Re: [PATCH v4 3/4] dhcp: Add --dhcp-opt with option table and value parser

[Anshu Kumari <anskuma@redhat.com>]

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On Fri, Jun 19, 2026 at 9:24 AM David Gibson <david@gibson.dropbear.id.au>
wrote:

> On Wed, Jun 17, 2026 at 06:52:37PM +0530, Anshu Kumari wrote:
> > Introduce the --dhcp-opt flag that allows setting arbitrary DHCP
> > options from command-line in the form [--dhcp-opt CODE,VALUE].
> >
> > Add a type lookup table mapping option codes to RFC 2132 value types
> > (IPv4, IPv4 list, integer, string) and dhcp_opt_parse() to convert
> > CLI strings to binary wire format.  Parsed options are stored in
> > struct ctx and injected into DHCP replies.  If the same option code
> > is given more than once, the last value wins.
> >
> > Link: https://bugs.passt.top/show_bug.cgi?id=192
> > Signed-off-by: Anshu Kumari <anskuma@redhat.com>
> > ---
> > v4:
> >   - Renamed custom_opts to dhcp_opts, 256 entries indexed by option
> >     code, removed MAX_CUSTOM_DHCP_OPTS and count field.
> >   - Changed str buffer from 256 to 255 bytes.
> >   - Moved function to conf.c as static conf_dhcp_option(), renamed
> >     from dhcp_add_option().
> >   - Made dhcp_opt_parse() non-static, declared in dhcp.h
> >   - Dropped val/len from ctx struct; conf_dhcp_option() validates
> >     with temp buffer, dhcp() parses str directly into opts[] at
> >     reply time.
>
> Hmm.  So each option is parsed twice.  What prevents you from parsing
> directly into the opts[] array at conf() time?
>

The first parse acts as a validation step to check that the user has
entered the
correct value format for the option. Without it, if the user passes
something
like *--dhcp-opt 3,notanip*, the error would surface only when the first
DHCP client connects, not at startup.

I think it's better to fail during startup if correct value format is not
entered in command-line rather than failing at later
stage during reply time.

>
> >   - Replaced strtok_r() + 256-byte buffer with strcspn() +
> >     INET_ADDRSTRLEN buffer.
> >   - Added DHCP_OPT_SINT32 for option 2 (Time Offset), uses strtol()
> >     per RFC 2132 Section 8.2.
> >   - All errors in dhcp_opt_parse() return -1, removed die() calls;
> >     caller handles error message consistently.
> >   - Removed redundant !slen check in DHCP_OPT_STR case.
> >   - Omitted explicit array size for dhcp_opt_types[], arraydded bounds
> >     check before lookup.
> >   - Added errno = 0 + errno check for strtoul() in case 34.
> >   - Fixed usage text: "Set DHCP option CODE to VAL".
> >   - Improved man page: added format description and examples
> >
> > v3:
> >   - Replaced DHCP_OPT_INTEGER with separate DHCP_OPT_INT8/INT16/INT32
> >     enums, removed dhcp_opt_int_width[] array.
> >   - Shared logic between DHCP_OPT_IPV4 and DHCP_OPT_IPV4_LIST — parse
> >     both as list, error if >1 in single case.
> >   - Added errno = 0 before strtoul() and check after.
> >   - Fixed range check: 1ULL << (width * 8) for all widths including
> >     width==4.
> >   - strncpy → memcpy for DHCP_OPT_STR.
> >   - Moved enum to dhcp.c since not used in other files.
> >   - Removed options 55, 61 (client-only), 119 (DNS compression, use
> >     --dhcp-search instead), 33 (IP pairs not supported).
> >   - DHCP_OPT_PARSE_BUF 1024 → char tmp[256].
> >   - Upgraded dhcp_add_option() to call dhcp_opt_parse() and populate
> >     val[]/len.
> >   - Aligned array entries for readability.
> >   - Added tab after @DHCP_OPT_IPV4_LIST: in kerneldoc.
> >   - Reject empty value strings before parsing
> >   - Reject leading/trailing/consecutive commas in IP list values.
> >
> > v2:
> >   - Replaced struct lookup table + dhcp_opt_type_lookup() function with
> flat dhcp_opt_types[256] array indexed by code.
> >   - Consolidated DHCP_OPT_UINT8/UINT16/UINT32 into single
> DHCP_OPT_INTEGER with dhcp_opt_int_width[256] table.
> >   - Dropped DHCP_OPT_ROUTES / option 121 entirely.
