dhcpc: cope with buggy DHCP servers which send oversized packets

(Cristian Ionescu-Idbohrn <cristian.ionescu-idbohrn@axis.com>)

3 files changed, 27 insertions, 3 deletions

diff --git a/networking/udhcp/Config.in b/networking/udhcp/Config.in
index 734db65cc..1aef69ba7 100644
--- a/networking/udhcp/Config.in
+++ b/networking/udhcp/Config.in
@@ -82,3 +82,25 @@ config FEATURE_RFC3397
         help
           If selected, both client and server will support passing of domain
           search lists via option 119, specified in RFC3397.
+
+config UDHCPC_SLACK_FOR_BUGGY_SERVERS
+        int "DHCP options slack buffer size"
+        default 80
+        range 0 924
+        depends on APP_UDHCPD || APP_UDHCPC
+        help
+          Some buggy DHCP servers will send DHCP offer packets with option
+          field larger than we expect (which might also be considered a
+          buffer overflow attempt). These packets are normally discarded.
+          If circumstances beyond your control force you to support such
+          servers, this may help. The upper limit (924) makes dhcpc accept
+          even 1500 byte packets (maximum-sized ethernet packets).
+
+          This options does not make dhcp[cd] emit non-standard
+          sized packets.
+
+          Known buggy DHCP servers:
+          3Com OfficeConnect Remote 812 ADSL Router:
+            seems to confuse maximum allowed UDP packet size with
+            maximum size of entire IP packet, and sends packets which are
+            28 bytes too large.
diff --git a/networking/udhcp/common.h b/networking/udhcp/common.h
index 4f2d31c98..eecb72ca2 100644
--- a/networking/udhcp/common.h
+++ b/networking/udhcp/common.h
@@ -21,6 +21,8 @@ extern const uint8_t MAC_BCAST_ADDR[6]; /* six all-ones */
 #include <netinet/udp.h>
 #include <netinet/ip.h>
 
+#define DHCP_OPTIONS_BUFSIZE 308
+
 struct dhcpMessage {
         uint8_t op;
         uint8_t htype;
@@ -37,7 +39,7 @@ struct dhcpMessage {
         uint8_t sname[64];
         uint8_t file[128];
         uint32_t cookie;
-        uint8_t options[308]; /* 312 - cookie */
+        uint8_t options[DHCP_OPTIONS_BUFSIZE + CONFIG_UDHCPC_SLACK_FOR_BUGGY_SERVERS];
 } ATTRIBUTE_PACKED;
 
 struct udp_dhcp_packet {
@@ -49,7 +51,7 @@ struct udp_dhcp_packet {
 /* Let's see whether compiler understood us right */
 struct BUG_bad_sizeof_struct_udp_dhcp_packet {
         char BUG_bad_sizeof_struct_udp_dhcp_packet
-                [sizeof(struct udp_dhcp_packet) != 576 ? -1 : 1];
+                [(sizeof(struct udp_dhcp_packet) != 576 + CONFIG_UDHCPC_SLACK_FOR_BUGGY_SERVERS) ? -1 : 1];
 };
 
 void udhcp_init_header(struct dhcpMessage *packet, char type);
diff --git a/networking/udhcp/options.c b/networking/udhcp/options.c
index 2b4f1644f..6744e2ad0 100644
--- a/networking/udhcp/options.c
+++ b/networking/udhcp/options.c
@@ -145,7 +145,7 @@ int add_option_string(uint8_t *optionptr, uint8_t *string)
         int end = end_option(optionptr);
 
         /* end position + string length + option code/length + end option */
-        if (end + string[OPT_LEN] + 2 + 1 >= 308) {
+        if (end + string[OPT_LEN] + 2 + 1 >= DHCP_OPTIONS_BUFSIZE) {
                 bb_error_msg("option 0x%02x did not fit into the packet",
                                 string[OPT_CODE]);
                 return 0;