Re: [Qemu-devel] slirp, incoming packets get truncated

[Chris Heinze]

hi,

On 9/8/19 1:29 AM, Samuel Thibault wrote:
>> 16:49:39.430959 IP 10.0.2.2.33294 > 10.0.2.15.19003: UDP, bad length 9000 > 
>> 1472
> 
> tcpdump seems to be showing dumb output here. The packet is fragmented
> by slirp, which makes tcpdump confused and show only the first
> fragment. If you let tcpdump print everything, you will see the other
> fragments. In reality, everything is going fine here.

oh ok. for some reason the receiving process is not happy with these packets 
anyway. thanks for the hint, i'll have to look into that deeper then.

>> #define IF_MRU 9000
>>
>> but the resulting qemu-system-x86_64 binary did not behave differently.
> 
> Did you explicitly remove the qemu-system-x86_64 binary? As mentioned

yup, i mean no, but i initially tested by copying binary and executing it 
directly, so it was the patched binary.

> try the attached patch? The lowest layer of slirp was indeed limited to
> 1600-byte frames for no good reason. With this and the virtio driver, I
> could exchange 9000-byte packets.

ah, thanks! looks good:

root@test-host:~# head -c 9000 /dev/zero | nc -u 127.0.0.1 19003
root@test-guest:~# tcpdump -ni eth0 port 19003
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
12:20:56.484044 IP 10.0.2.2.37331 > 10.0.2.15.19003: UDP, bad length 9000 > 8968

root@test-host:~# head -c 8000 /dev/zero | nc -u 127.0.0.1 19003
root@test-guest:~# tcpdump -ni eth0 port 19003
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
12:25:43.967476 IP 10.0.2.2.47243 > 10.0.2.15.19003: UDP, length 8000

...with mtu 9000 everywhere. mtu is l2, and complete frame size i assume, so: 
ip header = 20B, dst-mac + src-mac = 12B

great, thanx 1e6!

-- 

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