> >   - Added kerneldoc for enum dhcp_opt_type values.
> >   - Removed curly braces from switch cases, declarations before switch.
> >   - Added newlines before return statements.
> >   - Changed IP list delimiter from space to comma (--dhcp-opt
> 6,1.1.1.1,8.8.8.8).
> >   - Defined DHCP_OPT_PARSE_BUF constant for bare 1024.
> >   - Added len and val[255] fields to struct here (moved from patch 1).
> >   - Added kerneldoc for @custom_opts.len and @custom_opts.val.
> >   - Wired dhcp_opt_parse() into case 32 (--dhcp-boot) to populate
> val/len.
> > ---
> >  conf.c  |  45 ++++++++++++-
> >  dhcp.c  | 191 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  dhcp.h  |   2 +
> >  passt.1 |  42 +++++++++++++
> >  passt.h |   6 ++
> >  5 files changed, 285 insertions(+), 1 deletion(-)
> >
> > diff --git a/conf.c b/conf.c
> > index cd05adf..836b297 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 CODE to VAL\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 = 1; i < 255; i++)
> > +                             if (*c->dhcp_opts[i].str)
> > +                                     info("    option %u: %s", i,
> > +                                          c->dhcp_opts[i].str);
> >               }
> >
> >               for (i = 0; i < ARRAY_SIZE(c->ip4.dns); i++) {
> > @@ -1150,6 +1156,25 @@ static void conf_sock_listen(const struct ctx *c)
> >               die_perror("Couldn't add configuration socket to epoll");
> >  }
> >
> > +/**
> > + * conf_dhcp_option() - Set value for a DHCP option in configuration
> > + * @c:               Execution context
> > + * @code:    DHCP option code
> > + * @val_str: Value string from command line
> > + */
> > +static void conf_dhcp_option(struct ctx *c, uint8_t code, const char
> *val_str)
> > +{
> > +     uint8_t tmp[255];
> > +
> > +     if (dhcp_opt_parse(code, val_str, tmp, sizeof(tmp)) < 0)
> > +             die("Invalid value for DHCP option %u: %s", code, val_str);
> > +
> > +     if (snprintf_check(c->dhcp_opts[code].str,
> > +                        sizeof(c->dhcp_opts[0].str),
> > +                        "%s", val_str))
> > +             die("DHCP option value too long: %s", val_str);
> > +}
> > +
> >  /**
> >   * conf() - Process command-line arguments and set configuration
> >   * @c:               Execution context
> > @@ -1233,6 +1258,7 @@ 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,           34
> },
> >               { 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 +1274,13 @@ 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;
> >
> > @@ -1467,6 +1496,20 @@ 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 34:
> > +                     comma = strchr(optarg, ',');
> > +                     if (!comma)
> > +                             die("--dhcp-opt requires CODE,VALUE
> format");
> > +
> > +                     errno = 0;
> > +                     optcode = strtoul(optarg, &end, 0);
> > +                     if (end != comma || errno ||
> > +                         optcode < 1 || optcode > 254)
> > +                             die("DHCP option code must be 1-254: %s",
> > +                                 optarg);
> > +
> > +                     conf_dhcp_option(c, optcode, comma + 1);
> > +                     break;
> >               case 'd':
> >                       c->debug = 1;
> >                       c->quiet = 0;
> > diff --git a/dhcp.c b/dhcp.c
> > index 78790d8..47bb524 100644
> > --- a/dhcp.c
> > +++ b/dhcp.c
> > @@ -23,6 +23,7 @@
> >  #include <unistd.h>
> >  #include <string.h>
> >  #include <limits.h>
> > +#include <errno.h>
> >
> >  #include "util.h"
> >  #include "ip.h"
> > @@ -130,6 +131,189 @@ struct msg {
> >       uint8_t o[OPT_MAX + 1 /* End option */ ];
> >  } __attribute__((__packed__));
> >
> > +/**
> > + * enum dhcp_opt_type - DHCP option value types per RFC 2132
> > + * @DHCP_OPT_NONE:   Unsupported or unknown option
> > + * @DHCP_OPT_STR:    Variable-length string
> > + * @DHCP_OPT_IPV4:   Single IPv4 address
> > + * @DHCP_OPT_IPV4_LIST:      Multiple IPv4 addresses, comma-separated
> > + * @DHCP_OPT_INT8:   Unsigned 8-bit integer
> > + * @DHCP_OPT_INT16:  Unsigned 16-bit integer
> > + * @DHCP_OPT_INT32:  Unsigned 32-bit integer
> > + * @DHCP_OPT_SINT32: Signed 32-bit integer
>
> For consistency with C conventions, I'd suggset UINT{8,16,32} and just
> INT32 for the signed case.
