Re: [PATCH 2/2] dhcp: Add --dhcp-boot option to set boot filename in DHCP replies

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

[-- Attachment #1: Type: text/plain, Size: 4195 bytes --]

On Fri, Apr 03, 2026 at 10:02:04AM +0200, Laurent Vivier wrote:
> Add a --dhcp-boot option that populates the 'file' field in DHCP reply
> messages with the given filename.
> 
> Using --dhcp-boot together with --no-dhcp is rejected at startup.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  conf.c  | 10 ++++++++++
>  dhcp.c  |  5 ++++-
>  passt.1 |  6 ++++++
>  passt.h |  1 +
>  4 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/conf.c b/conf.c
> index ae37bf96dac4..2cf1bc7420d3 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -984,6 +984,7 @@ static void usage(const char *name, FILE *f, int status)
>  		"  --no-udp		Disable UDP protocol handler\n"
>  		"  --no-icmp		Disable ICMP/ICMPv6 protocol handler\n"
>  		"  --no-dhcp		Disable DHCP server\n"
> +		"  --dhcp-boot FILE	DHCP boot filename option\n"
>  		"  --no-ndp		Disable NDP responses\n"
>  		"  --no-dhcpv6		Disable DHCPv6 server\n"
>  		"  --no-ra		Disable router advertisements\n"
> @@ -1537,6 +1538,7 @@ void conf(struct ctx *c, int argc, char **argv)
>  		{"migrate-exit", no_argument,		NULL,		29 },
>  		{"migrate-no-linger", no_argument,	NULL,		30 },
>  		{"stats", required_argument,		NULL,		31 },
> +		{"dhcp-boot", required_argument,	NULL,		32 },
>  		{ 0 },
>  	};
>  	const char *optstring = "+dqfel:hs:F:I:p:P:m:a:n:M:g:i:o:D:S:H:461t:u:T:U:";
> @@ -1775,6 +1777,11 @@ void conf(struct ctx *c, int argc, char **argv)
>  				die("Can't display statistics if not running in foreground");
>  			c->stats = strtol(optarg, NULL, 0);
>  			break;
> +		case 32:
> +			if (snprintf_check(c->dhcp_boot, sizeof(c->dhcp_boot),
> +					   "%s", optarg))
> +				die("Invalid DHCP bootfile name: %s", optarg);
> +			break;
>  		case 'd':
>  			c->debug = 1;
>  			c->quiet = 0;
> @@ -2206,6 +2213,9 @@ void conf(struct ctx *c, int argc, char **argv)
>  		c->no_dhcpv6 = 1;
>  	}
>  
> +	if (c->no_dhcp && c->dhcp_boot[0])
> +		die("--dhcp-boot cannot be used with --no-dhcp");
> +
>  	get_dns(c);
>  
>  	if (!*c->pasta_ifn) {
> diff --git a/dhcp.c b/dhcp.c
> index 1ff8cba9f93d..7fdb6e051dec 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -344,6 +344,9 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
>  	    m->op != BOOTREQUEST)
>  		return -1;
>  
> +	static_assert(sizeof(reply.file) == sizeof(c->dhcp_boot),
> +		      "dhcp_boot must have the same size as reply.file");
> +
>  	reply.op		= BOOTREPLY;
>  	reply.htype		= m->htype;
>  	reply.hlen		= m->hlen;
> @@ -357,7 +360,7 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
>  	reply.giaddr		= m->giaddr;
>  	memcpy(&reply.chaddr,	m->chaddr,	sizeof(reply.chaddr));
>  	memset(&reply.sname,	0,		sizeof(reply.sname));
> -	memset(&reply.file,	0,		sizeof(reply.file));
> +	memcpy(&reply.file,	c->dhcp_boot,	sizeof(reply.file));
>  	reply.magic		= m->magic;
>  
>  	for (i = 0; i < ARRAY_SIZE(opts); i++)
> diff --git a/passt.1 b/passt.1
> index 13e8df9de9f3..a6ac2884bf3f 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -342,6 +342,12 @@ Disable the DHCP server. DHCP client requests coming from guest or target
>  namespace will be silently dropped. Implied if there is no gateway on the
>  selected IPv4 default route.
>  
> +.TP
> +.BR \-\-dhcp-boot " " \fIfile
> +Set the boot filename in DHCP replies to \fIfile\fR. This populates the
> +\fIfile\fR field in BOOTP/DHCP reply messages, which can be used for
> +network booting (PXE). Cannot be used with \fB--no-dhcp\fR.
> +
>  .TP
>  .BR \-\-no-ndp
>  Disable Neighbor Discovery. NDP messages coming from guest or target
> diff --git a/passt.h b/passt.h
> index 62b8dcdf0a41..4044a4c07ea3 100644
> --- a/passt.h
> +++ b/passt.h
> @@ -255,6 +255,7 @@ struct ctx {
>  
>  	char hostname[PASST_MAXDNAME];
>  	char fqdn[PASST_MAXDNAME];
> +	char dhcp_boot[128];
>  
>  	int ifi6;
>  	struct ip6_ctx ip6;
> -- 
> 2.53.0
> 

-- 
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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]