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Accept Force Renew messages with zero xid #719

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1d3adf9
Support Force Renew with zero xid
ColinMcInnes Sep 1, 2026
f570917
DHCP: Authenticate Force Renew before xid check
ColinMcInnes Sep 1, 2026
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117 changes: 76 additions & 41 deletions src/dhcp.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3091,6 +3091,7 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
bool bootp_copied;
uint32_t v6only_time = 0;
bool use_v6only = false, has_auto_conf = false;
bool bootp_must_reject_dhcp = false;
struct dhcp_policy dp = {
.ctx = ifp->ctx,
.bootp = bootp,
Expand Down Expand Up @@ -3128,13 +3129,53 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
return;
}

/* We may have found a BOOTP server */
if (get_option_uint8(ifp->ctx, &type, bootp, bootp_len,
DHO_MESSAGETYPE) == -1)
type = 0;
else
bootp_must_reject_dhcp = (ifo->options & DHCPCD_BOOTP) != 0;

#ifdef AUTH
if (type == DHCP_FORCERENEW) {
auth = get_option(ifp->ctx, bootp, bootp_len,
DHO_AUTHENTICATION, &auth_len);
if (auth == NULL) {
LOGDHCP(LOG_ERR, "unauthenticated Force Renew");
return;
}
if (dhcp_auth_validate(&state->auth, &ifo->auth,
(uint8_t *)bootp, bootp_len, 4, type, auth,
auth_len) == NULL) {
LOGDHCP0(LOG_ERR, "authentication failed");
return;
}
if (state->auth.token)
logdebugx("%s: validated using 0x%08" PRIu32, ifp->name,
state->auth.token->secretid);
else
loginfox("%s: accepted reconfigure key", ifp->name);

@ColinMcInnes ColinMcInnes Sep 1, 2026

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drop this to debug as renew is now debug

}
#endif

if (state->xid != ntohl(bootp->xid)) {
if (IS_STATE_ACTIVE(state))
logdebugx("%s: wrong xid 0x%x (expecting 0x%x) from %s",
ifp->name, ntohl(bootp->xid), state->xid,
inet_ntoa(*from));
dhcp_redirect_dhcp(ifp, bootp, bootp_len, from);
return;
/* AUTH has already run; this only skips redirect for
* MikroTik xid 0 FORCERENEW. Non-FORCERENEW xid
* mismatches still redirect. */
if (bootp->xid == 0 && type == DHCP_FORCERENEW) {
Comment thread
coderabbitai[bot] marked this conversation as resolved.
if (IS_STATE_ACTIVE(state))
logdebugx(
"%s: xid 0 in a BOOTP reply FORCERENEW from %s",
ifp->name, inet_ntoa(*from));
} else {
if (IS_STATE_ACTIVE(state))
logdebugx(
"%s: wrong xid 0x%x (expecting 0x%x) from %s",
ifp->name, ntohl(bootp->xid), state->xid,
inet_ntoa(*from));
dhcp_redirect_dhcp(ifp, bootp, bootp_len, from);
return;
}
}

if (ifp->hwlen <= sizeof(bootp->chaddr) &&
Expand Down Expand Up @@ -3170,38 +3211,31 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
}
}

/* We may have found a BOOTP server */
if (get_option_uint8(ifp->ctx, &type, bootp, bootp_len,
DHO_MESSAGETYPE) == -1)
type = 0;
else if (ifo->options & DHCPCD_BOOTP) {
logdebugx("%s: ignoring DHCP reply (expecting BOOTP)",
ifp->name);
return;
}

#ifdef AUTH
/* Authenticate the message */
auth = get_option(ifp->ctx, bootp, bootp_len, DHO_AUTHENTICATION,
&auth_len);
if (auth) {
if (dhcp_auth_validate(&state->auth, &ifo->auth,
(uint8_t *)bootp, bootp_len, 4, type, auth,
auth_len) == NULL) {
LOGDHCP0(LOG_ERR, "authentication failed");
return;
}
if (state->auth.token)
logdebugx("%s: validated using 0x%08" PRIu32, ifp->name,
state->auth.token->secretid);
else
loginfox("%s: accepted reconfigure key", ifp->name);
} else if (ifo->auth.options & DHCPCD_AUTH_SEND) {
if (ifo->auth.options & DHCPCD_AUTH_REQUIRE) {
LOGDHCP0(LOG_ERR, "no authentication");
return;
if (type != DHCP_FORCERENEW) {
/* Authenticate the message */
auth = get_option(ifp->ctx, bootp, bootp_len,
DHO_AUTHENTICATION, &auth_len);
if (auth) {
if (dhcp_auth_validate(&state->auth, &ifo->auth,
(uint8_t *)bootp, bootp_len, 4, type, auth,
auth_len) == NULL) {
LOGDHCP0(LOG_ERR, "authentication failed");
return;
}
if (state->auth.token)
logdebugx("%s: validated using 0x%08" PRIu32,
ifp->name, state->auth.token->secretid);
else
loginfox("%s: accepted reconfigure key",

@ColinMcInnes ColinMcInnes Sep 1, 2026

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same, renew msgs are debug level, so consider dropping the log level here too

ifp->name);
} else if (ifo->auth.options & DHCPCD_AUTH_SEND) {
if (ifo->auth.options & DHCPCD_AUTH_REQUIRE) {
LOGDHCP0(LOG_ERR, "no authentication");
return;
}
LOGDHCP0(LOG_WARNING, "no authentication");
}
LOGDHCP0(LOG_WARNING, "no authentication");
}
#endif

Expand All @@ -3213,11 +3247,6 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
return;
}
#ifdef AUTH
if (auth == NULL) {
LOGDHCP(LOG_ERR, "unauthenticated Force Renew");
if (ifo->auth.options & DHCPCD_AUTH_REQUIRE)
return;
}
if (state->state != DHS_BOUND && state->state != DHS_INFORM) {
LOGDHCP(LOG_DEBUG, "not bound, ignoring Force Renew");
return;
Expand All @@ -3237,6 +3266,12 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
return;
}

if (bootp_must_reject_dhcp) {
logdebugx("%s: ignoring DHCP reply (expecting BOOTP)",
ifp->name);
return;
}

if (state->state == DHS_BOUND) {
LOGDHCP(LOG_DEBUG, "bound, ignoring");
return;
Expand Down