Issues with using ioctl and SIOCGARP to get ARP entry on Linux Kernal

bostan · 02-19-2010, 10:52 AM

Hi ,
I am using Linux 2.6 Fedora 12.
I am using ioctl(s,SIOCGARP,&areq,sizeof(struct arpreq) ) command to query the kernel to get the mac address for a given IP address.
What I have seen is whenever I query the kernel to get the MAC I do get the MAC but for some reason after the ARP age out I don't see the arp entry again back when I get the new packet. If I commented out the IOCTL code after ARP age-out when next packet comes from the SRC IP Kernel do add it back to the ARP table.
Does anybody has seen this issue ?
Any help is appreciated.
-Thanks

boatbodger · 08-04-2014, 09:59 AM

Did you ever get to the bottom of this? I am seeing the same problem (albeit Fedora 14).
For my application, I need to find the MAC address of an incoming packet, and - bizzarely - the kernel does not seem to glean MAC from incoming packets - only as a result of ARP.

My not-so-elegant workaround is to set the arp cache timer up to something quite large and then run an IP scan just before the arp cache entry would time out. I'd love to find a better way...