dhcp: add configurable backoff parameters for DHCPv4

In cloud and virtual environments the DHCP service is typically
ready within hundreds of milliseconds of the interface coming up,
and once ready, responds within single-digit milliseconds.  The
RFC 2131 defaults (4s initial interval, 64s backoff cap) are
designed for congested broadcast networks and are unnecessarily
conservative in this context.

Add three new configuration options to tune DHCPv4 retransmission:

  initial_interval  - initial retransmission interval (default 4s)
  backoff_cutoff    - exponential backoff cap (default 64s)
  backoff_jitter    - random jitter per retry (default ±1000ms)

Defaults match RFC 2131 so existing behaviour is unchanged.
Option naming aligns with dhclient (initial-interval, backoff-cutoff).
Minimum of 1 is enforced at parse time for interval and cutoff;
invalid values are logged and the default is used.

These options are DHCPv4-only; DHCPv6 retransmission follows
RFC 8415 constants and is not user-configurable.

A test harness (tests/backoff/test_backoff.c) was used to validate
timing assumptions and correctness across multiple scenarios
(defaults, min-latency, cloud-recommended) and to confirm rejection
of invalid values.  It requires root and network namespaces so may
not be suitable for all CI environments.  It can be removed if it
doesn't have long-term value.

Example test output:

```
$ sudo ./tests/backoff/test_backoff -b ./src/dhcpcd -n 128
dhcpcd DHCPDISCOVER backoff integration test
=============================================
Binary: /home/cpatterson/git/dhcpcd/src/dhcpcd
Runs:   128 per test

--- defaults (N=128) ---
  dhcpcd.conf: timeout=75
  PASS: 128/128 runs produced data

  Retry     Expected    Min      Avg      Max      N     Result
  ------------------------------------------------------
  Init      1±1.0s      0.0s     1.0s     2.0s     128   PASS
  1         4±1.0s      3.0s     4.0s     5.0s     128   PASS
  2         8±1.0s      7.0s     8.0s     9.0s     128   PASS
  3         16±1.0s     15.0s    16.0s    17.0s    128   PASS
  4         32±1.0s     31.1s    32.1s    33.0s    128   PASS
  5         64±1.0s     63.0s    64.0s    65.0s    128   PASS
  Elapsed: 75.9s

--- min-latency (N=128) ---
  dhcpcd.conf: timeout=12, nodelay, initial_interval=1, backoff_cutoff=1, backoff_jitter=0
  PASS: 128/128 runs produced data

  Retry     Expected    Min      Avg      Max      N     Result
  ------------------------------------------------------
  Init      0           -        -        -        128   PASS
  1         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  2         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  3         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  4         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  5         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  6         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  7         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  8         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  9         1±0.0s      1.0s     1.0s     1.0s     128   PASS
  10        1±0.0s      1.0s     1.0s     1.0s     128   PASS
  Elapsed: 12.9s

--- cloud (N=128) ---
  dhcpcd.conf: timeout=12, nodelay, initial_interval=1, backoff_cutoff=1, backoff_jitter=100
  PASS: 128/128 runs produced data

  Retry     Expected    Min      Avg      Max      N     Result
  ------------------------------------------------------
  Init      0           -        -        -        128   PASS
  1         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  2         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  3         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  4         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  5         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  6         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  7         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  8         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  9         1±0.1s      0.9s     1.0s     1.1s     128   PASS
  10        1±0.1s      0.9s     1.0s     1.1s     128   PASS
  Elapsed: 13.0s

--- reject initial_interval=0 ---
  dhcpcd.conf: initial_interval 0 (expect rejection)
  PASS: dhcpcd rejected initial_interval 0 (exit=0)
    output: invalid initial interval: 0

--- reject backoff_cutoff=0 ---
  dhcpcd.conf: backoff_cutoff 0 (expect rejection)
  PASS: dhcpcd rejected backoff_cutoff 0 (exit=0)
    output: invalid backoff cutoff: 0

=============================================
All tests passed.
```

Signed-off-by: Chris Patterson <cpatterson@microsoft.com>

Author: cjp256

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