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 <anskuma@redhat.com>

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 (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
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