dhcp: add configurable backoff parameters for DHCPv4

src/dhcp.c

@@ -1173,7 +1173,7 @@ make_message(struct bootp **bootpm, const struct interface *ifp, uint8_t type)
 				*lp = (uint8_t)(*lp + vivco->len + 1);
 			}
 		}
-		
+
 		if (ifo->vsio_len &&
 		    !has_option_mask(ifo->nomask, DHO_VIVSO))
 		{
@@ -1864,15 +1864,21 @@ send_message(struct interface *ifp, uint8_t type,
 		    state->xid);
 		RT = 0; /* bogus gcc warning */
 	} else {
+		unsigned int jitter = ifo->backoff_jitter;
+
 		if (state->interval == 0)
-			state->interval = 4;
+			state->interval = ifo->initial_interval;
 		else {
+			unsigned int cutoff = ifo->backoff_cutoff;
+
 			state->interval *= 2;
-			if (state->interval > 64)
-				state->interval = 64;
+			if (state->interval > cutoff)
+				state->interval = cutoff;
 		}
+
 		RT = (state->interval * MSEC_PER_SEC) +
-		    (arc4random_uniform(MSEC_PER_SEC * 2) - MSEC_PER_SEC);
+		    (arc4random_uniform(jitter * 2) - jitter);
+
 		/* No carrier? Don't bother sending the packet.
 		 * However, we do need to advance the timeout. */
 		if (!if_is_link_up(ifp))

src/dhcpcd.conf.5.in

@@ -304,6 +304,42 @@ You can use this option to stop this from happening.
 .It Ic fallback Ar profile
 Fall back to using this profile if DHCP fails.
 This allows you to configure a static profile instead of using ZeroConf.
+.It Ic initial_interval Ar seconds
+Set the initial DHCPv4 retransmission interval to
+.Ar seconds .
+The minimum value is 1.
+The default is 4 seconds as per RFC 2131.
+This option only affects DHCPv4;
+DHCPv6 retransmission is governed by RFC 8415.
+See also
+.Ic backoff_cutoff
+and
+.Ic backoff_jitter .
+.It Ic backoff_cutoff Ar seconds
+Cap the DHCPv4 exponential backoff interval at
+.Ar seconds .
+The minimum value is 1.
+The default is 64 seconds as per RFC 2131.
+Setting this to 1 effectively disables exponential growth, so
+retransmissions use only the initial interval plus jitter.
+This option only affects DHCPv4;
+DHCPv6 retransmission is governed by RFC 8415.
+See also
+.Ic initial_interval
+and
+.Ic backoff_jitter .
+.It Ic backoff_jitter Ar milliseconds
+Set the random jitter applied to each DHCPv4 retransmission interval.
+The jitter is applied as \(+-
+.Ar milliseconds .
+The default is 1000 (\(+-1 second) as per RFC 2131.
+A value of 0 disables jitter, producing deterministic retransmission timing.
+This option only affects DHCPv4;
+DHCPv6 retransmission is governed by RFC 8415.
+See also
+.Ic backoff_cutoff
+and
+.Ic initial_interval .
 .It Ic fallback_time Ar seconds
 Start fallback after
 .Ar seconds .

src/if-options.c

@@ -177,6 +177,9 @@ const struct option cf_options[] = {
 	{"fallback_time",   required_argument, NULL, O_FALLBACK_TIME},
 	{"ipv4ll_time",     required_argument, NULL, O_IPV4LL_TIME},
 	{"nosyslog",        no_argument,       NULL, O_NOSYSLOG},
+	{"initial_interval",	required_argument, NULL, O_INITIAL_INTERVAL},
+	{"backoff_cutoff",  required_argument, NULL, O_BACKOFF_CUTOFF},
+	{"backoff_jitter",  required_argument, NULL, O_BACKOFF_JITTER},
 	{NULL,              0,                 NULL, '\0'}
 };
 
