From: Roy Marples <roy@marples.name>
Date: Thu, 19 Jun 2025 10:28:58 +0000 (+0100)
Subject: DHCP: In the IP header set TTL of 128 and TOS to low delay
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2c2058b02e94ef6d2306c4108c8c7d705747e5cc;p=thirdparty%2Fdhcpcd.git

DHCP: In the IP header set TTL of 128 and TOS to low delay

And set ID to zero because DHCP cannot be fragmented and the
ID is only for re-assembly of fragmented packets.

This matches the behaviour of other DHCP clients.

Fixes #511
---

diff --git a/src/dhcp.c b/src/dhcp.c
index 637f5a86..37ef48af 100644
--- a/src/dhcp.c
+++ b/src/dhcp.c
@@ -1786,10 +1786,16 @@ dhcp_makeudppacket(size_t *sz, const uint8_t *data, size_t length,
 	ip->ip_len = udp->uh_ulen;
 	udp->uh_sum = in_cksum(udpp, sizeof(*ip) + sizeof(*udp) + length, NULL);
 
+	/* RFC 6864 4.1
+	 * The IPv4 ID field MUST NOT be used for purposes other than
+	 * fragmentation and reassembly.
+	 *
+	 * DHCP cannot be fragmented.
+	 */
 	ip->ip_v = IPVERSION;
 	ip->ip_hl = sizeof(*ip) >> 2;
-	ip->ip_id = (uint16_t)arc4random_uniform(UINT16_MAX);
-	ip->ip_ttl = IPDEFTTL;
+	ip->ip_tos = DHCP_TOS;
+	ip->ip_ttl = DHCP_TTL;
 	ip->ip_len = htons((uint16_t)(sizeof(*ip) + sizeof(*udp) + length));
 	ip->ip_sum = in_cksum(ip, sizeof(*ip), NULL);
 	if (ip->ip_sum == 0)
diff --git a/src/dhcp.h b/src/dhcp.h
index e8c2a1e3..1f755220 100644
--- a/src/dhcp.h
+++ b/src/dhcp.h
@@ -45,6 +45,10 @@
 #include "auth.h"
 #include "dhcp-common.h"
 
+/* IP header values */
+#define DHCP_TTL		128	/* IPDEFTTL of 64 maybe to low */
+#define DHCP_TOS		IPTOS_LOWDELAY /* try to get DHCP fast */
+
 /* UDP port numbers for BOOTP */
 #define BOOTPS			67
 #define BOOTPC			68