I'm trying to learn the ARP protocol. My idea about ARP is that it is a protocol that help addressing the IP to the exact MAC Address.
But when I try to do:
arp -a
Which I think it will show the list of IP addresses and the matching MAC addresses. I understand everything until I see the word permanent. The other lines are marked ifscope, and one is marked permanent.
Can someone tell me what mean permanent? Why is this line different?
Thanks in advance.
According to the TCP/IP Guide:
There are two different ways that cache entries can be put into the
ARP cache:
Static ARP Cache Entries: These are address resolutions that are manually added to the cache table for a device and are kept in the
cache on a permanent basis. Static entries are typically managed using
a tool such as the arp software utility.
Dynamic ARP Cache Entries: These are hardware/IP address pairs that are added to the cache by the software itself as a result of
successfully-completed past ARP resolutions. They are kept in the
cache only for a period of time and are then removed.
So the word permanent in the arp -a output means that the entry is manually added to the cache (static).
Related
I have an app which checks whether a certain IP is on the local network. The IP belongs to a raspberry pi working as a homeautomation device.
Running the command
arp -a
from terminal or from within my app I get a list of all devices on the local network, but my raspberry pi is not always listed. After pinging the device, it appears on the list every time.
Any ideas what is going wrong?
That's the way ARP works. ARP entries are created only when they're needed. The first time you need to communicate with an IP on the local network, your device broadcasts an ARP query, then caches the response. There's nothing that proactively fills in the cache for all IPs on the LAN.
In addition, old entries may be removed from the cache if they haven't been used in a while.
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.
just as title says.
Let's assume this situation:
I have a network and I need to know IP adresses and everything else related of all computers connected to mantain socket sending via Winsock under Windows.
Is there better way than looping throught IP Adresses range and sending something like:
"Are You there?" and waiting for answer from client?
Performing an ARP scan would tell you which devices are active on the network, regardless of device type or operating system. So this would include PCs, laptops, phones, routers, and any other devices with a NIC. This is the most reliable way of scanning for live hosts on a network.
ARP is the protocol that translates IP addresses into MAC addresses, or vice versa. It allows routing devices to translate between the logical network and the physical network. When you send a packet to, say, 192.168.1.99, your system first sends out an ARP request for 192.168.1.99, saying something along the lines of "Who has 192.168.1.99? Tell [my MAC]". Other devices on the network see this packet, and the owner of that IP address will reply "Hi [MAC], I'm 192.168.1.99, my MAC is 00:12:34:56:78:90".
You can (ab)use this protocol to discover network devices. If you know you're on 192.168.1.x, you send ARP requests for 192.168.1.1 through 192.168.1.254, and record the results. Every reply you get indicates a live system at the defined IP address.
Keep in mind that not all your users will be on 192.168.1.0/24, so you'll need to check the current network configuration first.
As far as doing this in C goes, it's not a simple task. However, you can take a look at this example which implements ARP scanning.
An alternative to the ARP scan suggested by #Polynomial is the PING scan.
Same concept, PING each address in your subnet, and record the results.
Just as an ARP scan may give you false positives (because there is a router saying "I'll deal what that address" whether there is a device on it or not), so too, PING can give you false negatives (as some systems disable the ICMP/PING command).
There is a tool called fping that implements this kind of PING scan in a massively parallel way. Fping is originally a *NIX program, but there are Windows versions out there. You should be able to find the source to peruse if you want to learn the gory details.
I need to discover all network neighbors in Linux(they are running Linux too) and I need to get theirs IP addresses(3rd layer). Any ideas how to do that?
Btw, I need to do that in C, not in shell
Many thanks in advance!
What you should do is, have the neighbours run a daemon which responds (with a unicast response to the sender) to UDP multicasts.
Then send a UDP multicast with a TTL of 1 (so it will not be routed) and listen to see who responds. You will only receive responses from the neighbours which are running the agent.
Another possibility is to use an existing protocol which already does this, for example, mDNS.
There is no guaranteed way to do this if the machines in question aren't co-operating.
The best you can do is to scan likely addresses and probe each one to see if you can get a response - that probe could be anything from a simple ICMP echo request (a ping) up to a sophisticated malformed packet that attempts to elicit a response from the remote host.
The level of sophistication required, and whether it will work at all, depends entirely on how heavily firewalled etc the host in question is.
As a commenter has already observed, there are entire programs like nmap dedicated to attempting to discover this information, which gives some idea of how non-trivial this can be.
At the other extreme, if the hosts are co-operating, then a simple broadcast ICMP echo request might be enough.
If your segment uses reasonably decent switch, you can discover the link-layer neighbours by inspecting the forwarding database of one of the switches. You should be able to obtain this fairly automatically via SNMP, check your switch's documentation.
Once you have a list of link neighbours, you can try and find out their IP addresses, but remember that they may have many or none at all. For this you'd need some sort of reverse-ARP. Perhaps your router maintains a list of MAC-to-IP associations and you can query it (again SNMP would be the most convenient solution).
I'm using libpcap (and winpcap on Windows) in a C application to monitor network traffic. I need to differentiate between upload and download traffic on each network adapter, to produce connection speed stats, but the filter expressions used by the library don't seem to support this very easily (ie there are no 'incoming'/'outgoing' operators).
One approach that I have considered is to query the IP address of each adapter, and then use filters such as src host 1.2.3.4 (to measure uploads) and dst host 1.2.3.4 (to measure downloads).
My questions are:
Is there a better/simpler approach than the one above (something that would let me use the same filter expression for each adapter would be nice)?
If the above approach is the way to go, then is there any chance that a single adapter could have more than 1 IP address associated with it? The reason I ask is that the pcap_addr struct which holds the address details of a single adapter (in struct pcap_if) has a 'next' member suggesting that this is possible.
Firstly, remember, pcap sees only packets. It doesn't see "outgoing" or "incoming" - simply packets. So yes, you must filter using the src/dst in the ip headers. There is no other way to tell whether the packet is incoming or outgoing.
Secondly, yes, there is nothing stopping an adapter having multiple IP addresses. So you need to grab the IP addresses configured from that adapter. pcap_findalldevs() (WinPCap Documentation) ought to help you here, from which you should be able to deduce which devices you want to monitor.
Have you considered looking at pmacct - I have personally contributed to this in time past. This is a C tool that uses libpcap to passively monitor network traffic for accounting purposes.
Try tcpdump