@@ -2580,6 +2583,33 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 			logsetopts(logopts);
 		}
 		break;
+	case O_INITIAL_INTERVAL:
+		ARG_REQUIRED;
+		ifo->initial_interval =
+		    (uint32_t)strtou(arg, NULL, 0, 1, UINT32_MAX, &e);
+		if (e) {
+			logerrx("invalid initial interval: %s", arg);
+			return -1;
+		}
+		break;
+	case O_BACKOFF_CUTOFF:
+		ARG_REQUIRED;
+		ifo->backoff_cutoff =
+		    (uint32_t)strtou(arg, NULL, 0, 1, UINT32_MAX, &e);
+		if (e) {
+			logerrx("invalid backoff cutoff: %s", arg);
+			return -1;
+		}
+		break;
+	case O_BACKOFF_JITTER:
+		ARG_REQUIRED;
+		ifo->backoff_jitter =
+		    (uint32_t)strtou(arg, NULL, 0, 0, UINT32_MAX, &e);
+		if (e) {
+			logerrx("invalid backoff jitter: %s", arg);
+			return -1;
+		}
+		break;
 	default:
 		return 0;
 	}
@@ -2667,6 +2697,9 @@ default_config(struct dhcpcd_ctx *ctx)
 #ifdef INET
 	ifo->fallback_time = DEFAULT_FALLBACK;
 	ifo->ipv4ll_time = DEFAULT_IPV4LL;
+	ifo->initial_interval = DEFAULT_INITIAL_INTERVAL;
+	ifo->backoff_cutoff = DEFAULT_BACKOFF_CUTOFF;
+	ifo->backoff_jitter = DEFAULT_BACKOFF_JITTER;
 #endif
 	ifo->metric = -1;
 	ifo->auth.options |= DHCPCD_AUTH_REQUIRE;

src/if-options.h

@@ -52,6 +52,9 @@
 #define DEFAULT_REQUEST		180	/* secs to request, mirror DHCP6 */
 #define DEFAULT_FALLBACK	5	/* secs until fallback */
 #define DEFAULT_IPV4LL		5	/* secs until ipv4ll */
+#define DEFAULT_INITIAL_INTERVAL	4	/* DHCP_BASE per RFC 2131 */
+#define DEFAULT_BACKOFF_CUTOFF	64	/* DHCP_MAX per RFC 2131 */
+#define DEFAULT_BACKOFF_JITTER	1000	/* +/- milliseconds */
 
 #ifndef HOSTNAME_MAX_LEN
 #define HOSTNAME_MAX_LEN	250	/* 255 - 3 (FQDN) - 2 (DNS enc) */
@@ -190,6 +193,9 @@
 #define O_VSIO			O_BASE + 57
 #define O_VSIO6			O_BASE + 58
 #define O_NOSYSLOG		O_BASE + 59
+#define O_INITIAL_INTERVAL	O_BASE + 60
+#define O_BACKOFF_CUTOFF	O_BASE + 61
+#define O_BACKOFF_JITTER	O_BASE + 62
 
 extern const struct option cf_options[];
 
@@ -257,6 +263,9 @@ struct if_options {
 	uint32_t request_time;
 	uint32_t fallback_time;
 	uint32_t ipv4ll_time;
+	uint32_t initial_interval;
+	uint32_t backoff_cutoff;
+	uint32_t backoff_jitter;
 	unsigned long long options;
 	bool randomise_hwaddr;
 

tests/Makefile

@@ -1,6 +1,6 @@
-SUBDIRS=	crypt eloop-bench
+SUBDIRS=	crypt eloop-bench backoff
 
-all: 
+all:
 	for x in ${SUBDIRS}; do cd $$x; ${MAKE} $@ || exit $$?; cd ..; done
 
 install:

tests/backoff/Makefile

@@ -0,0 +1,18 @@
+PROG=		test_backoff
+SRCS=		test_backoff.c
+
+CFLAGS+=	-Wall -Wextra -std=c99 -D_DEFAULT_SOURCE
+LDFLAGS+=	-lm
+
+all: ${PROG}
+
+${PROG}: ${SRCS}
+	${CC} ${CFLAGS} -o ${PROG} ${SRCS} ${LDFLAGS}
+
+test: ${PROG}
+	@echo "Run as root: sudo ./${PROG} [-b path-to-dhcpcd]"
+
+clean:
+	rm -f ${PROG}
+
+distclean: clean