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

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

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On Thu, May 28, 2026 at 10:32:54AM +0530, Anshu Kumari wrote:
> On Wed, May 27, 2026 at 8:56 AM David Gibson <david@gibson.dropbear.id.au>
> 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 <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 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

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