netdev - Re: [PATCH iproute2-next v2] man8: ip-sr: Document that passphrase must be high-entropy

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Message-ID: <20250820184317.GA1838@quark>
Date: Wed, 20 Aug 2025 11:43:17 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Stephen Hemminger <stephen@...workplumber.org>
Cc: netdev@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Andrea Mayer <andrea.mayer@...roma2.it>,
	David Lebrun <dlebrun@...gle.com>
Subject: Re: [PATCH iproute2-next v2] man8: ip-sr: Document that passphrase
 must be high-entropy

On Wed, Aug 20, 2025 at 09:25:35AM -0700, Stephen Hemminger wrote:
> On Fri, 15 Aug 2025 20:18:46 -0700
> Eric Biggers <ebiggers@...nel.org> wrote:
> 
> > diff --git a/man/man8/ip-sr.8 b/man/man8/ip-sr.8
> > index 6be1cc54..cd8c5d18 100644
> > --- a/man/man8/ip-sr.8
> > +++ b/man/man8/ip-sr.8
> > @@ -1,6 +1,6 @@
> > -.TH IP\-SR 8 "14 Apr 2017" "iproute2" "Linux"
> > +.TH IP\-SR 8 "15 Aug 2025" "iproute2" "Linux"
> 
> NAK - do not change man page date for each change.

Sure, if that's the convention for this project.  Note that this differs
from the convention used by most projects with dated man pages.  The
purpose of the date is normally to indicate how fresh the man page is.

> >  .SH "NAME"
> >  ip-sr \- IPv6 Segment Routing management
> >  .SH SYNOPSIS
> >  .sp
> >  .ad l
> > @@ -32,13 +32,21 @@ internal parameters.
> >  .PP
> >  Those parameters include the mapping between an HMAC key ID and its associated
> >  hashing algorithm and secret, and the IPv6 address to use as source for encapsulated
> >  packets.
> >  .PP
> > -The \fBip sr hmac set\fR command prompts for a passphrase that will be used as the
> > -HMAC secret for the corresponding key ID. A blank passphrase removes the mapping.
> > -The currently supported algorithms for \fIALGO\fR are \fBsha1\fR and \fBsha256\fR.
> > +The \fBip sr hmac set\fR command prompts for a newline-terminated "passphrase"
> 
> That implies that newline is part of the pass phrase.

Not really.  "NUL-terminated" strings don't include the NUL in the
string content.  If you prefer, it could be made explicit as follows:

    The \fBip sr hmac set\fR command prompts for a "passphrase" that
    will be used as the HMAC secret for the corresponding key ID. The
    passphrase is terminated by a newline, but the terminating newline
    is not included in the resulting passphrase.

But I don't think it's very useful, as it's not needed to know how to
use the command correctly.

> The code to read password is using getpass() which is marked as obsolete
> in glibc. readpassphrase is preferred.

Is that relevant to this documentation patch?

> > +that will be used as the HMAC secret for the corresponding key ID. This
> > +"passphrase" is \fInot\fR stretched, and it is used directly as the HMAC key.
> > +Therefore it \fImust\fR have enough entropy to be used as a key. For example, a
> > +correct use would be to use a passphrase that was generated using
> > +\fBhead\~-c\~32\~/dev/urandom\~|\~base64\~-w\~0\fR.
> 
> Shouldn't /dev/random be used instead of /dev/urandom for keys.

If you prefer that as the maintainer of this code, sure.  There are
reasons to use either one, and it's been an endless debate for years.

- Eric