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Add hook for TCP Initial Sequence Number generation

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lwIP produces a TCP Initial Sequence Number (ISN) for each new TCP
connection. The current algorithm is simple and predictable however.
The result is that lwIP TCP connections may be the target of TCP
spoofing attacks.  The problem of such attacks is well known, and a
recommended ISN generation algorithm is standardized in RFC 6528.
This algorithm requires a high-resolution timer and cryptographic
hashing function, though.  The implementation (or best-effort
approximation) of both of these aspects is well beyond the scope of
lwIP itself.

For that reason, this patch adds LWIP_HOOK_TCP_ISN, a hook that
allows each platform to implement its own ISN generation using
locally available means.  The hook provides full flexibility, in
that the hook may generate anything from a simple random number
(by being set to LWIP_RAND()) to a full RFC 6528 implementation.

Implementation note:

Users of the hook would typically declare the function prototype of
the hook function in arch/cc.h, as this is the last place where such
prototypes can be supplied.  However, at that point, the ip_addr_t
type has not yet been defined.  For that reason, this patch removes
the leading underscore from "struct _ip_addr", so that a prototype
of the hook function can use "struct ip_addr" instead of "ip_addr_t".

Signed-off-by: sg <goldsimon@gmx.de>
This commit is contained in:
David van Moolenbroek
2016-10-26 21:24:46 +00:00
committed by sg
parent da15132aa0
commit a8b986bbb6
5 changed files with 40 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

17
src/core/tcp.c
View File

@@ -880,10 +880,11 @@ tcp_connect(struct tcp_pcb *pcb, const ip_addr_t *ipaddr, u16_t port,
#endif /* SO_REUSE */
}
iss = tcp_next_iss();
iss = tcp_next_iss(pcb);
pcb->rcv_nxt = 0;
pcb->snd_nxt = iss;
pcb->lastack = iss - 1;
pcb->snd_wl2 = iss - 1;
pcb->snd_lbb = iss - 1;
/* Start with a window that does not need scaling. When window scaling is
enabled and used, the window is enlarged when both sides agree on scaling. */
@@ -1491,7 +1492,6 @@ struct tcp_pcb *
tcp_alloc(u8_t prio)
{
struct tcp_pcb *pcb;
u32_t iss;
pcb = (struct tcp_pcb *)memp_malloc(MEMP_TCP_PCB);
if (pcb == NULL) {
@@ -1554,11 +1554,6 @@ tcp_alloc(u8_t prio)
pcb->sv = 3000 / TCP_SLOW_INTERVAL;
pcb->rtime = -1;
pcb->cwnd = 1;
iss = tcp_next_iss();
pcb->snd_wl2 = iss;
pcb->snd_nxt = iss;
pcb->lastack = iss;
pcb->snd_lbb = iss;
pcb->tmr = tcp_ticks;
pcb->last_timer = tcp_timer_ctr;
@@ -1826,12 +1821,18 @@ tcp_pcb_remove(struct tcp_pcb **pcblist, struct tcp_pcb *pcb)
* @return u32_t pseudo random sequence number
*/
u32_t
tcp_next_iss(void)
tcp_next_iss(struct tcp_pcb *pcb)
{
#ifdef LWIP_HOOK_TCP_ISN
return LWIP_HOOK_TCP_ISN(&pcb->local_ip, pcb->local_port, &pcb->remote_ip, pcb->remote_port);
#else /* LWIP_HOOK_TCP_ISN */
static u32_t iss = 6510;
LWIP_UNUSED_ARG(pcb);
iss += tcp_ticks; /* XXX */
return iss;
#endif /* LWIP_HOOK_TCP_ISN */
}
#if TCP_CALCULATE_EFF_SEND_MSS

6
src/core/tcp_in.c
View File

@@ -542,6 +542,7 @@ static void
tcp_listen_input(struct tcp_pcb_listen *pcb)
{
struct tcp_pcb *npcb;
u32_t iss;
err_t rc;
if (flags & TCP_RST) {
@@ -589,6 +590,11 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
npcb->state = SYN_RCVD;
npcb->rcv_nxt = seqno + 1;
npcb->rcv_ann_right_edge = npcb->rcv_nxt;
iss = tcp_next_iss(npcb);
npcb->snd_wl2 = iss;
npcb->snd_nxt = iss;
npcb->lastack = iss;
npcb->snd_lbb = iss;
npcb->snd_wl1 = seqno - 1;/* initialise to seqno-1 to force window update */
npcb->callback_arg = pcb->callback_arg;
#if LWIP_CALLBACK_API || TCP_LISTEN_BACKLOG