[PATCH v2] tap: always set the no_frag flag in IPv4 headers

[PATCH v2] 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>
---
v2: Updated checksum algorithm to consider the change
---
 ip.h  | 3 ++-
 tap.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/ip.h b/ip.h
index 1544dbf..858cc89 100644
--- a/ip.h
+++ b/ip.h
@@ -36,13 +36,14 @@
 		.tos		= 0,					\
 		.tot_len	= 0,					\
 		.id		= 0,					\
-		.frag_off	= 0,					\
+		.frag_off	= htons(IP_DF), 			\
 		.ttl		= 0xff,					\
 		.protocol	= (proto),				\
 		.saddr		= 0,					\
 		.daddr		= 0,					\
 	}
 #define L2_BUF_IP4_PSUM(proto)	((uint32_t)htons_constant(0x4500) +	\
+				 (uint32_t)htons_constant(IP_DF) +	\
 				 (uint32_t)htons(0xff00 | (proto)))
 
 
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 v2] tap: always set the no_frag flag in IPv4 headers

[Stefano Brivio]

On Wed, 19 Feb 2025 10:20:41 -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>
> ---
> v2: Updated checksum algorithm to consider the change

Applied.

-- 
Stefano