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

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 15 Aug 2025 20:18:46 -0700
Eric Biggers <ebiggers@kernel.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.

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

> +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.
Also, I would prefer original author evaluate this

> +.PP
> +A blank "passphrase" removes the mapping.
> +.PP
> +The currently supported algorithms for \fIALGO\fR are \fBsha1\fR and
> +\fBsha256\fR.