dhcp: add configurable backoff parameters for DHCPv4#593
dhcp: add configurable backoff parameters for DHCPv4#593cjp256 wants to merge 1 commit intoNetworkConfiguration:masterfrom
Conversation
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.
```
Closes NetworkConfiguration#406
Signed-off-by: Chris Patterson <cpatterson@microsoft.com>
WalkthroughIntroduces three new DHCPv4-only configuration directives ( Changes
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Actionable comments posted: 4
🤖 Prompt for all review comments with AI agents
Verify each finding against the current code and only fix it if needed.
Inline comments:
In `@src/dhcp.c`:
- Around line 1867-1880: The jitter subtraction currently happens in unsigned
space causing wrap; change the jitter computation to use a signed/wider integer:
compute a signed jitter_offset =
(int64_t)arc4random_uniform((uint32_t)ifo->backoff_jitter * 2) -
(int64_t)ifo->backoff_jitter, clamp jitter_offset to a reasonable range if
needed, then compute RT using signed/wider arithmetic as RT =
(int64_t)state->interval * MSEC_PER_SEC + jitter_offset and assign back ensuring
types match (use explicit casts) to avoid unsigned wrap; update the code around
variables jitter, state->interval, RT and the arc4random_uniform call.
In `@src/if-options.c`:
- Around line 2586-2603: The new options backoff_cutoff and initial_interval
must be validated together in finish_config(): after parsing, check if
ifo->backoff_cutoff is less than ifo->initial_interval and either clamp
backoff_cutoff to initial_interval or return an error; update finish_config() to
perform this comparison and adjust or reject the configuration (modify the
function that finalizes options, e.g., finish_config()) and provide a clear
logerrx or warning message indicating the clamp or failure including both
values.
In `@tests/backoff/test_backoff.c`:
- Around line 847-859: The test currently tears down host dhcpcd state by
calling run_cmd("pkill -9 dhcpcd ...") and manipulating /var/run/dhcpcd and
/var/db/dhcpcd; change this to operate only inside the per-test temporary
directory created via tmpdir/mkdtemp: remove the pkill and any run_cmd
invocations that create/delete /var/run/dhcpcd or /var/db/dhcpcd, and instead
create and use equivalent per-test directories under tmpdir (e.g., snprintf
paths under tmpdir and call run_cmd or mkdir -p on those) so cleanup is limited
to tmpdir and the host daemon is not affected; update any code that expects
global paths to point to the tmpdir equivalents so the timing/reject case stays
isolated.
- Around line 730-742: Help text for the -n option is incorrect: update the
fprintf format string in tests/backoff/test_backoff.c (the multi-line usage
string passed to fprintf) so the "-n N" line shows the actual default of
num_runs (128) instead of 1 (e.g., change "(default: 1, max: %d)" to "(default:
128, max: %d)"). Ensure the format still uses MAX_RUNS for the max placeholder
and that the rest of the usage string remains unchanged.
🪄 Autofix (Beta)
Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
ℹ️ Review info
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📒 Files selected for processing (7)
src/dhcp.csrc/dhcpcd.conf.5.insrc/if-options.csrc/if-options.htests/Makefiletests/backoff/Makefiletests/backoff/test_backoff.c
| 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); |
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Fix unsigned integer arithmetic wrapping in the DHCP retransmission timeout calculation.
The expression arc4random_uniform(jitter * 2) - jitter wraps when the random value is less than jitter, since both operands are unsigned int. With default config (initial_interval 4 and backoff_jitter 1000), a random value under 500 causes RT to wrap to near UINT_MAX, scheduling an incorrect timeout. The scenario with initial_interval 1 and backoff_jitter 2000 reliably triggers wrap on any random value below 2000.
Perform the jitter calculation in signed or wider integer space and clamp the result before assigning to RT.
🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed.
In `@src/dhcp.c` around lines 1867 - 1880, The jitter subtraction currently
happens in unsigned space causing wrap; change the jitter computation to use a
signed/wider integer: compute a signed jitter_offset =
(int64_t)arc4random_uniform((uint32_t)ifo->backoff_jitter * 2) -
(int64_t)ifo->backoff_jitter, clamp jitter_offset to a reasonable range if
needed, then compute RT using signed/wider arithmetic as RT =
(int64_t)state->interval * MSEC_PER_SEC + jitter_offset and assign back ensuring
types match (use explicit casts) to avoid unsigned wrap; update the code around
variables jitter, state->interval, RT and the arc4random_uniform call.
| 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; |
There was a problem hiding this comment.
Validate the new knobs together, not just individually.
Right now backoff_cutoff can be set below initial_interval. That makes the sequence shrink (4s -> 1s -> 1s) instead of cap, which contradicts the new manpage text and the expected meaning of a cutoff. Please clamp or reject that combination in finish_config() once both values are known.
🛠️ Possible clamp in finish_config()
static void
finish_config(struct if_options *ifo)
{
+ if (ifo->backoff_cutoff < ifo->initial_interval) {
+ logerrx("backoff_cutoff must be >= initial_interval");
+ ifo->backoff_cutoff = ifo->initial_interval;
+ }
+
/* Terminate the encapsulated options */
if (ifo->vendor[0] && !(ifo->options & DHCPCD_VENDORRAW)) {🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed.
