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I am looking for a way to programmatically (in C) send gratuitous ARP messages in a Linux user-space application. I noticed there are are some files for configuring ARP in procfs at
/proc/sys/net/ipv4/conf/<interface_name>
Is there a way to do this with an ioctl call to the corresponding network interface?
573
A gratuitous ARP is a broadcast request for a router's own IP address. If a router or switch sends an ARP request for its own IP address and no ARP replies are received, the router- or switch-assigned IP address is not being used by other nodes.
Gratuitous ARPs are useful for four reasons: They can help detect IP conflicts. When a machine receives an ARP request containing a source IP that matches its own, then it knows there is an IP conflict. They assist in the updating of other machines' ARP table.
The process is pretty straight forward, send a few ARP Probes (typically 3), and if no one responds, officially claim the IP address with an ARP Announcement. Both the ARP Probes and the ARP Announcements are sent as Broadcast frames – using the destination MAC address of ffff. ffff. ffff in the Ethernet header.
Gratuitous arp is when a device will send an arp reply that is not a response to a request. Depending on the particular IP stack, some devices will send gratiutous arp when they boot up, which announces their presence to the rest of the network.
Using scapy:
from scapy.all import *
SELF_MAC = '02:02:02:02:02:02' # fill in with your MAC address
BCAST_MAC = 'ff:ff:ff:ff:ff:ff'
def create_ARP_request_gratuituous(ipaddr_to_broadcast):
arp = ARP(psrc=ipaddr_to_broadcast,
hwsrc=SELF_MAC,
pdst=ipaddr_to_broadcast)
return Ether(dst=BCAST_MAC) / arp
# and then call sendp() with the result
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