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tap: always set the no_frag flag in IPv4 headers

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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>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
Jon Maloy 2025-02-19 10:20:41 -05:00 committed by Stefano Brivio
parent 4dac2351fa
commit ea69ca6a20
2 changed files with 3 additions and 2 deletions
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3
ip.h
View File

@ -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)))

2
tap.c
View File

@ -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;