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[WIP] dhcp[46]: Support custom options #262

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5f3a43b
odchpd: support custom DHCPv6 options
Alphix Sep 22, 2025
9dd6bc5
odhcpd: support custom DHCPv4 options
Alphix Sep 22, 2025
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2 changes: 2 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -88,6 +88,8 @@ and may also receive information from ubus
| dhcpv6_na |bool | 1 | DHCPv6 stateful addressing hands out IA_NA - Internet Address - Network Address |
| dhcpv6_pd |bool | 1 | DHCPv6 stateful addressing hands out IA_PD - Internet Address - Prefix Delegation |
| dhcpv6_pd_min_len |integer| - | Minimum prefix length to delegate with IA_PD (value is adjusted if needed to be greater than the interface prefix length). Range [1,62] |
| dhcpv6_option |list | - | Custom DHCPv6 options in the form of strings formatted as `<optcode>,<encoding>:<data>`. For example: `42,hex:4575726F70652F4265726C696E` (timezone = "Europe/Berlin", RFC4833, §3) |
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@systemcrash systemcrash Nov 5, 2025

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I guess a new example is better now we have RFC4833 support? :)

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@Alphix Alphix Nov 5, 2025

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Haha...yes...but don't waste time on reviewing this in detail just yet...I have a much more comprehensive idea in mind, it needs some more time though.

| dhcpv4_option |list | - | Custom DHCPv4 options in the same format |
| router |list |`<local address>`| Routers to announce, accepts IPv4 only |
| dns |list |`<local address>`| DNS servers to announce, accepts IPv4 and IPv6 |
| dnr |list |disabled| Encrypted DNS servers to announce, `<priority> <domain name> [<comma separated IP addresses> <SvcParams (key=value)>...]` |
Expand Down
114 changes: 114 additions & 0 deletions src/config.c
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Original file line number Diff line number Diff line change
Expand Up @@ -97,6 +97,8 @@ enum {
IFACE_ATTR_DHCPV6_PD_MIN_LEN,
IFACE_ATTR_DHCPV6_NA,
IFACE_ATTR_DHCPV6_HOSTID_LEN,
IFACE_ATTR_DHCPV6_OPTION,
IFACE_ATTR_DHCPV4_OPTION,
IFACE_ATTR_RA_DEFAULT,
IFACE_ATTR_RA_MANAGEMENT,
IFACE_ATTR_RA_FLAGS,
Expand Down Expand Up @@ -151,6 +153,8 @@ static const struct blobmsg_policy iface_attrs[IFACE_ATTR_MAX] = {
[IFACE_ATTR_DHCPV6_PD_MIN_LEN] = { .name = "dhcpv6_pd_min_len", .type = BLOBMSG_TYPE_INT32 },
[IFACE_ATTR_DHCPV6_NA] = { .name = "dhcpv6_na", .type = BLOBMSG_TYPE_BOOL },
[IFACE_ATTR_DHCPV6_HOSTID_LEN] = { .name = "dhcpv6_hostidlength", .type = BLOBMSG_TYPE_INT32 },
[IFACE_ATTR_DHCPV6_OPTION] = { .name = "dhcpv6_option", .type = BLOBMSG_TYPE_ARRAY },
[IFACE_ATTR_DHCPV4_OPTION] = { .name = "dhcpv4_option", .type = BLOBMSG_TYPE_ARRAY },
[IFACE_ATTR_PD_MANAGER] = { .name = "pd_manager", .type = BLOBMSG_TYPE_STRING },
[IFACE_ATTR_PD_CER] = { .name = "pd_cer", .type = BLOBMSG_TYPE_STRING },
[IFACE_ATTR_RA_DEFAULT] = { .name = "ra_default", .type = BLOBMSG_TYPE_INT32 },
Expand Down Expand Up @@ -281,6 +285,10 @@ static void set_interface_defaults(struct interface *iface)
iface->dhcpv6_pd_min_len = 0;
iface->dhcpv6_na = true;
iface->dhcpv6_hostid_len = HOSTID_LEN_DEFAULT;
iface->dhcpv6_options = NULL;
iface->dhcpv6_option_cnt = 0;
iface->dhcpv4_options = NULL;
iface->dhcpv4_option_cnt = 0;
iface->dns_service = true;
iface->ra_flags = ND_RA_FLAG_OTHER;
iface->ra_slaac = true;
Expand All @@ -303,6 +311,12 @@ static void clean_interface(struct interface *iface)
free(iface->dhcpv4_ntp);
free(iface->dhcpv6_ntp);
free(iface->dhcpv6_sntp);
for (unsigned i = 0; i < iface->dhcpv6_option_cnt; i++)
free(iface->dhcpv6_options[i]);
free(iface->dhcpv6_options);
for (unsigned i = 0; i < iface->dhcpv4_option_cnt; i++)
free(iface->dhcpv4_options[i]);
free(iface->dhcpv4_options);
for (unsigned i = 0; i < iface->dnr_cnt; i++) {
free(iface->dnr[i].adn);
free(iface->dnr[i].addr4);
Expand Down Expand Up @@ -1270,6 +1284,106 @@ int config_parse_interface(void *data, size_t len, const char *name, bool overwr