In `@src/if-options.c` around lines 2586 - 2603, The new options backoff_cutoff
and initial_interval must be validated together in finish_config(): after
parsing, check if ifo->backoff_cutoff is less than ifo->initial_interval and
either clamp backoff_cutoff to initial_interval or return an error; update
finish_config() to perform this comparison and adjust or reject the
configuration (modify the function that finalizes options, e.g.,
finish_config()) and provide a clear logerrx or warning message indicating the
clamp or failure including both values.
| fprintf(stderr, | ||
| "Usage: %s [-b path-to-dhcpcd] [-n num-runs] [-s name] [-v]\n" | ||
| "\n" | ||
| "Options:\n" | ||
| " -b path Path to dhcpcd binary " | ||
| "(default: ../../src/dhcpcd)\n" | ||
| " -n N Number of parallel runs per test " | ||
| "(default: 1, max: %d)\n" | ||
| " -s name Run only scenario name " | ||
| "(defaults, min-latency, cloud, reject)\n" | ||
| " -v Print all observed values per retry\n" | ||
| " -h Show this help\n", | ||
| prog, MAX_RUNS); |
There was a problem hiding this comment.
Fix the documented default for -n.
The help text says -n defaults to 1, but num_runs is initialized to 128. That makes the harness much heavier than the CLI advertises.
📝 Possible fix
- " -n N Number of parallel runs per test "
- "(default: 1, max: %d)\n"
+ " -n N Number of parallel runs per test "
+ "(default: 128, max: %d)\n"📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| fprintf(stderr, | |
| "Usage: %s [-b path-to-dhcpcd] [-n num-runs] [-s name] [-v]\n" | |
| "\n" | |
| "Options:\n" | |
| " -b path Path to dhcpcd binary " | |
| "(default: ../../src/dhcpcd)\n" | |
| " -n N Number of parallel runs per test " | |
| "(default: 1, max: %d)\n" | |
| " -s name Run only scenario name " | |
| "(defaults, min-latency, cloud, reject)\n" | |
| " -v Print all observed values per retry\n" | |
| " -h Show this help\n", | |
| prog, MAX_RUNS); | |
| fprintf(stderr, | |
| "Usage: %s [-b path-to-dhcpcd] [-n num-runs] [-s name] [-v]\n" | |
| "\n" | |
| "Options:\n" | |
| " -b path Path to dhcpcd binary " | |
| "(default: ../../src/dhcpcd)\n" | |
| " -n N Number of parallel runs per test " | |
| "(default: 128, max: %d)\n" | |
| " -s name Run only scenario name " | |
| "(defaults, min-latency, cloud, reject)\n" | |
| " -v Print all observed values per retry\n" | |
| " -h Show this help\n", | |
| prog, MAX_RUNS); |
🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed.
In `@tests/backoff/test_backoff.c` around lines 730 - 742, Help text for the -n
option is incorrect: update the fprintf format string in
tests/backoff/test_backoff.c (the multi-line usage string passed to fprintf) so
the "-n N" line shows the actual default of num_runs (128) instead of 1 (e.g.,
change "(default: 1, max: %d)" to "(default: 128, max: %d)"). Ensure the format
still uses MAX_RUNS for the max placeholder and that the rest of the usage
string remains unchanged.
| /* Create temp directory for all test artifacts */ | ||
| snprintf(tmpdir, sizeof(tmpdir), "/tmp/testbackoff.XXXXXX"); | ||
| if (mkdtemp(tmpdir) == NULL) { | ||
| fprintf(stderr, "mkdtemp: %s\n", strerror(errno)); | ||
| return 1; | ||
| } | ||
|
|
||
| /* Clean stale state */ | ||
| run_cmd("pkill -9 dhcpcd 2>/dev/null; true"); | ||
| run_cmd("mkdir -p /var/db/dhcpcd 2>/dev/null"); | ||
| run_cmd("mkdir -p /var/run/dhcpcd 2>/dev/null"); | ||
| run_cmd("find /var/run/dhcpcd/ -type f -delete 2>/dev/null; true"); | ||
| run_cmd("find /var/run/dhcpcd/ -type s -delete 2>/dev/null; true"); |
There was a problem hiding this comment.
Don't tear down the host's real dhcpcd state from this harness.
pkill -9 dhcpcd plus deleting /var/run/dhcpcd and /var/db/dhcpcd can kill the system daemon and disrupt networking on the machine running the test. The timing runs already use private namespaces and state; the reject case should too, so cleanup can stay under tmpdir.
🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed.
In `@tests/backoff/test_backoff.c` around lines 847 - 859, The test currently
tears down host dhcpcd state by calling run_cmd("pkill -9 dhcpcd ...") and
manipulating /var/run/dhcpcd and /var/db/dhcpcd; change this to operate only
inside the per-test temporary directory created via tmpdir/mkdtemp: remove the
pkill and any run_cmd invocations that create/delete /var/run/dhcpcd or
/var/db/dhcpcd, and instead create and use equivalent per-test directories under
tmpdir (e.g., snprintf paths under tmpdir and call run_cmd or mkdir -p on those)
so cleanup is limited to tmpdir and the host daemon is not affected; update any
code that expects global paths to point to the tmpdir equivalents so the
timing/reject case stays isolated.
1 participant
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:
Closes #406