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

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

[Jon Maloy]

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);
 	ip4h->ttl = 255;
 	ip4h->protocol = proto;
 	ip4h->saddr = src.s_addr;
-- 
@@ -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);
 	ip4h->ttl = 255;
 	ip4h->protocol = proto;
 	ip4h->saddr = src.s_addr;
-- 
2.48.1

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

[Stefano Brivio]

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

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

[Jon Maloy]

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

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

[Stefano Brivio]

On Fri, 14 Feb 2025 17:46:21 -0500
Jon Maloy <jmaloy@redhat.com> wrote:

> 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

We precalculate that part, see L2_BUF_IP4_PSUM() (and also
L2_BUF_IP4_INIT()).

> so does it matter?

Well, I think it matters that we send out valid IPv4 packets. Try this
change and see for yourself.

-- 
Stefano

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

[Jon Maloy]

On 2025-02-14 17:57, Stefano Brivio wrote:
> On Fri, 14 Feb 2025 17:46:21 -0500
> Jon Maloy <jmaloy@redhat.com> wrote:
> 
>> 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
> 
> We precalculate that part, see L2_BUF_IP4_PSUM() (and also
> L2_BUF_IP4_INIT()).
> 
>> so does it matter?
Brain fart. I was thinking about the UDP header checksum, which
is optional we don't caluclate in current code.
Of course it matters.
> 
> Well, I think it matters that we send out valid IPv4 packets. Try this
> change and see for yourself.
> 

Ok. Now I see what you mean. I will try this.

///jon