Getting ping 'DUP' response from host machine running raspbian buster lite image with QEMU

Issue

I ran the raspbian image with the following command:

qemu-system-arm -kernel kernel-qemu-4.19.50-buster -cpu arm1176 -m 256 -M versatilepb -dtb versatile-pb.dtb -no-reboot -serial stdio -append "root=/dev/sda2 panic=1 rootfstype=ext4 rw" -drive "file=2020-02-13-raspbian-buster-lite.img,index=0,media=disk,format=raw" -net user,hostfwd=tcp::5022-:22 -net nic -net user,smb=/dev/shm/

Booting the image completed successfully. Withing guest machine I get the following routing table:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.0.2.2        0.0.0.0         UG    202    0        0 eth0
10.0.2.0        0.0.0.0         255.255.255.0   U     202    0        0 eth0

Now when pinging the gateway at 10.0.2.2 works fine, but when pinging the host machine or the host gateway at 10.0.0.138 I get:

pi@raspberrypi:~$ ping 10.0.0.138
PING 10.0.0.138 (10.0.0.138) 56(84) bytes of data.
64 bytes from 10.0.0.138: icmp_seq=1 ttl=255 time=1.19 ms
64 bytes from 10.0.0.138: icmp_seq=1 ttl=255 time=1.23 ms (DUP!)

I verified that 10.0.0.138 isn't defined as broadcast address, and there are no IP duplications. Any idea how to debug from here? Thanks

Solution

As Peter Maydell suggested, merging the two options into one "-net user,smb=/dev/shm/,hostfwd=tcp::5022-:22" solved the case.

This is because QEMU creates a new 'user' network backend for each use of '-net user' on the command line, so in the original commandline there were two backends, each of which was responding to ping packets.

Answered By - AutomatedOwl