New option --traceroute to send a traceroute with fping #451

gsnw-sebast · 2026-01-03T15:21:17Z

The traceroute function is now complete. There is no JSON output in the current version, and testing is still pending, if it is even possible given the required root privileges.
IPv6 also works.

./fping --traceroute 8.8.8.8

Output

fping traceroute to 127.0.0.1 (127.0.0.1), 30 hops max, 84 byte packets
127.0.0.1: hop 1 reached DESTINATION 127.0.0.1 (0.055 ms)

Test Checklist

Option	Test	Supported
--ipv4 (-4)	Yes	Yes
--ipv6 (-6)	Yes	Yes
--alive (-a)	Yes	No (No effect)
--addr (-A)	Yes	Yes
--size (-b)	Yes	Yes (I couldn't really check the results and effects.)
--backoff (-B)	Yes	No (No effect)
--count (-c)	Yes	No
--vcount (-C)	Yes	No
--rdns (-d)	Yes	Yes
--timestamp (-D)	Yes	No
--elapsed (-e)	Yes	No (No effect)
--file (-f)
--generate (-g)
--ttl (-H)	Yes	Yes (1-30)
--interval (-i)	Yes	Yes
--iface (-I)	No	Yes (Untested)
--json (-j)	No	No (Next version)
--icmp-timestamp
--fwmark (-k)	No	No (Untested)
--loop (-l)	Yes	No
--all (-m)	Yes	No (No effect)
--dontfrag (-M)	Yes	Yes
--name (-n)	Yes	Yes
--netdata (-N)	Yes	No
--outage (-o)
--tos (-O)	Yes	Yes
--period (-p)	Yes	Yes (same effect as -i)
--quiet (-q)	Yes	No
--squiet (-Q)	Yes	No
--retry (-r)	Yes	No (No effect)
--random (-R)	Yes	Yes
--stats (-s)	Yes	No (Works, but the output is misleading)
--src (-S)	No	Yes (Untested)
--timeout (-t)	Yes	No (No effect)
(-T)
--unreach (-u)	Yes	No (No effect)
--retry (-r)	Yes	No (No effect)
--fast-reachable (-X)
--check-source	Yes	Yes
--print-tos	Yes	No
--print-ttl	Yes	No
--seqmap-timeout
(-z)

gemini-code-assist · 2026-01-03T15:21:34Z

Summary of Changes

Hello @gsnw-sebast, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates a new traceroute capability into the fping utility, accessible via the --traceroute option. The implementation supports both IPv4 and IPv6, providing hop-by-hop path discovery to a target host. It includes necessary checks for root privileges and manages Time-To-Live (TTL) values to control hop limits. While the core functionality is complete, the current version focuses on standard output, with JSON output slated for a later release.

Highlights

New Traceroute Option: Introduces a new --traceroute command-line option to fping, allowing users to perform traceroute functionality.
IPv4 and IPv6 Support: The traceroute feature is implemented to work with both IPv4 and IPv6 addresses.
Root Privileges Required: The --traceroute option requires root privileges to operate, particularly for raw socket access on Linux.
Option Compatibility: The --traceroute option cannot be combined with several existing fping options, such as -c, -C, -l, -N, -q, -Q, -s, or -J.
TTL Management: A default TTL of 30 hops is set for traceroute, and any user-specified TTL exceeding 30 will be clamped to 30.
No JSON Output (Current Version): The initial implementation of the traceroute feature does not include JSON output, which is planned for a future version.

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gemini-code-assist

Code Review

This pull request introduces a --traceroute option to fping, which is a significant and useful feature. The implementation correctly handles both IPv4 and IPv6, including the necessary ICMP message processing and TTL manipulation. The code is generally well-structured and follows the existing patterns in the codebase. I've provided a few suggestions to improve maintainability by reducing code duplication, enhance efficiency by optimizing memory allocation, and increase correctness in option parsing. Overall, this is a solid contribution.

src/fping.c

coveralls · 2026-01-03T15:24:22Z

coverage: 84.76% (-3.3%) from 88.037%
when pulling 6f7cc42 on gsnw:issue/62
into 7dcd667 on schweikert:develop.

gsnw-sebast · 2026-01-04T05:55:41Z

The code block for traceroute in the function int wait_for_reply(int64_t wait_time) can still be optimized and is currently structured purely functionally.

Update: Optimization completed

gsnw-sebast · 2026-01-04T07:15:27Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces a new --traceroute feature to fping. The implementation is comprehensive, covering both IPv4 and IPv6, and handling various fping options. However, I've found several issues that need to be addressed. There is a critical race condition when using traceroute with multiple hosts due to improper handling of socket options. I've also identified a high-severity buffer over-read vulnerability and a bug that affects IPv6 traceroute output. Additionally, there are several opportunities to improve code quality by removing hardcoded values and refactoring duplicated code.

src/fping.c

gsnw-sebast · 2026-01-04T11:59:43Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces a new --traceroute feature, which is a great addition to fping's capabilities. The implementation is mostly complete and handles both IPv4 and IPv6. I've identified a few issues related to option parsing, the logic for the traceroute loop, and error handling that should be addressed to ensure correctness and robustness. Specifically, the check for the number of hosts in traceroute mode is currently ineffective, there's an off-by-one error in the traceroute loop, a potential infinite loop on the last hop, and the error handling for setsockopt could be strengthened. The documentation and general structure of the changes are well done.

