It seems sendto() can return EACCES for UDP as well; the current man page in git only says it can return EACCES for Unix sockets. I was able to make sendto() return EACCES if I try to send from 192.168.1.1/24 to 192.168.1.0. I think the relevant code (in kernel 2.6.38, but also present in 2.6.7 and 2.6.32, the 2 kernels we use) is this (net/ipv4/udp.c, udp_sendmsg()): 910 err = -EACCES; 911 if ((rt->rt_flags & RTCF_BROADCAST) && 912 !sock_flag(sk, SOCK_BROADCAST)) 913 goto out; So I guess if the kernel finds a route to the destination and it's a broadcast route (and the socket doesn't have the broadcast flag), then it returns EACCES. I can verify the behavior with a very simple program (attached). I've run it on my Ubuntu 10.10 (2.6.35 kernel) and got this: stefan@spuiu-vml2:~/src/test/broadcast$ ./broadcast_test 10.205.20.94 10.205.20.1 sendto() returned 4 stefan@spuiu-vml2:~/src/test/broadcast$ ./broadcast_test 10.205.20.94 10.205.20.0 sendto() returned negative, errno: 13/Permission denied (10.205.20.94 is my local IP, of course). ===== #include <stdio.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <errno.h> #include <stdlib.h> int main(int argc, char **argv) { int sock; if (argc < 2) { printf("Usage: %s local_address destination_address\n", argv[0]); exit(1); } sock = socket(AF_INET, SOCK_DGRAM, 0); if (sock < 0) { perror("socket"); return -1; } struct sockaddr_in local_addr; local_addr.sin_family = AF_INET; local_addr.sin_port = htons(1234); local_addr.sin_addr.s_addr = inet_addr(argv[1]); int ret = bind(sock, (struct sockaddr *) &local_addr, sizeof(local_addr)); if (ret < 0) { perror("bind"); return -1; } struct sockaddr_in remote_addr; remote_addr.sin_family = AF_INET; remote_addr.sin_port = htons(1234); remote_addr.sin_addr.s_addr = inet_addr(argv[2]); ret = sendto(sock, "blah", 4, 0, (struct sockaddr *)&remote_addr, sizeof(remote_addr)); if (ret < 0) { printf("sendto() returned negative, errno: %d/%m\n", errno); } else { printf("sendto() returned %d\n", ret); } return 0; } ===== Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>