Re: [PATCH v3 2/2] doc: Document CIDR notation support for -a/--address option

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Dec 18, 2025 at 05:22:13PM -0500, Jon Maloy wrote:
> Update the man page to document the new CIDR notation support:
> 
> - Update -a/--address to show addr[/prefix_len] syntax
> - Document that prefix length (0-32) can be appended for IPv4
> - Note that mixing CIDR notation with -n results in an error
> - Update -n/--netmask to mention CIDR alternative
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

I don't see much reason to keep this a separate patch from the
previous one.

> ---
>  passt.1 | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/passt.1 b/passt.1
> index db0d662..117239f 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -156,10 +156,14 @@ By default, the advertised MTU is 65520 bytes, that is, the maximum 802.3 MTU
>  minus the length of a 802.3 header, rounded to 32 bits (IPv4 words).
>  
>  .TP
> -.BR \-a ", " \-\-address " " \fIaddr
> +.BR \-a ", " \-\-address " " \fIaddr\fR[\fB/\fR\fIprefix_len\fR]
>  Assign IPv4 \fIaddr\fR via DHCP (\fByiaddr\fR), or \fIaddr\fR via DHCPv6 (option
>  5) and an \fIaddr\fR-based prefix via NDP Router Advertisement (option type 3)
>  for an IPv6 \fIaddr\fR.
> +For IPv4 addresses, an optional \fB/\fR\fIprefix_len\fR (0-32) can be
> +appended in CIDR notation (e.g., 192.168.1.1/24). This is an alternative to
> +using the \fB-n\fR, \fB--netmask\fR option. Mixing CIDR notation with
> +\fB-n\fR results in an error.

This seems to imply that the CIDR notation is only for Ipv4, which is
not what's implemented.

>  This option can be specified zero (for defaults) to two times (once for IPv4,
>  once for IPv6).
>  By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces
> @@ -172,7 +176,9 @@ is assigned for IPv4, and no additional address will be assigned for IPv6.
>  .TP
>  .BR \-n ", " \-\-netmask " " \fImask
>  Assign IPv4 netmask \fImask\fR, expressed as dot-decimal or number of bits, via
> -DHCP (option 1).
> +DHCP (option 1). Alternatively, the prefix length can be specified using CIDR
> +notation with the \fB-a\fR, \fB--address\fR option (e.g., \fB-a\fR 192.168.1.1/24).
> +Mixing \fB-n\fR with CIDR notation results in an error.
>  By default, the netmask associated to the host address matching the assigned one
>  is used. If there's no matching address on the host, the netmask is determined
>  according to the CIDR block of the assigned address (RFC 4632).

Pre-existing, but this seems a bit misleading.  If we *don't* specify
-a, then, yes, the netmask will be copied from the host, along with
the address.  If we specify a specific address with -a, I think we'll
always use the network class (which is rarely useful these days), even
if the specified address happens to be one that's on the host.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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