}

if ((c = tb[IFACE_ATTR_DHCPV6_OPTION])) {
int opt_count;

opt_count = blobmsg_check_array_len(c, BLOBMSG_TYPE_STRING, blob_raw_len(c));
if (opt_count > 0) {
struct blob_attr *cur;
size_t rem;

iface->dhcpv6_option_cnt = 0;
iface->dhcpv6_options = realloc(iface->dhcpv6_options, opt_count * sizeof(*iface->dhcpv6_options));
blobmsg_for_each_attr(cur, c, rem) {
const char *opt_data;
char *opt_code_end;
unsigned long opt_code;
size_t opt_len;
struct dhcpv6_option *opt;

/* syntax: <code>,<encoding>:<data> */
opt_data = strchr(blobmsg_get_string(cur), ',');
if (!opt_data)
goto err_option;

opt_code = strtoul(blobmsg_get_string(cur), &opt_code_end, 10);
if (opt_code < 1 || opt_code > UINT16_MAX || opt_code_end != opt_data)
goto err_option;

opt_data++;
if (strncmp(opt_data, "hex:", strlen("hex:")))
goto err_option;

opt_data += strlen("hex:");
opt_len = strlen(opt_data);
if (opt_len % 2 || opt_len / 2 > UINT16_MAX)
goto err_option;

opt_len /= 2;
opt = malloc(sizeof(*opt) + opt_len);
opt->code = opt_code;
opt->length = opt_len;
odhcpd_unhexlify(opt->data, opt_len, opt_data);
iface->dhcpv6_options[iface->dhcpv6_option_cnt++] = opt;
continue;

err_option:
syslog(LOG_ERR, "Invalid %s value configured for interface '%s'",
iface_attrs[IFACE_ATTR_DHCPV6_OPTION].name, iface->name);
}
}
}

if ((c = tb[IFACE_ATTR_DHCPV4_OPTION])) {
int opt_count;

opt_count = blobmsg_check_array_len(c, BLOBMSG_TYPE_STRING, blob_raw_len(c));
if (opt_count > 0) {
struct blob_attr *cur;
size_t rem;

iface->dhcpv4_option_cnt = 0;
iface->dhcpv4_options = realloc(iface->dhcpv4_options, opt_count * sizeof(*iface->dhcpv4_options));
blobmsg_for_each_attr(cur, c, rem) {
const char *opt_data;
char *opt_code_end;
unsigned long opt_code;
size_t opt_len;
struct dhcpv4_option *opt;

/* syntax: <code>,<encoding>:<data> */
opt_data = strchr(blobmsg_get_string(cur), ',');
if (!opt_data)
goto err_option4;

opt_code = strtoul(blobmsg_get_string(cur), &opt_code_end, 10);
if (opt_code < 1 || opt_code > UINT8_MAX || opt_code_end != opt_data)
goto err_option4;

opt_data++;
if (strncmp(opt_data, "hex:", strlen("hex:")))
goto err_option4;

opt_data += strlen("hex:");
opt_len = strlen(opt_data);
if (opt_len % 2 || opt_len / 2 > UINT8_MAX)
goto err_option4;

opt_len /= 2;
opt = malloc(sizeof(*opt) + opt_len);
opt->type = opt_code;
opt->len = opt_len;
odhcpd_unhexlify(opt->data, opt_len, opt_data);
iface->dhcpv4_options[iface->dhcpv4_option_cnt++] = opt;
continue;

err_option4:
syslog(LOG_ERR, "Invalid %s value configured for interface '%s'",
iface_attrs[IFACE_ATTR_DHCPV4_OPTION].name, iface->name);
}
}
}

if ((c = tb[IFACE_ATTR_RA_DEFAULT]))
iface->default_router = blobmsg_get_u32(c);

