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ip4: tcp: send zero IP_ID for small packets

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... especially the TCP control ones (SYNACK, RST, etc).

This change makes the system useless as a zombie machine in idle-scan
attempts.

This approach is inspired by what is done in other network stack (such
as the linux kernel one).

This approach is a low-cost CPU solution, compared to generating a
random IP ID value for every single IP packet.

See patch #10270

Signed-off-by: Samuel Martin <samuel.martin@netatmo.com>
Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>
This commit is contained in:
Samuel Martin
2022-06-30 17:27:09 +02:00
committed by Simon Goldschmidt
parent ad897db7ea
commit 13d7ac3363
1 changed files with 12 additions and 2 deletions
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14
src/core/ipv4/ip4.c
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@@ -961,11 +961,21 @@ ip4_output_if_opt_src(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *d
chk_sum += iphdr->_len;
#endif /* CHECKSUM_GEN_IP_INLINE */
IPH_OFFSET_SET(iphdr, 0);
IPH_ID_SET(iphdr, lwip_htons(ip_id));
if ((proto == IP_PROTO_TCP) && (p->tot_len <= IP4_MIN_MTU_LENGTH))
{
/* For small TCP packets, e.g. protocol handshake,
* do not bother generating IP_ID (just use 0 ip_id)
*/
IPH_ID_SET(iphdr, lwip_htons(0));
}
else
{
IPH_ID_SET(iphdr, lwip_htons(ip_id));
++ip_id;
}
#if CHECKSUM_GEN_IP_INLINE
chk_sum += iphdr->_id;
#endif /* CHECKSUM_GEN_IP_INLINE */
++ip_id;
if (src == NULL) {
ip4_addr_copy(iphdr->src, *IP4_ADDR_ANY4);