gemini-code-assist · 2026-01-04T12:01:17Z

src/fping.c

+            if (traceroute_flag) {
+                printf("%s: hop %d no reply\n", h->host, h->trace_ttl);
+                h->trace_ttl++;
+                if (h->trace_ttl > (int)ttl) h->trace_ttl = (int)ttl;


This line, which clamps h->trace_ttl to ttl, can cause an infinite loop. When the main loop condition is corrected to h->trace_ttl <= (int)ttl, if the last hop (ttl) times out, h->trace_ttl is incremented to ttl + 1 on the previous line, then clamped back to ttl here. The next ping event will then re-send a ping for the same hop ttl, leading to an infinite loop. This clamp should be removed, as the loop termination is correctly handled by the scheduler condition.

gemini-code-assist · 2026-01-04T12:01:17Z

src/fping.c

+        if (h->trace_ttl > (int)ttl) {
+            h->trace_ttl = (int)ttl;
+        }


This block, which clamps h->trace_ttl to ttl, can cause an infinite loop. When the main loop condition is corrected to h->trace_ttl <= (int)ttl, if the last hop (ttl) does not reach the destination, h->trace_ttl is incremented to ttl + 1, then clamped back to ttl here. The next ping event will then re-send a ping for the same hop ttl, leading to an infinite loop if that hop also doesn't reach the destination. This clamp should be removed.

src/fping.c

gemini-code-assist · 2026-01-04T12:01:17Z

src/fping.c

+                    if (setsockopt(socket4, IPPROTO_IP, IP_TTL, &ttl_set, sizeof(ttl_set)))
+                        perror("setsockopt IP_TTL");


Failing to set the IP_TTL socket option is a fatal error for traceroute mode, as it won't be able to control the TTL of outgoing packets. The program should exit with an error instead of just printing to perror and continuing, which would lead to incorrect traceroute behavior.

if (setsockopt(socket4, IPPROTO_IP, IP_TTL, &ttl_set, sizeof(ttl_set))) errno_crash_and_burn("setsockopt IP_TTL");

gemini-code-assist · 2026-01-04T12:01:17Z

src/fping.c

+                    if (setsockopt(socket6, IPPROTO_IPV6, IPV6_UNICAST_HOPS, &ttl_set, sizeof(ttl_set))) {
+                        perror("setsockopt IPV6_UNICAST_HOPS");
+                    }


Failing to set the IPV6_UNICAST_HOPS socket option is a fatal error for traceroute mode, as it won't be able to control the hop limit of outgoing packets. The program should exit with an error instead of just printing to perror and continuing, which would lead to incorrect traceroute behavior.

if (setsockopt(socket6, IPPROTO_IPV6, IPV6_UNICAST_HOPS, &ttl_set, sizeof(ttl_set))) { errno_crash_and_burn("setsockopt IPV6_UNICAST_HOPS"); }

gemini-code-assist · 2026-01-04T12:01:17Z

src/fping.c

-            /* Loop and count mode: schedule next ping */
-            if (loop_flag || (count_flag && event->ping_index + 1 < count)) {
+            /* Loop, count and traceroute mode: schedule next ping */
+            if (loop_flag || (count_flag && event->ping_index + 1 < count) || (traceroute_flag && h->trace_ttl < (int)ttl)) {


The loop condition for traceroute, h->trace_ttl < (int)ttl, is an off-by-one error. If a user specifies --ttl 30, they expect fping to try all hops from 1 to 30. With the current condition, it will only try up to TTL 29. The condition should be h->trace_ttl <= (int)ttl to include the final hop.

if (loop_flag || (count_flag && event->ping_index + 1 < count) || (traceroute_flag && h->trace_ttl <= (int)ttl)) {

src/fping.c

schweikert · 2026-01-04T13:35:34Z

I appreciate the effort, but I am a bit sceptical about this change. It adds quite a bit of complexity, and I don't yet understand why it would be a useful addition to fping. In particular, the limitation of only allowing one host seems to defeat the purpose of adding this to fping, which is focused on doing parallel requests to multiple hosts.

gsnw-sebast · 2026-01-04T15:08:31Z

I wouldn't have expected such complexity, and querying different hosts doesn't make it any easier.

I would have no problem closing the function change or, if necessary, creating something like ftraceroute, i.e., an extra program based on fping outside of fping itself.

Especially since the tests are currently very complex

auerswal · 2026-01-06T23:34:56Z

I also appreciate the effort, but it seems to me as if this functionality does not really fit into the existing fping code. Please note that issue #71 explicitly asks for parallel traceroutes to multiple hosts.

gsnw-sebast · 2026-01-08T09:23:20Z

I took another look at the whole thing, and if sendmsg() is used instead of sendto(), a parallel query should be possible.
From my side, I couldn't find any reason why sendto() must be used.

I've now made the change. The output is still a bit ugly, but it works.

gsnw-sebast linked an issue Jan 3, 2026 that may be closed by this pull request

RFE: option for "ftraceroute" functionality #71

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New option --traceroute to send a traceroute with fping

6f7cc42

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		if (setsockopt(socket4, IPPROTO_IP, IP_TTL, &ttl_set, sizeof(ttl_set)))
		perror("setsockopt IP_TTL");

New option --traceroute to send a traceroute with fping #451

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