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Date:      Wed, 15 Jul 2020 12:07:20 -0700 (PDT)
From:      "Rodney W. Grimes" <freebsd-rwg@gndrsh.dnsmgr.net>
To:        bugzilla-noreply@freebsd.org
Cc:        net@freebsd.org
Subject:   Re: [Bug 248002] ping(8): TTL DETAILS update
Message-ID:  <202007151907.06FJ7KB5018502@gndrsh.dnsmgr.net>
In-Reply-To: <bug-248002-7501@https.bugs.freebsd.org/bugzilla/>

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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@FreeBSD.org
>           Reporter: jlduran@gmail.com
>                 CC: doc@FreeBSD.org, net@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.

Why hard code another constant?  Refer to net.inet.ip.ttl.

> 
> 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.

And it probably shouldnt, sticking hard constants in man pages leads
to out of date information any time that constant evolves.

> 
> 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

FreeBSD should not document what other OS's do.

> 
> > 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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-- 
Rod Grimes                                                 rgrimes@freebsd.org



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