Re: [PATCH] tap: always set the no_frag flag in IPv4 headers.

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 12 Feb 2025 18:50:23 -0500
Jon Maloy <jmaloy@redhat.com> wrote:

> When studying the Linux source code and Wireshark dumps it seems like
> the no_frag flag in the IPv4 header is always set. Following discussions
> in the Internet on this subject indicates that modern routers never
> fragment packets, and that it isn't even supported in many cases.
> 
> Adding to this that incoming messages forwarded on the tap interface
> never even pass through a router it seems safe to always set this flag.
> 
> This makes the IPv4 headers of forwarded messages identical to those
> sent by the external sockets, something we must consider desirable.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>  tap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tap.c b/tap.c
> index d0673e5..44b0fc0 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -153,7 +153,7 @@ static void *tap_push_ip4h(struct iphdr *ip4h, struct in_addr src,
>  	ip4h->tos = 0;
>  	ip4h->tot_len = htons(l3len);
>  	ip4h->id = 0;
> -	ip4h->frag_off = 0;
> +	ip4h->frag_off = htons(IP_DF);

$ tshark -r test/test_logs/pasta.pcap -V -Y frame.number==9 | grep "Header Checksum"
    Header Checksum: 0x07d4 incorrect, should be 0xc7d3(may be caused by "IP checksum offload"?)

See L2_BUF_IP4_PSUM().

-- 
Stefano