USB-NCM DHCP-Server shared with WiFi-AP DHCP-Server -> local link IP on AP #19685

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1. SciLor

   added

   bug

   on Sep 5, 2026
2. SciLor commented on Sep 9, 2026

   

   SciLor

   on Sep 9, 2026

   Last edited by SciLor

   Author

   More actions

   Possible patches (I currently use myself)
   cyw32-driver

   From 982b0ece8661cb2c9029121e23e81066bbbab331 Mon Sep 17 00:00:00 2001
   From: SciLor <[email protected]>
   Date: Sat, 5 Sep 2026 23:45:01 +0000
   Subject: [PATCH] usb-ncm dhcp fix: pass netif pointer to dhcp_server_init
   
   Pass the AP interface's struct netif pointer to dhcp_server_init() to
   allow binding and pinning the DHCP socket to the specific netif.
   ---
    src/cyw43_lwip.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)
   
   diff --git a/src/cyw43_lwip.c b/src/cyw43_lwip.c
   index 1bb5c6b..788169b 100644
   --- a/src/cyw43_lwip.c
   +++ b/src/cyw43_lwip.c
   @@ -235,7 +235,7 @@ void cyw43_cb_tcpip_init(cyw43_t *self, int itf) {
            #endif
        } else {
            #if CYW43_NETUTILS
   -        dhcp_server_init(&self->dhcp_server, &ipconfig[0], &ipconfig[1]);
   +        dhcp_server_init(&self->dhcp_server, &ipconfig[0], &ipconfig[1], n);
            #endif
        }
    
   -- 
   2.43.0
   
   

   Micropython

   From 920995b18a59e3af594274e980ad0d67fa76d26b Mon Sep 17 00:00:00 2001
   From: SciLor <[email protected]>
   Date: Sat, 5 Sep 2026 23:44:07 +0000
   Subject: [PATCH] extmod/dhcp: Update dhcp_server_init to accept netif
    parameter for improved binding
   
   Bind the DHCP server socket to the specific server IP and netif rather
   than 0.0.0.0 (INADDR_ANY). This allows multiple DHCP servers (such as
   WiFi AP and USBD_NCM) to operate concurrently without UDP port 67
   collisions.
   ---
    extmod/network_usbd_ncm.c    |  2 +-
    shared/netutils/dhcpserver.c | 25 +++++++++++++++++++------
    shared/netutils/dhcpserver.h |  4 +++-
    3 files changed, 23 insertions(+), 8 deletions(-)
   
   diff --git a/extmod/network_usbd_ncm.c b/extmod/network_usbd_ncm.c
   index d6ae152a8..ab1e5b402 100644
   --- a/extmod/network_usbd_ncm.c
   +++ b/extmod/network_usbd_ncm.c
   @@ -201,7 +201,7 @@ static void ncm_init(void) {
        #endif
    
        #if MICROPY_PY_NETWORK_USBD_NCM_DHCP_SERVER
   -    dhcp_server_init(&ncm_obj.dhcp_server, &ncm_obj.ipaddr, &ncm_obj.netmask);
   +    dhcp_server_init(&ncm_obj.dhcp_server, &ncm_obj.ipaddr, &ncm_obj.netmask, netif);
        // Don't advertise a default gateway -- this is a point-to-point USB link
        // and advertising a router causes the host to route all traffic over USB.
        ncm_obj.dhcp_server.send_router = false;
   diff --git a/shared/netutils/dhcpserver.c b/shared/netutils/dhcpserver.c
   index c54dad357..95680312f 100644
   --- a/shared/netutils/dhcpserver.c
   +++ b/shared/netutils/dhcpserver.c
   @@ -109,11 +109,9 @@ static void dhcp_socket_free(struct udp_pcb **udp) {
        }
    }
    
   -static int dhcp_socket_bind(struct udp_pcb **udp, uint32_t ip, uint16_t port) {
   -    ip_addr_t addr;
   -    IP_ADDR4(&addr, ip >> 24 & 0xff, ip >> 16 & 0xff, ip >> 8 & 0xff, ip & 0xff);
   +static int dhcp_socket_bind(struct udp_pcb **udp, ip_addr_t *ip, uint16_t port) {
        // TODO convert lwIP errors to errno
   -    return udp_bind(*udp, &addr, port);
   +    return udp_bind(*udp, ip, port);
    }
    
    static int dhcp_socket_sendto(struct udp_pcb **udp, struct netif *netif, const void *buf, size_t len, uint32_t ip, uint16_t port) {
   @@ -293,7 +291,7 @@ ignore_request:
        pbuf_free(p);
    }
    
   -void dhcp_server_init(dhcp_server_t *d, ip_addr_t *ip, ip_addr_t *nm) {
   +void dhcp_server_init(dhcp_server_t *d, ip_addr_t *ip, ip_addr_t *nm, struct netif *netif) {
        ip_addr_copy(d->ip, *ip);
        ip_addr_copy(d->nm, *nm);
        memset(d->lease, 0, sizeof(d->lease));
   @@ -301,7 +299,22 @@ void dhcp_server_init(dhcp_server_t *d, ip_addr_t *ip, ip_addr_t *nm) {
        if (dhcp_socket_new_dgram(&d->udp, d, dhcp_server_process) != 0) {
            return;
        }
   -    dhcp_socket_bind(&d->udp, 0, PORT_DHCP_SERVER);
   +    // Bind to this server's own specific address, not the wildcard
   +    // 0.0.0.0 the old `dhcp_socket_bind(&d->udp, 0, ...)` used - and pin
   +    // the socket to its own netif. Without both of these, a second
   +    // dhcp_server_init() call for a different interface (e.g. a WiFi AP
   +    // and USBD_NCM both active at once) silently fails to bind at all:
   +    // lwIP's udp_bind() rejects a second PCB on the same port whenever
   +    // either side of the conflict is the wildcard address, so only the
   +    // first-initialized server ever actually receives anything, and every
   +    // client on every interface ends up served from that one server's
   +    // pool regardless of which physical link it actually asked on - see
   +    // the "AP-mode DHCP server hands out addresses from another active
   +    // interface's DHCP server" issue this fixes.
   +    dhcp_socket_bind(&d->udp, &d->ip, PORT_DHCP_SERVER);
   +    if (netif != NULL) {
   +        udp_bind_netif(d->udp, netif);
   +    }
    }
    
    void dhcp_server_deinit(dhcp_server_t *d) {
   diff --git a/shared/netutils/dhcpserver.h b/shared/netutils/dhcpserver.h
   index 24224d6aa..f06794c6c 100644
   --- a/shared/netutils/dhcpserver.h
   +++ b/shared/netutils/dhcpserver.h
   @@ -28,6 +28,8 @@
    
    #include "lwip/ip_addr.h"
    
   +struct netif;
   +
    #define DHCPS_BASE_IP (16)
    #define DHCPS_MAX_IP (8)
    
   @@ -44,7 +46,7 @@ typedef struct _dhcp_server_t {
        bool send_router; // advertise server IP as default gateway
    } dhcp_server_t;
    
   -void dhcp_server_init(dhcp_server_t *d, ip_addr_t *ip, ip_addr_t *nm);
   +void dhcp_server_init(dhcp_server_t *d, ip_addr_t *ip, ip_addr_t *nm, struct netif *netif);
    void dhcp_server_deinit(dhcp_server_t *d);
    
    #endif // MICROPY_INCLUDED_LIB_NETUTILS_DHCPSERVER_H
   -- 
   2.43.0
   
   
3. dpgeorge

   added this to the release-1.30 milestone on Sep 9, 2026