[PATCH v2 2/2] dhcp: Add --dhcp-boot option to set boot filename in DHCP replies

Laurent Vivier
2026-04-16T16:21:48Z

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