[Bug 248002] ping(8): TTL DETAILS update

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=248002

            Bug ID: 248002
           Summary: ping(8): TTL DETAILS update
           Product: Documentation
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Manual Pages
          Assignee: bugs at FreeBSD.org
          Reporter: jlduran at gmail.com
                CC: doc at FreeBSD.org, net at FreeBSD.org

Reading ping(8), section "TTL DETAILS", I have seen a number of
inconsistencies regarding this value in FreeBSD.  This section of the man page
has not been updated to reflect the current state. For instance:

> Not change it; this is what BSD systems did before the 4.3BSD-Tahoe release.
> In this case the TTL value in the received packet will be 255 minus the
> number of routers in the round-trip path.

This paragraph implies that FreeBSD is still using 255 as its default TTL,
it should be updated to reflect the current value (64) [1], perhaps:

Not change it; this is what BSD systems did before the 4.3BSD-Tahoe release.
In this case the TTL value in the received packet will be 64 minus the number
of routers in the round-trip path.

Other paragraphs could also be updated, for example:

> The TCP/IP specification recommends setting the TTL field for IP packets to
> 64, but many systems use smaller values (4.3BSD uses 30, 4.2BSD used 15).

The TCP/IP specification recommends setting the TTL field for IP packets to
64, FreeBSD uses this value...

The DESCRIPTION mentions that the sysctl net.inet.ip.ttl is used, but
it does not say that this value is 64 by default.

Maybe also include some current OSs? I have compiled a quick list that can be
easily verified pinging the hosts inside the brackets, this should give you
the TTL (plus the number of hops):

Windows: 128 [live.com]
BSDs (NetBSD and OpenBSD): 255 [netbsd.org, openbsd.org]
Linux: 64 [linux.org]

FreeBSD, macOS: 64

> Set it to 255; this is what current BSD systems do.  In this case the TTL
> value in the received packet will be 255 minus the number of routers in the
> path from the remote system to the pinging host.

Set it to 255; this is what some BSD systems do...

Some current BSD systems do use 255 (at least NetBSD and OpenBSD); however,
FreeBSD uses the IANA-recommended value of 64 [2].

References:

[1] 
https://github.com/freebsd/freebsd/blob/3f4da6d3e7a2a8cc34a94938bd1c4f80c8d1d449/sys/netinet/ip.h#L212
[2] 
https://www.iana.org/assignments/ip-parameters/ip-parameters.xhtml#ip-parameters-2

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