From a172c6c1235680cb014f5f5f5e0bb5b904c4e031 Mon Sep 17 00:00:00 2001
From: Matthew Harkness <matthew.harkness@spacex.com>
Date: Mon, 21 Sep 2026 09:48:57 -0700
Subject: [PATCH] dhcp: add dhcp-hold-offered to reserve offered addresses briefly

---
 man/dnsmasq.8 | 4 ++++
 src/dnsmasq.h | 3 ++-
 src/option.c  | 3 +++
 src/rfc2131.c | 9 ++++++++-
 4 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/man/dnsmasq.8 b/man/dnsmasq.8
index 8923325..7e5ac0e 100644
--- a/man/dnsmasq.8
+++ b/man/dnsmasq.8
@@ -1801,6 +1801,10 @@ option with a DHCPACK including a Rapid Commit option and fully committed
 address and configuration information. Should only be enabled if either the
 server is  the only server for the subnet, or multiple servers are present and they each commit a binding for all clients.
 .TP
+.B --dhcp-hold-offered
+Create a 30s lease when sending DHCPOFFER so the address is not offered
+to another client before DHCPREQUEST.
+.TP
 .B --dhcp-alternate-port[=<server port>[,<client port>]]
 (IPv4 only) Change the ports used for DHCP from the default. If this option is
 given alone, without arguments, it changes the ports used for DHCP
diff --git a/src/dnsmasq.h b/src/dnsmasq.h
index ecff18d..2288557 100644
--- a/src/dnsmasq.h
+++ b/src/dnsmasq.h
@@ -296,7 +296,8 @@ struct event_desc {
 #define OPT_LEASEQUERY     77
 #define OPT_LOG_ONLY_FAILED  78
 #define OPT_LOG_MALLOC     79
-#define OPT_LAST           80
+#define OPT_HOLD_OFFERED   80
+#define OPT_LAST           81
 
 #define OPTION_BITS (sizeof(unsigned int)*8)
 #define OPTION_SIZE ( (OPT_LAST/OPTION_BITS)+((OPT_LAST%OPTION_BITS)!=0) )
diff --git a/src/option.c b/src/option.c
index 44bd522..61c91ed 100644
--- a/src/option.c
+++ b/src/option.c
@@ -201,6 +201,7 @@ struct myoption {
 #define LOPT_LEASEQUERY    389
 #define LOPT_SPLIT_RELAY   390
 #define LOPT_LOG_MALLOC    391
+#define LOPT_HOLD_OFFERED  392
 
 #ifdef HAVE_GETOPT_LONG
 static const struct option opts[] =  
@@ -389,6 +390,7 @@ static const struct myoption opts[] =
     { "dhcp-ttl", 1, 0 , LOPT_DHCPTTL },
     { "dhcp-reply-delay", 1, 0, LOPT_REPLY_DELAY },
     { "dhcp-rapid-commit", 0, 0, LOPT_RAPID_COMMIT },
+    { "dhcp-hold-offered", 0, 0, LOPT_HOLD_OFFERED },
     { "dumpfile", 1, 0, LOPT_DUMPFILE },
     { "dumpmask", 1, 0, LOPT_DUMPMASK },
     { "dhcp-ignore-clid", 0, 0,  LOPT_IGNORE_CLID },
@@ -600,6 +602,7 @@ static struct {
   { LOPT_DHCPTTL, ARG_ONE, "<ttl>", gettext_noop("Set TTL in DNS responses with DHCP-derived addresses."), NULL }, 
   { LOPT_REPLY_DELAY, ARG_ONE, "<integer>", gettext_noop("Delay DHCP replies for at least number of seconds."), NULL },
   { LOPT_RAPID_COMMIT, OPT_RAPID_COMMIT, NULL, gettext_noop("Enables DHCPv4 Rapid Commit option."), NULL },
+  { LOPT_HOLD_OFFERED, OPT_HOLD_OFFERED, NULL, gettext_noop("Hold DHCPOFFER addresses until REQUEST."), NULL },
   { LOPT_DUMPFILE, ARG_ONE, "<path>", gettext_noop("Path to debug packet dump file."), NULL },
   { LOPT_DUMPMASK, ARG_ONE, "<hex>", gettext_noop("Mask which packets to dump."), NULL },
   { LOPT_SCRIPT_TIME, OPT_LEASE_RENEW, NULL, gettext_noop("Call dhcp-script when lease expiry changes."), NULL },
diff --git a/src/rfc2131.c b/src/rfc2131.c
index ec0194b..5d0094f 100644
--- a/src/rfc2131.c
+++ b/src/rfc2131.c
@@ -1376,7 +1376,14 @@ size_t dhcp_reply(struct dhcp_context *context, char *iface_name, int int_index,
 
       daemon->metrics[METRIC_DHCPOFFER]++;
       log_packet("DHCPOFFER" , &mess->yiaddr, emac, emac_len, iface_name, NULL, NULL, mess->xid);
-      
+      if (!lease && option_bool(OPT_HOLD_OFFERED))
+	{
+	  if (!(lease = lease4_allocate(mess->yiaddr)))
+	    return 0;
+	  lease_set_hwaddr(lease, mess->chaddr, clid, mess->hlen, mess->htype, clid_len, now, 0);
+	  lease_set_expires(lease, 30, now);
+	}
+
       time = calc_time(context, config, option_find(mess, sz, OPTION_LEASE_TIME, 4));
       clear_packet(mess, end);
       option_put(mess, end, OPTION_MESSAGE_TYPE, 1, DHCPOFFER);
-- 
2.52.0