>
> > + */
> > +enum dhcp_opt_type {
> > +     DHCP_OPT_NONE,
> > +     DHCP_OPT_STR,
> > +     DHCP_OPT_IPV4,
> > +     DHCP_OPT_IPV4_LIST,
> > +     DHCP_OPT_INT8,
> > +     DHCP_OPT_INT16,
> > +     DHCP_OPT_INT32,
> > +     DHCP_OPT_SINT32,
> > +};
> > +
> > +/**
> > + * dhcp_opt_types - Maps option code to RFC 2132 value type, indexed by
> code
> > + */
> > +static const enum dhcp_opt_type dhcp_opt_types[] = {
> > +     [1]   = DHCP_OPT_IPV4,          /* Subnet Mask */
> > +     [2]   = DHCP_OPT_SINT32,        /* Time Offset */
> > +     [3]   = DHCP_OPT_IPV4_LIST,     /* Router */
> > +     [4]   = DHCP_OPT_IPV4_LIST,     /* Time Server */
> > +     [5]   = DHCP_OPT_IPV4_LIST,     /* Name Server */
> > +     [6]   = DHCP_OPT_IPV4_LIST,     /* Domain Name Server */
> > +     [7]   = DHCP_OPT_IPV4_LIST,     /* Log Server */
> > +     [8]   = DHCP_OPT_IPV4_LIST,     /* Cookie Server */
> > +     [9]   = DHCP_OPT_IPV4_LIST,     /* LPR Server */
> > +     [10]  = DHCP_OPT_IPV4_LIST,     /* Impress Server */
> > +     [11]  = DHCP_OPT_IPV4_LIST,     /* Resource Location Server */
> > +     [12]  = DHCP_OPT_STR,           /* Host Name */
> > +     [13]  = DHCP_OPT_INT16,         /* Boot File Size */
> > +     [15]  = DHCP_OPT_STR,           /* Domain Name */
> > +     [16]  = DHCP_OPT_IPV4,          /* Swap Server */
> > +     [17]  = DHCP_OPT_STR,           /* Root Path */
> > +     [19]  = DHCP_OPT_INT8,          /* IP Forwarding */
> > +     [23]  = DHCP_OPT_INT8,          /* Default IP TTL */
> > +     [26]  = DHCP_OPT_INT16,         /* Interface MTU */
> > +     [28]  = DHCP_OPT_IPV4,          /* Broadcast Address */
> > +     [37]  = DHCP_OPT_INT8,          /* TCP Default TTL */
> > +     [38]  = DHCP_OPT_INT32,         /* TCP Keepalive Interval */
> > +     [40]  = DHCP_OPT_STR,           /* NIS Domain Name */
> > +     [41]  = DHCP_OPT_IPV4_LIST,     /* NIS Servers */
> > +     [42]  = DHCP_OPT_IPV4_LIST,     /* NTP Servers */
> > +     [44]  = DHCP_OPT_IPV4_LIST,     /* NetBIOS Name Server */
> > +     [50]  = DHCP_OPT_IPV4,          /* Requested IP Address */
> > +     [51]  = DHCP_OPT_INT32,         /* IP Address Lease Time */
> > +     [53]  = DHCP_OPT_INT8,          /* DHCP Message Type */
> > +     [54]  = DHCP_OPT_IPV4,          /* Server Identifier */
> > +     [57]  = DHCP_OPT_INT16,         /* Max DHCP Message Size */
> > +     [58]  = DHCP_OPT_INT32,         /* Renewal (T1) Time */
> > +     [59]  = DHCP_OPT_INT32,         /* Rebinding (T2) Time */
> > +     [60]  = DHCP_OPT_STR,           /* Vendor Class Identifier */
> > +     [66]  = DHCP_OPT_STR,           /* TFTP Server Name */
> > +     [67]  = DHCP_OPT_STR,           /* Bootfile Name */
> > +     [252] = DHCP_OPT_STR,           /* WPAD URL */
> > +};
> > +
> > +/**
> > + * dhcp_opt_parse() - Parse a DHCP option value
> > + * @code:    DHCP option code
> > + * @str:     Value string from command line
> > + * @buf:     Output buffer for binary value
> > + * @buf_len: Size of output buffer
> > + *
> > + * Return: number of bytes written to @buf, or -1 on error
> > + */
> > +int dhcp_opt_parse(uint8_t code, const char *str,
> > +                uint8_t *buf, size_t buf_len)
> > +{
> > +     enum dhcp_opt_type type;
> > +     unsigned long val;
> > +     unsigned int i;
> > +     uint8_t width;
> > +     size_t slen;
> > +     char *end;
> > +     int len;
> > +
> > +     if (code >= ARRAY_SIZE(dhcp_opt_types))
> > +             return -1;
> > +
> > +     type = dhcp_opt_types[code];
> > +
> > +     if (!*str)
> > +             return -1;
> > +
> > +     switch (type) {
> > +     case DHCP_OPT_NONE:
> > +             return -1;
> > +     case DHCP_OPT_IPV4:
