Notes on arpresolve error

[Ted]

I recently ran into the console error "arpresolve:can't allocate llinfo for <ip addr> on <intf>". I'm writing a brief note on my experience. My primary ISP is broadband cable with static IPv4. I had been using a DSL circuit as a failover, and while it was very reliable, it was not very fast. I had a chance to try out a Netgear NightHawk cellular modem/wireless router, which promised better speed.

I started by connecting a PC (configured to use DHCP) to the Ethernet port on the Netgear device. I was able to determine the public IP by browsing to 'whatismyip.com'. I set the device up using information gathered from the Internet, which consisted mainly of turning WiFi off, and putting it in "passthrough mode" (I figured that was Netgear's term for bridged). Everything seemed to work as expected.

On the OPNSense side, I configured DHCPv4 and DHCPv6 on an open interface. According to OPNsense, the IPv4 interface address (from DHCP) was 10.155.146.127 and the gateway was 10.155.146.1. I assumed that the cellular carrier was using CGNAT. Then I noticed that the OPNsense console was being spammed with "arpresolve: can't allocate llinfo for 10.155.146.1 on igb1" errors. Re-reading the documentation, I noted that the private address of the router was available, even in "passthrough mode" (so, not quite a bridge). I changed the OPNsense interface to use static IPv4 addressing in the 192.168.1.x subnet, and the arpresolve error went away. I was able to use Dynamic DNS to register the public IP behind the CGNAT, and so far the device seems to be working OK.

One other oddity is that DHCPv6 doesn't appear to work quite right. However, that is something for another day. Hopefully this will be helpful to someone.