Allow compiling out the DHCP client and UDP

.github/workflows/linux.yml

@@ -122,6 +122,18 @@ jobs:
           set -euo pipefail
           timeout --preserve-status 2m sudo ./build/packet_ping wtcp0 10.10.10.1
 
+      - name: Build without DHCP and with no UDP sockets
+        run: |
+          set -euo pipefail
+          mkdir -p build/noudp
+          sed 's/^#define MAX_UDPSOCKETS .*/#define MAX_UDPSOCKETS 0/' config.h > build/noudp/config.h
+          gcc -Ibuild/noudp -I. -D_GNU_SOURCE -DWOLFIP_ENABLE_DHCP=0 -Wall -Werror -Wextra -c src/wolfip.c -o build/noudp/wolfip.o
+          nm build/noudp/wolfip.o > build/noudp/syms.txt
+          if grep -qi dhcp build/noudp/syms.txt; then
+            echo "DHCP symbols left in a WOLFIP_ENABLE_DHCP=0 build"
+            exit 1
+          fi
+
       - name: Install check
         run: |
           sudo apt-get install -y check

docs/API.md

@@ -246,6 +246,8 @@ For `non_ethernet` devices this value remains the internal frame budget; the max
 
 ## DHCP Client Functions
 
+Compiled unless `WOLFIP_ENABLE_DHCP` is set to 0.
+
 ```c
 int dhcp_client_init(struct wolfIP *s);
 ```

docs/dhcp_dns_howto.md

@@ -77,6 +77,8 @@ retransmit and lease timers never fire.
 
 ## 3. The API
 
+The DHCP client is compiled by default. A static-address build can drop it by setting `WOLFIP_ENABLE_DHCP` to 0; the functions below are then still declared, but not defined, so a call fails at link time.
+
 All declarations are in `wolfip.h`:
 
 ```c
@@ -351,6 +353,8 @@ if your application needs UDP sockets of its own. If the pool is exhausted,
 `dhcp_client_init()` returns negative and `nslookup()` fails to allocate its
 socket.
 
+A target that needs no UDP at all can set `MAX_UDPSOCKETS 0` together with `WOLFIP_ENABLE_DHCP 0`. Every UDP path then finds no socket to use, DNS lookups fail to allocate one, and the per-socket UDP buffers disappear from RAM. Leaving DHCP enabled with no UDP sockets stops the build with an `#error`.
+
 ## 10. Troubleshooting
 
 **DHCP never gets bound.** Confirm `now_ms` advances between `wolfIP_poll()`

src/wolfip.c

@@ -40,6 +40,13 @@
 #define WOLFIP_MAX_ROUTES 16U
 #endif
 
+#ifndef WOLFIP_ENABLE_DHCP
+#define WOLFIP_ENABLE_DHCP 1
+#endif
+#if WOLFIP_ENABLE_DHCP && defined(MAX_UDPSOCKETS) && (MAX_UDPSOCKETS < 1)
+#error "WOLFIP_ENABLE_DHCP requires MAX_UDPSOCKETS >= 1"
+#endif
+
 #define WOLFIP_LOOPBACK_IP 0x7F000001U
 #define WOLFIP_LOOPBACK_MASK 0xFF000000U
 #if WOLFIP_ENABLE_LOOPBACK
@@ -1513,6 +1520,7 @@ struct wolfIP {
     char dns_ptr_name[256];
     struct timers_binheap timers;
     struct tsocket tcpsockets[MAX_TCPSOCKETS];
+    /* Zero-length (GNU extension) when MAX_UDPSOCKETS is 0. */
     struct tsocket udpsockets[MAX_UDPSOCKETS];
     struct tsocket icmpsockets[MAX_ICMPSOCKETS];
 #if WOLFIP_RAWSOCKETS
@@ -9159,6 +9167,7 @@ static void icmp_input(struct wolfIP *s, unsigned int if_idx, struct wolfIP_ip_p
     icmp_try_recv(s, if_idx, icmp, len);
 }
 
+#if WOLFIP_ENABLE_DHCP
 static int dhcp_send_discover(struct wolfIP *s);
 static int dhcp_send_request(struct wolfIP *s);
 #ifdef ETHERNET
@@ -10337,6 +10346,7 @@ static void dhcp_dad_conflict(struct wolfIP *s)
     dhcp_schedule_timer_at(s, s->last_tick + DHCP_DECLINE_WAIT_MS);
 }
 #endif
+#endif /* WOLFIP_ENABLE_DHCP */
 
 /* ARP */
 #ifdef ETHERNET
@@ -10585,6 +10595,7 @@ static void arp_request(struct wolfIP *s, unsigned int if_idx, ip4 tip)
  * reply is detected in arp_recv (dhcp_dad_conflict). Deliberately bypasses
  * the 1 req/s rate limit: DAD is at most 3 probes per acquisition, one
  * per second, and must not starve behind ordinary traffic. */
+#if WOLFIP_ENABLE_DHCP
 static int dhcp_send_dad_probe(struct wolfIP *s)
 {
     struct arp_packet arp;
@@ -10613,6 +10624,7 @@ static int dhcp_send_dad_probe(struct wolfIP *s)
     return wolfIP_ll_send_frame(s, WOLFIP_PRIMARY_IF_IDX, &arp,
                                 sizeof(struct arp_packet));
 }
+#endif /* WOLFIP_ENABLE_DHCP */
 
 static void arp_recv(struct wolfIP *s, unsigned int if_idx, void *buf, int len)
 {
@@ -10640,6 +10652,7 @@ static void arp_recv(struct wolfIP *s, unsigned int if_idx, void *buf, int len)
     if (arp->sma[0] & 0x01)
         return;
 
+#if WOLFIP_ENABLE_DHCP
     /* RFC 4331/5227 DAD: on the probing interface, a request from a
      * foreign MAC that claims the candidate (sender IP, including a
      * gratuitous announcement with sip==tip) or probes for it is a
@@ -10654,6 +10667,7 @@ static void arp_recv(struct wolfIP *s, unsigned int if_idx, void *buf, int len)
             return;
         }
     }
+#endif /* WOLFIP_ENABLE_DHCP */
 
     /* An unconfigured interface (no assigned address) must not answer
      * ARP requests: matching tip against a zero conf->ip would let a
@@ -10690,6 +10704,7 @@ static void arp_recv(struct wolfIP *s, unsigned int if_idx, void *buf, int len)
     else if (arp->opcode == ee16(ARP_REPLY)) {
         ip4 sip = ee32(arp->sip);
         int pending;
+#if WOLFIP_ENABLE_DHCP
         /* RFC 4331 DAD: a reply on the probing interface claiming the
          * candidate is a conflict, unless it is our own MAC (looped probe).
          * Bound to the DAD interface + recorded candidate so a reply on
@@ -10700,6 +10715,7 @@ static void arp_recv(struct wolfIP *s, unsigned int if_idx, void *buf, int len)
                 dhcp_dad_conflict(s);
             return;
         }
+#endif /* WOLFIP_ENABLE_DHCP */
         /* Validate sender IP: reject broadcast, multicast, zero, and
          * our own address -- same checks as the ARP request handler. */
         if (sip == IPADDR_ANY || sip == conf->ip ||
@@ -12876,7 +12892,9 @@ int wolfIP_poll(struct wolfIP *s, uint64_t now)
 
     /* Handle timers */
     handle_timers(s, now);
+#if WOLFIP_ENABLE_DHCP
     dhcp_timer_recover(s);
+#endif
 #ifdef IP_MULTICAST
     igmp_timer_recover(s);
 #endif