> > +     case DHCP_OPT_IPV4_LIST:
> > +             len = 0;
> > +
> > +             while (*str) {
> > +                     char ipbuf[INET_ADDRSTRLEN];
> > +                     size_t chunk;
> > +
> > +                     chunk = strcspn(str, ",");
> > +
> > +                     if (!chunk || chunk >= sizeof(ipbuf))
> > +                             return -1;
> > +
> > +                     memcpy(ipbuf, str, chunk);
> > +                     ipbuf[chunk] = '\0';
> > +
> > +                     if (len + (int)sizeof(struct in_addr) >
> (int)buf_len)
>
> Both sides are necessarily non-negative, so it would make more sense
> to make len unsigned than to cast the other things to signed.
>
> > +                             return -1;
> > +
> > +                     if (inet_pton(AF_INET, ipbuf, buf + len) != 1)
> > +                             return -1;
> > +
> > +                     len += sizeof(struct in_addr);
> > +
> > +                     if (type == DHCP_OPT_IPV4) {
> > +                             if (str[chunk] == ',')
> > +                                     return -1;
> > +                             break;
> > +                     }
> > +
> > +                     str += chunk + (str[chunk] == ',');
> > +             }
> > +
> > +             if (!len)
> > +                     return -1;
> > +
> > +             return len;
> > +     case DHCP_OPT_INT8:
> > +     case DHCP_OPT_INT16:
> > +     case DHCP_OPT_INT32:
> > +     case DHCP_OPT_SINT32:
> > +             if (type == DHCP_OPT_INT8)
> > +                     width = 1;
> > +             else if (type == DHCP_OPT_INT16)
> > +                     width = 2;
> > +             else
> > +                     width = 4;
> > +
> > +             if (buf_len < width)
> > +                     return -1;
> > +
> > +             errno = 0;
> > +             if (type == DHCP_OPT_SINT32) {
> > +                     long sval;
> > +
> > +                     sval = strtol(str, &end, 0);
> > +                     if (*end || errno ||
> > +                         sval < INT32_MIN || sval > INT32_MAX)
> > +                             return -1;
> > +                     val = (uint32_t)sval;
> > +             } else {
> > +                     val = strtoul(str, &end, 0);
> > +                     if (*end || errno ||
> > +                         val >= (1ULL << (width * 8)))
> > +                             return -1;
> > +             }
> > +
> > +             for (i = width; i > 0; i--) {
> > +                     buf[i - 1] = val & 0xff;
> > +                     val >>= 8;
> > +             }
> > +
> > +             return width;
> > +     case DHCP_OPT_STR:
> > +             slen = strlen(str);
> > +
> > +             if (slen >= buf_len)
> > +                     return -1;
> > +
> > +             memcpy(buf, str, slen);
> > +
> > +             return slen;
> > +     }
> > +
> > +     return -1;
> > +}
> > +
> >  /**
> >   * fill_one() - Fill a single option into a buffer
> >   * @buf:     Buffer to write option
> > @@ -541,6 +725,13 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
> >       if (!c->no_dhcp_dns_search)
> >               opt_set_dns_search(c, OPT_MAX - 3);
> >
> > +     for (i = 1; i < 255; i++) {
> > +             if (!c->dhcp_opts[i].str[0])
> > +                     continue;
> > +             opts[i].slen = dhcp_opt_parse(i, c->dhcp_opts[i].str,
> > +                                           opts[i].s,
> sizeof(opts[i].s));
> > +     }
> > +
> >       /* RFC 2132, Section 9.5: put boot file name in the 'file' header
> >        * field.  Suppress option 67 from the options area and reserve
> >        * the file field from overload.
