connect-bug-stage1.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <arpa/inet.h>

int main() {
	int fd = socket(PF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP);
	if (fd == -1)
		exit(1);
	if (bind(fd, (struct sockaddr*)&(struct sockaddr_in){.sin_family=AF_INET, .sin_port=htons(57997), .sin_addr=inet_addr("0.0.0.0")}, 16))
		exit(2);
	if (connect(fd, (struct sockaddr*)&(struct sockaddr_in){.sin_family=AF_INET, .sin_port=htons(53), .sin_addr=inet_addr("127.0.0.2")}, 16))
		exit(3);
	if (-1 == sendto(fd, "test", 4, 0, NULL, 0))
		exit(4);
	char buf[200];
	if (-1 == recvfrom(fd, buf, 200, 0, 0, 0))
		exit(5);
	exit(0);
}

connect-bug-stage2.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <arpa/inet.h>
#include <errno.h>

int main() {
	int fd = socket(PF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP);
	if (fd == -1)
		exit(1);
	if (bind(fd, (struct sockaddr*)&(struct sockaddr_in){.sin_family=AF_INET, .sin_port=htons(57997), .sin_addr=inet_addr("0.0.0.0")}, 16))
		exit(2);
	if (connect(fd, (struct sockaddr*)&(struct sockaddr_in){.sin_family=AF_INET, .sin_port=htons(53), .sin_addr=inet_addr("127.0.0.2")}, 16))
		exit(3);
	if (-1 == sendto(fd, "test", 4, 0, NULL, 0))
		exit(4);
	char buf[200];
	if (-1 != recvfrom(fd, buf, 200, 0, 0, 0) && errno != ECONNREFUSED)
		exit(5);
	/* okay, try next server... */
	if (connect(fd, (struct sockaddr*)&(struct sockaddr_in){.sin_family=AF_INET, .sin_port=htons(53), .sin_addr=inet_addr("8.8.8.8")}, 16))
		exit(6);
	exit(0);
}

output.txt

$ gcc connect-bug-stage1.c 
$ strace ./a.out 
execve("./a.out", ["./a.out"], [/* 56 vars */]) = 0
brk(NULL)                               = 0x1fa7000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=215350, ...}) = 0
mmap(NULL, 215350, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f6d1962f000
close(3)                                = 0
open("/usr/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260\3\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1951744, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f6d1962d000
mmap(NULL, 3791152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f6d190a4000
mprotect(0x7f6d19239000, 2093056, PROT_NONE) = 0
mmap(0x7f6d19438000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x194000) = 0x7f6d19438000
mmap(0x7f6d1943e000, 14640, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f6d1943e000
close(3)                                = 0
arch_prctl(ARCH_SET_FS, 0x7f6d1962e400) = 0
mprotect(0x7f6d19438000, 16384, PROT_READ) = 0
mprotect(0x600000, 4096, PROT_READ)     = 0
mprotect(0x7f6d19664000, 4096, PROT_READ) = 0
munmap(0x7f6d1962f000, 215350)          = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
bind(3, {sa_family=AF_INET, sin_port=htons(57997), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.2")}, 16) = 0
sendto(3, "test", 4, 0, NULL, 0)        = 4
recvfrom(3, 0x7ffd2bcbfb60, 200, 0, NULL, NULL) = -1 ECONNREFUSED (Connection refused)
exit_group(5)                           = ?
+++ exited with 5 +++
$ gcc connect-bug-stage2.c 
$ strace ./a.out 
execve("./a.out", ["./a.out"], [/* 56 vars */]) = 0
brk(NULL)                               = 0x860000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=215350, ...}) = 0
mmap(NULL, 215350, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fc978791000
close(3)                                = 0
open("/usr/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260\3\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1951744, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc97878f000
mmap(NULL, 3791152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc978206000
mprotect(0x7fc97839b000, 2093056, PROT_NONE) = 0
mmap(0x7fc97859a000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x194000) = 0x7fc97859a000
mmap(0x7fc9785a0000, 14640, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fc9785a0000
close(3)                                = 0
arch_prctl(ARCH_SET_FS, 0x7fc978790400) = 0
mprotect(0x7fc97859a000, 16384, PROT_READ) = 0
mprotect(0x600000, 4096, PROT_READ)     = 0
mprotect(0x7fc9787c6000, 4096, PROT_READ) = 0
munmap(0x7fc978791000, 215350)          = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
bind(3, {sa_family=AF_INET, sin_port=htons(57997), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.2")}, 16) = 0
sendto(3, "test", 4, 0, NULL, 0)        = 4
recvfrom(3, 0x7ffcc2c9a3a0, 200, 0, NULL, NULL) = -1 ECONNREFUSED (Connection refused)
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("8.8.8.8")}, 16) = -1 EINVAL (Invalid argument)
exit_group(6)                           = ?
+++ exited with 6 +++