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Accept Force Renew messages with zero xid #719

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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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42 changes: 26 additions & 16 deletions src/dhcp.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3128,15 +3128,35 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
return;
}

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);
/* 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;
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}

if (state->xid != ntohl(bootp->xid)) {
/* If the xid is 0 in a BOOTP reply FORCERENEW, move on and
* check AUTH. MikroTik dhpc servers send xid 0. */
if (bootp->xid == 0 && type == DHCP_FORCERENEW) {
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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) &&
memcmp(bootp->chaddr, ifp->hwaddr, ifp->hwlen)) {
if (IS_STATE_ACTIVE(state)) {
Expand Down Expand Up @@ -3170,16 +3190,6 @@ 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,
Expand Down