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Date: Wed, 8 Apr 2026 17:08:01 +0200
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Subject: Re: FreeBSD guest on Linux KVM, some vtnet(4) packets don't arrive
To: net@freebsd.org
References: <adZocDxDXSBFXQGA@amaryllis.le-fay.org>
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Cc: ivy@freebsd.org
From: Miroslav Lachman <000.fbsd@quip.cz>
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On 08/04/2026 16:38, Lexi Winter wrote:
> hello,
> 
> i'm using FreeBSD main:
> 
> FreeBSD 16.0-CURRENT #0 main-n284768-e6eba5076929: Mon Mar 30 16:07:28 UTC 2026
> root@releng3.nyi.freebsd.org:/usr/obj/usr/src/powerpc.powerpc64/sys/GENERIC64

[..]

> vtnet0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 1500
>          options=cc00ba<TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,VLAN_HWTSO,LINKSTATE,TXCSUM_IPV6,HWSTATS>
>          ether 52:54:00:12:34:56
>          inet6 fe80::2%vtnet0 prefixlen 64 scopeid 0x1
>          media: Ethernet autoselect (10Gbase-T <full-duplex>)
>          status: active
>          nd6 options=821<PERFORMNUD,AUTO_LINKLOCAL,STABLEADDR>


> 15:34:39.106898 IP6 fd00:0:0:1::12 > fd00:0:0:1::b: ICMP6, echo request, id 49664, seq 3, length 64
> 15:34:40.106951 IP6 fd00:0:0:1::12 > fd00:0:0:1::b: ICMP6, echo request, id 49664, seq 4, length 64
> 
> but these packets never arrive on the guest; tcpdump shows nothing.
> 
> however, 'netstat -in' shows that the packets *are* arriving on the guest,
> since Ipkts increments for each ICMP packet.  it seems like they're just
> getting lost somewhere in the network stack. (Ierrs/Oerrs is zero.)
> 
> does anyone have any ideas what might be causing this?

Can you try disabling rxcsum / txcsum? (add  -rxcsum -txcsum to 
ifconfig_vtnet0 in rc.conf)
I had similar problem in the past on KVM hosted FreeBSD guest and this 
solved the problem for me.

Kind regards
Miroslav Lachman