I've been using tcpdump for about a month now, and recently, it has stopped capturing any packets that were not sent to or from the computer running tcpdump. I've stripped down my command to just:
sudo tcpdump -i en2
I've checked my interfaces with ifconfig, and en2 is in "PROMISC" mode. When specifying a specific host as a filter, I only see a few "arp" messages but nothing compared to what is actually going on in the network.
Any ideas why this would be happening? Much appreciated if anyone can offer some advice!
Richard
ps, sorry for the re-post, i wanted to register this time! (new to s.o.)
Do you know what network equipment is used \ if there has been a change recently?
One possible explanation is that your computer is connected to a switch (and not a hub) the switch sends to your adapter only traffic intended to your MAC address, and broad casts (hence the ARP)
one way to check this is to send broad casts from other computers in the network (just use ping 255.255.255.255) and see if you can see anything.
Related
Is there a foolproof way to discover the MAC address of a device that is freshly plugged into an ethernet network in linux?
I'm looking specifically for something that works on the MAC address level. Assume everything at the ip address level is broken.
i.e. maybe ethernet chip drivers expose a list of the MAC's they've seen in /proc?
Taken from a similar question, this gets pretty close:
sudo tshark -i eth0 -e eth.src -Tfields
However, it doesn't de-duplicate lines, and it seems there are intermittent line number prefixes.
I have UDP network traffic arriving on my machine (OSC traffic from an iPad, to be exact) and I want two programs to be able to receive that traffic. The problem is that I can't bind to the same network port with two programs at once and I can't send to multiple ports with the iOS app I'm using. How can I solve this problem?
You can use the power of the command line for this. The following snippet uses socat (probably needs to be installed beforehand) and tee (should be preinstalled on any OS X or Linux).
socat -u UDP4-RECVFROM:8123,fork - | tee >(socat -u - UDP4-SENDTO:localhost:8223) | socat -u - UDP4-SENDTO:localhost:8323
Explanation: socat listens for traffic on UDP port 8123, pipes it to tee, which pipes it to two other instances of socat forwarding it to ports 8223 and 8323 on localhost respectively. With your two programs you need to listen to those ports on localhost.
While the answer with using socat is elegant it is not clear for me, what you are trying to do:
both programs should receive all parts of the traffic and they will only receive and not reply. This can be done with the proposed socat way
both program should receive all parts of the traffic and there reply will be mixed together (how?)
each of the programs should only receive parts of the traffic, e.g. the one which the other did not get. This should be possible if both of your programs use SO_REUSEADDR, SO_REUSEPORT. Replies will then be mixed together.
or do you actually want to communicate with each of the programs seperatly - then you would have to use either multiple sockets in the iOS app (which you don't want to do) or built your own protocol which does multiplexing, e.g. each message is prefixed with there target app and on the target machine a demultiplexer application will receive all packets and forward them to the appropriate application and wrap the replies back in the multiplexing protocol.
In summary: please describe the problem your are trying to solve, not only one small technical detail of it.
The problem is that I can't bind to the same network port with two programs at once
Yes you can. Just set SO_REUSEADDR and maybe SO_REUSEPORT on both of them before you bind.
There is an old program a member of this online community made for everyone to use, but he is no longer around and no longer supports it. I wanted to help the community by extending it's features somewhat. To do that, I need to know how data is sent from it. How can I capture the network traffic it sends, and determine what it's sending?
WireShark is your friend. Available for UNIX and Windows.
Wireshark is a network packet analyzer. A network packet analyzer will
try to capture network packets and tries to display that packet data
as detailed as possible.
Wireshark User's Guide
#Kerrek SB's Tip: use tcpdump -s0 -w /tmp/data -i eth3 or so to create the dump file, and analyze it with Wireshark later as an unprivileged user.
I'm working on a Wake on LAN service that will run from a web site and should interact with many different platforms - therefore, no Windows-only solutions. When a user registers their system with the web site, I need to get the MAC address to use in constructing the "magic" packet. I have a Java Applet that is able to do this for me and am aware of an ActiveX control that will work, but I'm wondering if there is a way to do this server-side by querying routers/switches. Since the system may be on any of a number of different physical subnets, using ARP won't work -- unless there's a way to configure the router(s) to perform the ARP on my behalf.
Anyone know of any network APIs, proprietary or otherwise, that can be used to look up MAC addresses given an IP address? I think we're using Cisco routers, but it's a complicated network and there may be multiple vendors involved at various levels. I'd like to get some background information on possible solutions before I go to make a sacrifice to the network gods. No point in abasing myself if it's not technically possible. :-)
EDIT: We do have the network infrastructure set up to allow directed broadcast, though figuring out the exact broadcast address since netmasks are not always /24 is another conundrum that I need to solve.
If you are on a local network that uses DHCP you might be able to look in the servers database to get the MAC of the last user with that address. In the future you could watch the network for ARP requests and cache the responses in some sort of table. You might also look at using RMON or SNMP to try and query the Address Tables on the switches and routers.
