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?

>   - 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