Expand Down
12 changes: 12 additions & 0 deletions src/dhcpv4.c
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Expand Up @@ -871,6 +871,18 @@ void dhcpv4_handle_msg(void *addr, void *data, size_t len,
dhcpv4_put(&reply, &cookie, DHCPV4_OPT_DNR,
dnrs_len, dnrs);
break;

default:
for (size_t i = 0; i < iface->dhcpv4_option_cnt; i++) {
if (iface->dhcpv4_options[i]->type != a->reqopts[opt])
continue;
dhcpv4_put(&reply, &cookie,
iface->dhcpv4_options[i]->type,
iface->dhcpv4_options[i]->len,
iface->dhcpv4_options[i]->data);
break;
}
break;
}
}

Expand Down
7 changes: 0 additions & 7 deletions src/dhcpv4.h
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Expand Up @@ -91,12 +91,6 @@ struct dhcpv4_auth_forcerenew {
uint8_t key[16];
} _packed;

struct dhcpv4_option {
uint8_t type;
uint8_t len;
uint8_t data[];
};

/* DNR */
struct dhcpv4_dnr {
uint16_t len;
Expand All @@ -105,7 +99,6 @@ struct dhcpv4_dnr {
uint8_t body[];
};


#define dhcpv4_for_each_option(start, end, opt)\
for (opt = (struct dhcpv4_option*)(start); \
&opt[1] <= (struct dhcpv4_option*)(end) && \
Expand Down
67 changes: 58 additions & 9 deletions src/dhcpv6.c
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Expand Up @@ -185,6 +185,7 @@ enum {
IOV_DHCPV4O6_SERVER,
IOV_DNR,
IOV_BOOTFILE_URL,
IOV_CUSTOM_OPTIONS,
IOV_TOTAL
};

Expand Down Expand Up @@ -411,6 +412,12 @@ static void handle_client_request(void *addr, void *data, size_t len,
struct dhcpv6_dnr *dnrs = NULL;
size_t dnrs_len = 0;

/* Custom options */
struct dhcpv6_option *custom_options[iface->dhcpv6_option_cnt];
size_t custom_option_cnt = 0;
uint8_t *custom_option_buf = NULL;
size_t custom_option_buf_len = 0;

uint16_t otype, olen;
uint8_t *odata;
uint16_t *reqopts = NULL;
Expand Down Expand Up @@ -502,6 +509,37 @@ static void handle_client_request(void *addr, void *data, size_t len,
memcpy(&d6dnr->len, &d6dnr_len_be, sizeof(d6dnr_len_be));
}
break;

default:
for (size_t i = 0; i < iface->dhcpv6_option_cnt; i++) {
struct dhcpv6_option *copt = iface->dhcpv6_options[i];

if (opt != copt->code)
continue;

custom_options[custom_option_cnt++] = copt;
custom_option_buf_len += 2 * sizeof(uint16_t) + copt->length;
}
break;
}
}

if (custom_option_cnt > 0) {
uint8_t *tmp;

custom_option_buf = alloca(custom_option_buf_len);
tmp = custom_option_buf;

for (size_t i = 0; i < custom_option_cnt; i++) {
struct dhcpv6_option *copt = custom_options[i];
struct {
uint16_t type;
uint16_t len;
} copt_hdr = { htons(copt->code), htons(copt->length) };

memcpy(tmp, &copt_hdr, sizeof(copt_hdr));
memcpy(tmp + sizeof(copt_hdr), copt->data, copt->length);
tmp += sizeof(copt_hdr) + copt->length;
}
}

