dhcp: add configurable backoff parameters for DHCPv4

src/dhcp.c

@@ -1823,15 +1823,23 @@ send_message(struct interface *ifp, uint8_t type, void (*callback)(void *))
 		    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);
+
+		/* Jitter is bounded at config time to the smallest possible
+		 * interval, so the result can never underflow. */
+		RT = state->interval * 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

@@ -303,6 +303,52 @@ 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 and the maximum is 4.
+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 and the maximum is 64.
+The default is 64 seconds as per RFC 2131.
+If
+.Ar seconds
+is less than
+.Ic initial_interval
+it will be raised to match it, since the cutoff is a cap on the
+exponential growth and must not shrink the initial interval.
+Setting both
+.Ic initial_interval
+and
+.Ic backoff_cutoff
+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, which is also the maximum.
+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

@@ -175,6 +175,9 @@ const struct option cf_options[] = { { "background", no_argument, NULL, 'b' },
 	{ "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' } };
 
 static char *
@@ -2552,6 +2555,33 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 		logopts &= ~LOGERR_LOG;
 		logsetopts(logopts);
 	} break;
+	case O_INITIAL_INTERVAL:
+		ARG_REQUIRED;
+		ifo->initial_interval = (uint32_t)strtou(arg, NULL, 0, 1,
+		    MAX_INITIAL_INTERVAL, &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,
+		    MAX_BACKOFF_CUTOFF, &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,
+		    MAX_BACKOFF_JITTER, &e);
+		if (e) {
+			logerrx("invalid backoff jitter: %s", arg);
+			return -1;
+		}
+		break;
 	default:
 		return 0;
 	}
@@ -2618,6 +2648,19 @@ finish_config(struct if_options *ifo)
 	if (!(ifo->options & DHCPCD_IPV6RS))
 		ifo->options &= ~(
 		    DHCPCD_IPV6RA_AUTOCONF | DHCPCD_IPV6RA_REQRDNSS);
+
+#ifdef INET
+	/* The exponential backoff cutoff must not be lower than the initial
+	 * interval, otherwise the retransmission sequence would shrink rather
+	 * than grow up to the cap. Clamp the cutoff up to the initial
+	 * interval to preserve the documented "cap" semantics. */
+	if (ifo->backoff_cutoff < ifo->initial_interval) {
+		logwarnx("backoff_cutoff (%u) is less than initial_interval "
+			 "(%u); raising backoff_cutoff to match",
+		    ifo->backoff_cutoff, ifo->initial_interval);
+		ifo->backoff_cutoff = ifo->initial_interval;
+	}
+#endif
 }
 
 static struct if_options *
@@ -2637,6 +2680,9 @@ 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

@@ -54,6 +54,15 @@
 #define DEFAULT_REQUEST	 180 /* secs to request, mirror DHCP6 */
 #define DEFAULT_FALLBACK 5   /* secs until fallback */
 #define DEFAULT_IPV4LL	 5   /* secs until ipv4ll */
+/* DHCPv4 retransmission backoff defaults (RFC 2131); DHCPv4-only. */
+#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 */
+
+/* Upper bounds to keep the retransmit timeout arithmetic well within range. */
+#define MAX_INITIAL_INTERVAL 4	  /* DHCP_BASE per RFC 2131 */
+#define MAX_BACKOFF_CUTOFF   64	  /* DHCP_MAX per RFC 2131 */
+#define MAX_BACKOFF_JITTER   1000 /* +/- milliseconds, per RFC 2131 */
 
 #ifndef HOSTNAME_MAX_LEN
 #define HOSTNAME_MAX_LEN 250 /* 255 - 3 (FQDN) - 2 (DNS enc) */
@@ -191,6 +200,9 @@
 #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[];
 
@@ -258,6 +270,9 @@ 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;