Re: [PATCH v2 1/6] conf: Add --dhcp-opt command-line option

On Thu, May 28, 2026 at 10:32:54AM +0530, Anshu Kumari wrote:

On Wed, May 27, 2026 at 8:56 AM David Gibson wrote:

...

On Tue, May 26, 2026 at 06:01:08PM +0530, Anshu Kumari wrote:

...

Introduce the --dhcp-opt flag that allows setting arbitrary DHCP options from command-line in the form of [Option CODE,VALUE]. This patch adds the option storage in struct ctx and CLI parsing; the type-aware value parser and DHCP reply injection follow in subsequent patches.

Link: https://bugs.passt.top/show_bug.cgi?id=192 Signed-off-by: Anshu Kumari

Mostly LGTM, but one query below.

...

--- v2: - Added kerneldoc for @custom_opts, @custom_opts.code, @custom_opts.str, and @custom_opts_count in struct ctx - Removed len and val[255] fields from struct (moved to patch 3) - Removed braces from case 33, moved declarations (optcode, comma, end) to function scope - Renamed code → optcode to follow function-scope convention --- conf.c | 34 +++++++++++++++++++++++++++++++++- passt.h | 12 ++++++++++++ 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/conf.c b/conf.c index 029b9c7..89d2127 100644 --- a/conf.c +++ b/conf.c @@ -47,6 +47,7 @@ #include "lineread.h" #include "isolation.h" #include "log.h" +#include "dhcp.h" #include "vhost_user.h" #include "epoll_ctl.h" #include "conf.h" @@ -616,7 +617,8 @@ static void usage(const char *name, FILE *f, int status) " -S, --search LIST Space-separated list, search domains\n" " a single, empty option disables the DNS search list\n" " -H, --hostname NAME Hostname to configure client with\n" - " --fqdn NAME FQDN to configure client with\n"); + " --fqdn NAME FQDN to configure client with\n" + " --dhcp-opt CODE,VAL Set DHCP option by code\n"); if (strstr(name, "pasta")) FPRINTF(f, " default: don't use any search list\n"); else @@ -844,6 +846,10 @@ static void conf_print(const struct ctx *c) info(" router: %s", inet_ntop(AF_INET, &c->ip4.guest_gw, buf, sizeof(buf))); + for (i = 0; i < c->custom_opts_count; i++) + info(" option %u: %s", + c->custom_opts[i].code, + c->custom_opts[i].str); }

for (i = 0; i < ARRAY_SIZE(c->ip4.dns); i++) { @@ -1233,6 +1239,7 @@ void conf(struct ctx *c, int argc, char **argv) {"migrate-no-linger", no_argument, NULL, 30 }, {"stats", required_argument, NULL, 31 }, {"conf-path", required_argument, NULL, 'c' }, + {"dhcp-opt", required_argument, NULL, 33 }, { 0 }, }; const char *optstring = "+dqfel:hs:c:F:I:p:P:m:a:n:M:g:i:o:D:S:H:461t:u:T:U:"; @@ -1248,10 +1255,13 @@ void conf(struct ctx *c, int argc, char **argv) uint8_t prefix_len_from_opt = 0; unsigned int ifi4 = 0, ifi6 = 0; const char *logfile = NULL; + unsigned long optcode; char *runas = NULL; size_t logsize = 0; + const char *comma; long fd_tap_opt; int name, ret; + char *end; uid_t uid; gid_t gid;

@@ -1465,6 +1475,28 @@ 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 33: + comma = strchr(optarg, ','); + if (!comma) + die("--dhcp-opt requires Option CODE,VALUE format"); + + optcode = strtoul(optarg, &end, 0); + if (end != comma || optcode < 1 || optcode > 254) + die("DHCP option code must be 1-254: %s", + optarg); + + if (c->custom_opts_count >= MAX_CUSTOM_DHCP_OPTS) + die("Too many --dhcp-opt entries (max %d)", + MAX_CUSTOM_DHCP_OPTS); + + c->custom_opts[c->custom_opts_count].code = optcode;

What happens if the user specifies the same DHCP option code multiple times?

I don't know off-hand if DHCP allows the same option to be presented multiple times; I'm guessing not. In which case we probably want to check for already-specified options and overwrite them, instead of adding them to the array twice.

If a user specifies the same DHCP option code multiple times, the option value is overwritten. The last specified value is considered, like if we run below command

./passt -f --dhcp-opt 6,8.8.8.8,8.8.4.4 --dhcp-opt 6,1.1.1.1,9.9.9.9

then passt DHCP option stores the last value DHCP: assign: 192.168.1.9 mask: 255.255.255.0 router: 192.168.1.1 option 6: 1.1.1.1,9.9.9.9

Ok, that's good. It's not clear to me where that behaviour is implemented: the option parsing logic here seems to just add things to the list, even if they're the same option.

...

...

+ if (snprintf_check(c->custom_opts[c->custom_opts_count].str, + sizeof(c->custom_opts[0].str), + "%s", comma + 1)) + die("DHCP option value too long: %s", + comma + 1); + c->custom_opts_count++; + break; case 'd': c->debug = 1; c->quiet = 0; diff --git a/passt.h b/passt.h index 1726965..3a0816f 100644 --- a/passt.h +++ b/passt.h @@ -182,6 +182,10 @@ struct ip6_ctx { * @dns_search: DNS search list * @hostname: Guest hostname * @fqdn: Guest FQDN + * @custom_opts: User-specified DHCP options from --dhcp-opt + * @custom_opts.code: DHCP option code + * @custom_opts.str: Original string value from command line + * @custom_opts_count: Number of entries in @custom_opts * @ifi6: Template interface for IPv6, -1: none, 0: IPv6 disabled * @ip6: IPv6 configuration * @pasta_ifn: Name of namespace interface for pasta @@ -263,6 +267,14 @@ struct ctx { char hostname[PASST_MAXDNAME]; char fqdn[PASST_MAXDNAME];

+#define MAX_CUSTOM_DHCP_OPTS 32 + + struct { + uint8_t code; + char str[256]; + } custom_opts[MAX_CUSTOM_DHCP_OPTS]; + int custom_opts_count; + int ifi6; struct ip6_ctx ip6;

-- 2.54.0

-- 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

-- 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