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

[Jon Maloy <jmaloy@redhat.com>]

On 2025-02-14 06:00, Stefano Brivio wrote:
> 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().
> 

Not sure what to do about this. I don't even see we calculate the 
checksum in our code, so does it matter?

///jon