Let’s ignore my last reply, because there is no sendto, so it is more likely a health check.
haproxy got an request, accept as fd 2, and sendto 503 without tcp-rst-like message.
It seems to be that haproxy reply 503 automatically by internal logic without proxy request to backend server.
11:22:36.765962 accept4(8, {sa_family=AF_INET, sin_port=htons(31516), sin_addr=inet_addr(“42.120.75.138”)}, [16], SOCK_NONBLOCK) = 2
11:22:36.766073 setsockopt(2, SOL_TCP, TCP_NODELAY, [1], 4) = 0
11:22:36.766140 accept4(8, 0x7ffed5fc07a0, 0x7ffed5fc079c, SOCK_NONBLOCK) = -1 EAGAIN (Resource temporarily unavailable)
11:22:36.766227 recvfrom(2, 0x55690f3efe04, 15360, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
11:22:36.766295 epoll_ctl(0, EPOLL_CTL_ADD, 2, {EPOLLIN|EPOLLRDHUP, {u32=2, u64=2}}) = 0
11:22:36.766369 epoll_wait(0, [{EPOLLIN, {u32=76, u64=76}}], 200, 62) = 1
11:22:36.766460 recvfrom(76, “GET /agent/0d908a29-bf91-4aa7-97”…, 15360, 0, NULL, NULL) = 842
11:22:36.766606 getpid() = 22479
11:22:36.766663 sendmsg(5, {msg_name(110)={sa_family=AF_LOCAL, sun_path="/dev/log"}, msg_iov(8)=[{"<135>Jun 29 03:22:36 “, 21}, {“haproxy”, 7}, {”[", 1}, {“22479”, 5}, {"]: “, 3}, {”", 0}, {“add log-id header:01153024255676”…, 56}, {"\n", 1}], msg_controllen=0, msg_flags=0}, MSG_DONTWAIT|MSG_NOSIGNAL) = 94
11:22:36.766806 socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 68
11:22:36.766884 fcntl(68, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
11:22:36.766946 setsockopt(68, SOL_TCP, TCP_NODELAY, [1], 4) = 0
11:22:36.767005 connect(68, {sa_family=AF_INET, sin_port=htons(80), sin_addr=inet_addr(“9.0.5.130”)}, 16) = -1 EINPROGRESS (Operation now in progress)
11:22:36.767125 epoll_wait(0, [{EPOLLIN, {u32=2, u64=2}}], 200, 0) = 1
11:22:36.767249 sendto(68, “GET /agent/0d908a29-bf91-4aa7-97”…, 951, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 951
11:22:36.767339 recvfrom(2, “GET / HTTP/1.1\r\nHost: yanan-stor”…, 15360, 0, NULL, NULL) = 464
11:22:36.767490 getpid() = 22479
11:22:36.767549 sendmsg(5, {msg_name(110)={sa_family=AF_LOCAL, sun_path="/dev/log"}, msg_iov(8)=[{"<135>Jun 29 03:22:36 “, 21}, {“haproxy”, 7}, {”[", 1}, {“22479”, 5}, {"]: “, 3}, {”", 0}, {“add log-id header:01153024255676”…, 56}, {"\n", 1}], msg_controllen=0, msg_flags=0}, MSG_DONTWAIT|MSG_NOSIGNAL) = 94
11:22:36.767698 epoll_ctl(0, EPOLL_CTL_DEL, 2, 0x55690ed5f430) = 0
11:22:36.767738 epoll_wait(0, [], 200, 0) = 0
11:22:36.767764 recvfrom(68, 0x55690f3ebde4, 15360, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
11:22:36.767805 sendto(2, “HTTP/1.0 503 Service Unavailable”…, 212, MSG_DONTWAIT|MSG_NOSIGNAL|MSG_MORE, NULL, 0) = 212
11:22:36.767836 shutdown(2, SHUT_WR) = 0