> > diff --git a/dhcp.h b/dhcp.h
> > index cd50c99..cc8d5dd 100644
> > --- a/dhcp.h
> > +++ b/dhcp.h
> > @@ -8,5 +8,7 @@
> >
> >  int dhcp(const struct ctx *c, struct iov_tail *data);
> >  void dhcp_init(void);
> > +int dhcp_opt_parse(uint8_t code, const char *str,
> > +                uint8_t *buf, size_t buf_len);
> >
> >  #endif /* DHCP_H */
> > diff --git a/passt.1 b/passt.1
> > index 908fd4a..ccdcbb2 100644
> > --- a/passt.1
> > +++ b/passt.1
> > @@ -430,6 +430,48 @@ Send \fIname\fR as DHCP option 12 (hostname).
> >  FQDN to configure the client with.
> >  Send \fIname\fR as Client FQDN: DHCP option 81 and DHCPv6 option 39.
> >
> > +.TP
> > +.BR \-\-dhcp-opt " " \fICODE\fR,\fIVALUE\fR
> > +Set DHCP option \fICODE\fR (1\-254) to \fIVALUE\fR. The value format
> depends
> > +on the option type and is determined automatically from the option code.
> > +Multiple IPv4 addresses are comma-separated.
> > +This option can be specified multiple times. If the same option code is
> > +given more than once, the last value wins. Options set with
> > +\fB\-\-dhcp-opt\fR override built-in values.
> > +.PP
> > +Examples:
> > +.nf
> > +  \-\-dhcp-opt 6,8.8.8.8,4.4.4.4
> > +  \-\-dhcp-opt 12,myhostname
> > +.fi
> > +.PP
> > +Only the following option codes are supported (unsupported codes cause
> an error):
> > +.RS
> > +.TP
> > +.B IPv4 address options
> > +1 (Subnet Mask), 16 (Swap Server), 28 (Broadcast Address), 50
> (Requested IP),
> > +54 (Server Identifier)
> > +.TP
> > +.B IPv4 address list options (comma-separated)
> > +3 (Router), 4 (Time Server), 5 (Name Server), 6 (DNS), 7 (Log Server),
> > +8 (Cookie Server), 9 (LPR Server), 10 (Impress Server),
> > +11 (Resource Location Server), 41 (NIS Servers),
> > +42 (NTP Servers), 44 (NetBIOS Name Server)
> > +.TP
> > +.B Integer options
> > +2 (Time Offset, 32-bit), 13 (Boot File Size, 16-bit), 19 (IP
> Forwarding, 8-bit),
> > +23 (Default IP TTL, 8-bit), 26 (Interface MTU, 16-bit),
> > +37 (TCP Default TTL, 8-bit), 38 (TCP Keepalive Interval, 32-bit),
> > +51 (IP Address Lease Time, 32-bit),
> > +53 (DHCP Message Type, 8-bit), 57 (Max DHCP Message Size, 16-bit),
> > +58 (Renewal Time, 32-bit), 59 (Rebinding Time, 32-bit)
> > +.TP
> > +.B String options
> > +12 (Host Name), 15 (Domain Name), 17 (Root Path), 40 (NIS Domain Name),
> > +60 (Vendor Class Identifier), 66 (TFTP Server Name),
> > +67 (Bootfile Name), 252 (WPAD URL)
> > +.RE
> > +
> >  .TP
> >  .BR \-t ", " \-\-tcp-ports " " \fIspec
> >  Configure TCP port forwarding to guest or namespace. \fIspec\fR can be
> one of:
> > diff --git a/passt.h b/passt.h
> > index 3a07294..15e2d83 100644
> > --- a/passt.h
> > +++ b/passt.h
> > @@ -182,6 +182,8 @@ struct ip6_ctx {
> >   * @dns_search:              DNS search list
> >   * @hostname:                Guest hostname
> >   * @fqdn:            Guest FQDN
> > + * @dhcp_opts:               User-specified DHCP options from --dhcp-opt
> > + * @dhcp_opts.str:   String value from command line
> >   * @ifi6:            Template interface for IPv6, -1: none, 0: IPv6
> disabled
> >   * @ip6:             IPv6 configuration
> >   * @pasta_ifn:               Name of namespace interface for pasta
> > @@ -264,6 +266,10 @@ struct ctx {
> >       char hostname[PASST_MAXDNAME];
> >       char fqdn[PASST_MAXDNAME];
> >
> > +     struct {
> > +             char str[255];
> > +     } dhcp_opts[256];
> > +
> >       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
>

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