d2d23c848
Denys Vlasenko
2025-04-07
Subject
udhcpd: send DHCPOFFERs as unicast (unless clients specifically asks for bcast)
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networking/udhcp/dhcpd.c+42 -16
| @@ -575,29 +575,51 @@ static void send_packet_to_client(struct dhcp_packet *dhcp_pkt, int force_broadc | |
| const uint8_t *chaddr; | |
| uint32_t ciaddr; | |
| - | // Was: |
| + | // Logic: |
| //if (force_broadcast) { /* broadcast */ } | |
| //else if (dhcp_pkt->ciaddr) { /* unicast to dhcp_pkt->ciaddr */ } | |
| + | // ^^^ dhcp_pkt->ciaddr comes from client's request packet. |
| + | // We expect such clients to have an UDP socket listening on that IP. |
| //else if (dhcp_pkt->flags & htons(BROADCAST_FLAG)) { /* broadcast */ } | |
| //else { /* unicast to dhcp_pkt->yiaddr */ } | |
| - | // But this is wrong: yiaddr is _our_ idea what client's IP is |
| - | // (for example, from lease file). Client may not know that, |
| - | // and may not have UDP socket listening on that IP! |
| - | // We should never unicast to dhcp_pkt->yiaddr! |
| - | // dhcp_pkt->ciaddr, OTOH, comes from client's request packet, |
| - | // and can be used. |
| - | |
| - | if (force_broadcast |
| - | || (dhcp_pkt->flags & htons(BROADCAST_FLAG)) |
| - | || dhcp_pkt->ciaddr == 0 |
| + | // ^^^ The last case is confusing, but *should* work. |
| + | // It's a case where client have sent a DISCOVER |
| + | // and does not have a kernel UDP socket listening on the IP |
| + | // we are offering in yiaddr (it does not know the IP yet)! |
| + | // This *should* work because client *should* listen on a raw socket |
| + | // instead at this time (IOW: it should examine ALL IPv4 packets |
| + | // "by hand", not relying on kernel's UDP stack.) |
| + | |
| + | chaddr = dhcp_pkt->chaddr; |
| + | |
| + | if (dhcp_pkt->ciaddr == 0 |
| + | || force_broadcast /* sending DHCPNAK pkt? */ |
| ) { | |
| - | log1s("broadcasting packet to client"); |
| - | ciaddr = INADDR_BROADCAST; |
| - | chaddr = MAC_BCAST_ADDR; |
| + | if (dhcp_pkt->flags & htons(BROADCAST_FLAG) |
| + | || force_broadcast /* sending DHCPNAK pkt? */ |
| + | ) { |
| + | // RFC 2131: |
| + | // If 'giaddr' is zero and 'ciaddr' is zero, and the broadcast bit is |
| + | // set, then the server broadcasts DHCPOFFER and DHCPACK messages to |
| + | // 0xffffffff. ... |
| + | // In all cases, when 'giaddr' is zero, the server broadcasts any DHCPNAK |
| + | // messages to 0xffffffff. |
| + | ciaddr = INADDR_BROADCAST; |
| + | chaddr = MAC_BCAST_ADDR; |
| + | log1s("broadcasting packet to client"); |
| + | } else { |
| + | // If the broadcast bit is not set and 'giaddr' is zero and |
| + | // 'ciaddr' is zero, then the server unicasts DHCPOFFER and DHCPACK |
| + | // messages to the client's hardware address and 'yiaddr' address. |
| + | ciaddr = dhcp_pkt->yiaddr; |
| + | log1("unicasting packet to client %ciaddr", 'y'); |
| + | } |
| } else { | |
| - | log1s("unicasting packet to client ciaddr"); |
| + | // If the 'giaddr' |
| + | // field is zero and the 'ciaddr' field is nonzero, then the server |
| + | // unicasts DHCPOFFER and DHCPACK messages to the address in 'ciaddr'. |
| ciaddr = dhcp_pkt->ciaddr; | |
| - | chaddr = dhcp_pkt->chaddr; |
| + | log1("unicasting packet to client %ciaddr", 'c'); |
| } | |
| udhcp_send_raw_packet(dhcp_pkt, | |
| @@ -624,6 +646,10 @@ static void send_packet_to_relay(struct dhcp_packet *dhcp_pkt) | |
| static void send_packet(struct dhcp_packet *dhcp_pkt, int force_broadcast) | |
| { | |
| if (dhcp_pkt->gateway_nip) | |
| + | // RFC 2131: |
| + | // If the 'giaddr' field in a DHCP message from a client is non-zero, |
| + | // the server sends any return messages to the 'DHCP server' port on the |
| + | // BOOTP relay agent whose address appears in 'giaddr'. |
| send_packet_to_relay(dhcp_pkt); | |
| else | |
| send_packet_to_client(dhcp_pkt, force_broadcast); | |