New option --traceroute to send a traceroute with fping

[gsnw-sebast]

This is my first working version for sending a traceroute with fping.

It can be tested with the following command. I have omitted IPv6 for now, and some of the text still needs to be revised.
The primary focus was on the function itself.

./fping --traceroute 8.8.8.8

[gemini-code-assist]

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 enhances the fping utility by integrating a new --traceroute option. This feature allows users to trace the network path to a destination by sending ICMP echo requests with incrementally increasing Time-To-Live values. The changes involve modifications to host data structures, command-line argument parsing, and the core ping and reply handling logic to support the progressive discovery and reporting of network hops, providing a basic but functional traceroute capability.

Highlights

• New Traceroute Option: Introduced a new command-line option, --traceroute, allowing fping to perform traceroute-like functionality by progressively increasing the Time-To-Live (TTL) for outgoing ICMP echo requests.
• TTL Management: Implemented per-host trace_ttl tracking, dynamically setting the IP_TTL socket option for outgoing packets, and scheduling subsequent pings with incrementing TTLs to discover network hops.
• ICMP Time Exceeded Handling: Added specific logic to decode_icmp_ipv4 to correctly parse and extract information from ICMP_TIMXCEED messages, which are crucial for identifying intermediate routers in a traceroute.
• Traceroute Output and State: Modified wait_for_reply to print detailed output for both intermediate hops (identified by ICMP_TIMXCEED) and the final destination (identified by ICMP_ECHOREPLY), and to manage the traceroute state for each host.

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[gemini-code-assist]

Code Review

This pull request introduces a new --traceroute option to fping, adding a trace_ttl field to the host_entry structure and a global traceroute_flag. The changes involve parsing the new option, setting the number of trials to 255 for traceroute, dynamically setting the IP TTL for outgoing IPv4 packets using setsockopt in the main loop, and handling ICMP_TIMXCEED and ICMP_ECHOREPLY messages to report hop information and manage the traceroute process. Review comments suggest improving the option parsing by using strcmp instead of strstr for exact matching, optimizing TTL setting by using sendmsg with ancillary data instead of setsockopt per packet (and noting the current lack of IPv6 support), standardizing time formatting with sprint_tm(), and replacing the magic number 100 with a named constant for better code clarity.

[gemini-code-assist]

Using strcmp is safer here than strstr for checking the long option name. strstr could lead to incorrect behavior if another option name contains "traceroute" as a substring (e.g., --traceroute-foo). Since --traceroute doesn't take an argument, optparse_state.optlongname will be an exact match.

            } else if (strcmp(optparse_state.optlongname, "traceroute") == 0) {

[gemini-code-assist]

A couple of points on this section:

• Efficiency: Calling setsockopt() for every packet sent is a system call and can be inefficient. A more performant approach is to use sendmsg() with ancillary data (cmsg) to set the TTL on a per-packet basis. This would involve modifying send_ping() and socket_sendto_ping_ipv4().
• IPv6 Support: As noted in the PR description, this is IPv4-only. For completeness, IPv6 support would require adding similar logic for socket6 using setsockopt with IPPROTO_IPV6 and IPV6_UNICAST_HOPS.

[coveralls]

coverage: 83.788% (-2.9%) from 86.652%
when pulling 799a495 on gsnw:issue/62-dev
into abc8c09 on schweikert:develop.

[auerswal]

Thanks for working on this, @gsnw-sebast!
At least on Linux, receiving ICMP error messages requires the use of raw sockets. Please add checks that fping opened raw sockets when using --traceroute.
I have not yet looked at all code in detail, this was a first glance.

[gsnw-sebast]

Checklist with options that allow and tested

Option	Test	Supported
--ipv4 (-4)
--ipv6 (-6)
--alive (-a)
--addr (-A)
--size (-b)
--backoff (-B)
--count (-c)	Yes	No
--vcount (-C)	Yes	No
--rdns (-d)
--timestamp (-D)
--elapsed (-e)
--file (-f)
--generate (-g)
--ttl (-H)	Yes	Yes (1-30)
--interval (-i)
--iface (-I)
--json (-j)
--icmp-timestamp
--fwmark (-k)
--loop (-l)	Yes	No
--all (-m)
--dontfrag (-M)
--name (-n)
--netdata (-N)
--outage (-o)
--tos (-O)	Yes	Yes
--period (-p)
--quiet (-q)	Yes	No
--squiet (-Q)
--retry (-r)
--random (-R)
--stats (-s)
--src (-S)
--timeout (-t)
(-T)
--unreach (-u)
--reachable (-r)
--fast-reachable (-X)
--check-source
--print-tos	Yes	No
--print-ttl	Yes	No
--seqmap-timeout
(-z)