Expand Down Expand Up @@ -560,7 +598,8 @@ static void handle_client_request(void *addr, void *data, size_t len,
[IOV_DNR] = {dnrs, dnrs_len},
[IOV_RELAY_MSG] = {NULL, 0},
[IOV_DHCPV4O6_SERVER] = {&dhcpv4o6_server, 0},
[IOV_BOOTFILE_URL] = {NULL, 0}
[IOV_BOOTFILE_URL] = {NULL, 0},
[IOV_CUSTOM_OPTIONS] = {custom_option_buf, custom_option_buf_len},
};

if (hdr->msg_type == DHCPV6_MSG_RELAY_FORW)
Expand Down Expand Up @@ -730,15 +769,25 @@ static void handle_client_request(void *addr, void *data, size_t len,
}

if (iov[IOV_NESTED].iov_len > 0) /* Update length */
update_nested_message(data, len, iov[IOV_DEST].iov_len + iov[IOV_MAXRT].iov_len +
iov[IOV_RAPID_COMMIT].iov_len + iov[IOV_DNS].iov_len +
iov[IOV_DNS_ADDR].iov_len + iov[IOV_SEARCH].iov_len +
iov[IOV_SEARCH_DOMAIN].iov_len + iov[IOV_PDBUF].iov_len +
update_nested_message(data, len,
iov[IOV_DEST].iov_len +
iov[IOV_MAXRT].iov_len +
iov[IOV_RAPID_COMMIT].iov_len +
iov[IOV_DNS].iov_len +
iov[IOV_DNS_ADDR].iov_len +
iov[IOV_SEARCH].iov_len +
iov[IOV_SEARCH_DOMAIN].iov_len +
iov[IOV_PDBUF].iov_len +
iov[IOV_DHCPV4O6_SERVER].iov_len +
iov[IOV_CERID].iov_len + iov[IOV_DHCPV6_RAW].iov_len +
iov[IOV_NTP].iov_len + iov[IOV_NTP_ADDR].iov_len +
iov[IOV_SNTP].iov_len + iov[IOV_SNTP_ADDR].iov_len +
iov[IOV_DNR].iov_len + iov[IOV_BOOTFILE_URL].iov_len -
iov[IOV_CERID].iov_len +
iov[IOV_DHCPV6_RAW].iov_len +
iov[IOV_NTP].iov_len +
iov[IOV_NTP_ADDR].iov_len +
iov[IOV_SNTP].iov_len +
iov[IOV_SNTP_ADDR].iov_len +
iov[IOV_DNR].iov_len +
iov[IOV_BOOTFILE_URL].iov_len +
iov[IOV_CUSTOM_OPTIONS].iov_len -
(4 + opts_end - opts));

syslog(LOG_DEBUG, "Sending a DHCPv6-%s on %s", iov[IOV_NESTED].iov_len ? "relay-reply" : "reply", iface->name);
Expand Down
18 changes: 17 additions & 1 deletion src/odhcpd.h
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Expand Up @@ -270,6 +270,19 @@ struct dnr_options {
uint16_t svc_len;
};

// Generic DHCPv6 option
struct dhcpv6_option {
uint16_t code;
uint16_t length;
uint8_t data[];
};

// Generic DHCPv4 option
struct dhcpv4_option {
uint8_t type;
uint8_t len;
uint8_t data[];
};

// RA PIO - RFC9096
struct ra_pio {
Expand All @@ -278,7 +291,6 @@ struct ra_pio {
time_t lifetime;
};


struct interface {
struct avl_node avl;

Expand Down Expand Up @@ -375,6 +387,8 @@ struct interface {
struct in_addr *dhcpv4_dns;
size_t dhcpv4_dns_cnt;
bool dhcpv4_forcereconf;
struct dhcpv4_option **dhcpv4_options;
size_t dhcpv4_option_cnt;

// DNS
struct in6_addr *dns;
Expand All @@ -390,6 +404,8 @@ struct interface {
bool dhcpv6_na;
uint32_t dhcpv6_hostid_len;
uint32_t dhcpv6_pd_min_len; // minimum delegated prefix length
struct dhcpv6_option **dhcpv6_options;
size_t dhcpv6_option_cnt;

char *upstream;
size_t upstream_len;
Expand Down