When debugging https://github.com/daurnimator/lua-http/issues/73 which uses https://github.com/wahern/dns we ran into an issue where modern linux kernels return EINVAL if you try and re-use a udp socket. The issue seems to occur if you go from a local destination ip to a non-local one.
>From connect(2) man page: > connectionless protocol sockets may use connect() multiple times to change > their association (the following content is also available at https://gist.github.com/daurnimator/6765345776e87a3830ed101d1d983ee1) $ cat 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); } $ 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 +++
