Check for carrier before sendto() or write() on interface fd.

Linux will quietly proceed as if the data were sent even if the carrier
is down and nothing actually happened.  There is still a tiny race
condition where the carrier could drop between the check and the actual
write, but we really can't do anything about that and it is a very
small race.

src/arp.c

@@ -257,8 +257,14 @@ static int arp_send(struct client_state_t *cs, struct arpMsg *arp)
         return -1;
     }
 
-    ssize_t r = safe_sendto(cs->arpFd, (const char *)arp, sizeof *arp,
+    ssize_t r;
-                            0, (struct sockaddr *)&addr, sizeof addr);
+    if (!check_carrier(cs->arpFd)) {
+        log_error("%s: (%s) carrier down; sendto would fail",
+                  client_config.interface, __func__);
+        goto carrier_down;
+    }
+    r = safe_sendto(cs->arpFd, (const char *)arp, sizeof *arp, 0,
+                    (struct sockaddr *)&addr, sizeof addr);
     if (r < 0 || (size_t)r != sizeof *arp) {
         if (r < 0)
             log_error("%s: (%s) sendto failed: %s",
@@ -266,6 +272,7 @@ static int arp_send(struct client_state_t *cs, struct arpMsg *arp)
         else
             log_error("%s: (%s) sendto short write: %z < %zu",
                       client_config.interface, __func__, r, sizeof *arp);
+carrier_down:
         arp_reopen_fd(cs);
         return -1;
     }

src/dhcp.c

@@ -31,6 +31,7 @@
 #include <stdio.h>
 #include <unistd.h>
 #include <string.h>
+#include <sys/ioctl.h>
 #include <arpa/inet.h>
 #include <sys/types.h>
 #include <netpacket/packet.h>
@@ -107,6 +108,11 @@ static ssize_t send_dhcp_unicast(struct client_state_t *cs,
     }
     size_t payload_len =
         sizeof *payload - (sizeof payload->options - 1 - endloc);
+    if (!check_carrier(fd)) {
+        log_error("%s: (%s) carrier down; write would fail",
+                  client_config.interface, __func__);
+        goto out_fd;
+    }
     ret = safe_write(fd, (const char *)payload, payload_len);
     if (ret < 0 || (size_t)ret != payload_len)
         log_error("%s: (%s) write failed: %d", client_config.interface,
@@ -224,6 +230,22 @@ static ssize_t get_raw_packet(struct client_state_t *cs,
     return l;
 }
 
+int check_carrier(int fd)
+{
+    struct ifreq ifr;
+    memset(&ifr, 0, sizeof ifr);
+    memcpy(ifr.ifr_name, client_config.interface,
+           sizeof client_config.interface);
+    if (ioctl(fd, SIOCGIFFLAGS, &ifr) == -1) {
+        log_error("%s: (%s) ioctl failed: %s", client_config.interface,
+                  __func__, strerror(errno));
+        return 0;
+    }
+    if ((ifr.ifr_flags & IFF_RUNNING) && (ifr.ifr_flags & IFF_UP))
+        return 1;
+    return 0;
+}
+
 // Broadcast a DHCP message using a raw socket.
 static ssize_t send_dhcp_raw(struct dhcpmsg *payload)
 {
@@ -282,6 +304,11 @@ static ssize_t send_dhcp_raw(struct dhcpmsg *payload)
         .sll_halen = 6,
     };
     memcpy(da.sll_addr, "\xff\xff\xff\xff\xff\xff", 6);
+    if (!check_carrier(fd)) {
+        log_error("%s: (%s) carrier down; sendto would fail",
+                  client_config.interface, __func__);
+        goto carrier_down;
+    }
     ret = safe_sendto(fd, (const char *)&iudmsg, iud_len, 0,
                       (struct sockaddr *)&da, sizeof da);
     if (ret < 0 || (size_t)ret != iud_len) {
@@ -292,6 +319,7 @@ static ssize_t send_dhcp_raw(struct dhcpmsg *payload)
             log_error("%s: (%s) sendto short write: %z < %zu",
                       client_config.interface, __func__, ret, iud_len);
     }
+carrier_down:
     close(fd);
     return ret;
 }

src/dhcp.h

@@ -91,4 +91,6 @@ ssize_t send_rebind(struct client_state_t *cs);
 ssize_t send_decline(struct client_state_t *cs, uint32_t server);
 ssize_t send_release(struct client_state_t *cs);
 
+int check_carrier(int fd);
+
 #endif

src/seccomp.c

@@ -83,6 +83,11 @@ int enforce_seccomp_ndhc(void)
         ALLOW_SYSCALL(mmap),
         ALLOW_SYSCALL(munmap),
 
+        // For checking to see if the interface carrier is up.  Should be
+        // removed in favor of synchronous requests to ifchd, but it will
+        // work for now.
+        ALLOW_SYSCALL(ioctl),
+
         ALLOW_SYSCALL(rt_sigreturn),
 #ifdef __NR_sigreturn
         ALLOW_SYSCALL(sigreturn),