the same machine. Doing it by IP address is problematic since the OS is generating two IP addresses and the process only knows one. Anyway, verifying by IP address is a bit poor.
We want to do the verification by comparing the "remote" MAC address to the local MAC address. We already get the local MAC address. All I need to know is how to get the "remote" MAC address. It's in the packet that gets sent when forming the connection (and in all subsequent ones too). How do we drag it out of the ethernet layer?
Before anyone says this again, I KNOW you cannot get the MAC address of the remote host if it's not on the same subnet/LAN. That's fine. Presumably we'll get something like 00:00:00:00:00:00 and since that is different to the local MAC address it will be different - just what we want.
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So, to summarise, we have a TCP connection socket fd, we've received a data packet, how do we then find the MAC address of the remote host, the MAC address that was in the packet's header?
If I understand correctly, you are not trying to tell remote machines apart, but to use the idea that the source and destination MAC would match on traffic sent from a machine to itself in order to allow only local traffic.
This seems rather roundabout, and has been pointed out, insecure.
A somewhat better idea might be to have the TCP client listen only on the loopback interface (127.0.0.1) and not on INADDR_ANY. Or go a step further and use a unix-domain socket instead of a TCP one (a common method used by X servers today to prevent the possibility of remote connections)
The MAC address of a live same-subnet TCP connection will almost certainly be in the ARP cache.
On Linux, you could examine the ARP cache by looking in /proc/net/arp. Here is what it looks like on my Ubuntu box:
aix#aix:~$ cat /proc/net/arp
IP address HW type Flags HW address Mask Device
10.0.0.32 0x1 0x2 00:1e:4f:f5:be:dc * eth0
10.10.10.1 0x1 0x2 00:1f:6c:3e:02:e3 * eth0
There's probably some callable API that you could use to get to the same data if you're averse to parsing the pseudo-file.
How about configuring a firewall (internal or external) to block or MAC-filter external traffic on the port in question?
A loopback connection (whether it's over the loopback interface or some other interface) it not routed over any ethernet device and therefore does not have a MAC address associated with it.
I suggest you just use getsockname and getpeername to get the local and remote IP address and compare that they are equal. That will work without any a priori knowledge of the configured IP addresses of your system.
Further, if you want to be IPv4/v6 agnostic, you could use getnameinfo with the NI_NUMERIC flag to convert both addresses to numeric string representations and strcmp them.
If I use arp and arping on machines in my local network I get the mac addresses from them. In the same way I can construct and send a ARP request and collect the response to these machines. This is used since I build raw packets completely from scratchy (to allow spoofing of every possible field, including mac addresses if needed). But, when I try arping or arp on external ip's and hosts such as google.com it doesn't get any reply. What should the destination mac address be set to when sending packets to targets outside my local network? I guess the router since that's what passes it on... am I correct? Is there a quick way in ANSI C to collect the mac address of the router in use by the computer? Or at least the IP so I can send a ARP request to it.
Thanx in advance
MAC operations are limited to machines directly connected within your subnet. So you should use the router's MAC address for packets intended for hosts outside your subnet.
There are numerous ways to obtain the router's IP address.
You can parse the configuration files on your local host if the interface is statically configured.
You can see if your compute platform has an API that lets you access the interface configuration information directly. This would work in both static and dhcp cases.
You can write socket code to send an ICMP message to an outside address then parse the incoming responses. They will be from the router. The stack will, in this case, find the router for you.
It should be set to the gateway (assuming ethernet on that link...).
I have a need for some C or C++ code, compilable under Linux, to be able to take a list of IP addresses of some arbitrary number of remote hosts machines and obtain a ethernet MAC address for each one. These host machines may be on the same subnet or they could be on a different subnet behind a router. Its OK if the MAC address of some or all of the remote hosts is the address of the interface on the router. Ultimately, I want to hand off the IP address and MAC address to an FPGA who will use these pieces of information to format and send UDP/IP packets over ethernet to the hosts. Obviously, the FPGA will also be given its own MAC address and IP address to fill in the source MAC and source IP addresses in the packets.
Is there some code I can be pointed to that can create and broadcast ARP packets to these remote machines and receive back the ARP response packets such that the destination MAC addresses can be extracted?
Part of what you want to do requires some raw socket programming.
http://mixter.void.ru/rawip.html
The source for the linux arp command will give the rest of what you need. Here's a link:
http://www.comp.nus.edu.sg/~cs4236/readings/out/html/arp_8c-source.html
I recommend looking into arping which is doing a somewhat identical job. It takes IPs and MACs and tries to receive additional informationen. Or justs pings them.
http://freshmeat.net/projects/arping/