It should be noted that to use WoL across routers you either need to enable Directed Broadcasts or you need to have a relay server in the local segment.
Been a while since I played routers and swtiches but this might be a starting point for what to query using SNMP http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a00801c9199.shtml
Use the following:
getmac /s destIp
To get the remote session Mac address.
I don't know if these might be helpful but take a look:
http://www.webdeveloper.com/forum/showthread.php?t=134120
http://www.qualitycodes.com/tutorial.php?articleid=19
You've said everything I can think of...
The source MAC address changes as a packet hops from device to device so unless the client is on the same subnet, the server won't be able to get the MAC address. (You would do it via ARP)
A signed java applet or activex control would be the easiest solution. It would be able to (almost passively) get all the networking info you need (IE doesn't even prompt to run a signed applet)
If you are fully aware of the network that is using the service then you could probably query a gateway's client-list via SNMP or CDP. You would be able to map out IP-Addresses to MAC addresses... but this is really vendor dependent (but common) and wouldn't be much better (imo) than having an applet.
Currently the application is using a Java 6 applet that allows me to extract both the hostname and the MAC address from the remote system. I don't like having this dependency on Java 6, but Snow Leopard and Windows both support it, so I can probably live with it.
On a related-front our networking folks approached me for some help with converting some existing code to ASP.NET. During the conversation I asked if they had live MAC address information (since they do port shutoffs based on suspicious network activity -- viruses/worms). Turns out they do and we may be able to leverage this project to get access to the information from the network database.
I don't think there is any way to accomplish this. When the IP packet goes via the first router the host's MAC information is lost (as you know MAC is only used in ethernet layer). If the router most close to your PC was capable of telling the remote MAC code to you, again it would only see the MAC of the next router between your PC and the "other end".
Start sacrificing.
There's no general way to do this in terms of the network unless you have no routers involved. With a router involved, you will never see the MAC address of the originating system.
This assumes that the originating system only ever has a single network interface, so has only a single MAC address.
In fact, are you even sure that your "magic packet" (whatever that is) will reach the system you want it to reach, through the routers? That sounds like a function the routers or other network infrastructure should be performing.
Mac address is only used on network segments, and is lost at each hop. Only IP is preserved for end-to-end - and even then the from ip address is rewritten when Natted. I guess my answer is, not possible unless everything is on the same network segment, or your routers are set up for proxy arp (which is not really realistic).
You can only get MAC entries in the ARP table for machines on the same network. If you connect to a machine via a router then you will only see the routers MAC address in the ARP table. So there is no way of knowing the foreign host’s MAC address unless it's a host on the same network (no routers involved).
And by the way there are many similar question already on SO.
if it's a windows system you can use NBTSTAT -A
this will return the netbios info and the IP is there
any Management system like SMS or Altiris will have this info
The DHCP server is a good idea
If it's local you can ping it and then quickly run ARP -a
look for the IP and the MAC will be there.
you might need to write a small batch file.
if you have access to the PC you can use WMI to access the info for the Nic with DHCP.
As said above we can get mac address from a known IP address if that host is in the same subnet. First ping that ip; then look at arp -a | grep and parse the string on nix* to get mac address.
We can issue system command from all programming languages standard API's and can parse the output to get mac address.Java api can ping an IP but I am not sure if we parse the ping output(some library can do it).
It would be better to avoid issuing system command and find an alternative solution as it is not really Platform Independent way of doing it.
Courtesy: Professor Saleem Bhatti
I'm trying to confirm a user a unplugged my embedded device from a network before performing some maintenance. I'm considering "ping"ing all IP address on my sub-net, but that sounds crude. Is there a broadcast/ARP method that might work better?
You can try a broadcast ping (this is from linux):
ping -b 255.255.255.255
Another option is to download Nmap and do a ping-scan.
You could use nmap. It's still crude, but at least it's using a tool designed to do it so you don't have to spend time on it.
If you can't get reliable link state information from your Ethernet device (which most chipsets should support these days, BTW...), sending an ARP request for each IP on your local subnet is a decent substitute. The overhead is minimal, and as soon as you get a single response, you can be sure you're still connected to a network.
The only possible problem I see here, is that if your device is on a /8 subnet, it can take quite a while to loop through all 4294967296 possible IPs. So, you may want to consider some optimization, such as only sending ARP requests for your default gateway, as well as all IPs currently in your ARP table.
If there's a peer you know you were connected to recently you could try pinging or arping that first. That could cut down on the traffic you're generating.
you could also run tcpdump -n to see what's active on the network too.
Not receiving any responses to ICMP pings or ARP requests is not a 100% guarantee that there's no network connection. For instances, there might be devices on the network that are firewalled off.
EDIT: May be you could access some lower-level information on your embedded device to check whether the network interface has its link up without actually sending any data.
Is there any chance that your device supports UPnP or Bonjour? Beside of the low-level protocols your should also have a look at these protocols which support some kind of plug-&-Play functionality. A UPnP device for example sends a message on the LAN before it is switched off (though, this doesn't help if it is just removed by unplugging it...).