Linux, socket loopback after process crash, why?

Question

linux

Hello everyone, I ran into the following problem:
- debian 8
- REST service on nodejs (listens on port, 50100)
- PHP (curl) client, connects quite often, makes a request, disconnects

Everything works fine until the service is turned off (or it won't crash on its own for some reason), then the following happens:
$ netstat -apne|grep 50100
tcp 0 0 127.0.0.1:50100 127.0.0.1:50100 ESTABLISHED 10000 2375204860 8054/php

php process that was connected (or tried to connect?) at the time of disconnection, the service becomes connected to itself , while it goes to the eternal poll:
rt_sigaction(SIGPIPE, NULL, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x7fc404f7d8d0}, 8) =
0
[{fd=9, events=POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND}], 1, 0) = 0 (Timeout)
rt_sigaction(SIGPIPE, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x7fc404f7d8d0}, NULL, 8) = 0
poll([{fd=9, events=POLLIN}], 1, 1000) = 0 (Timeout)
rt_sigaction(SIGPIPE, NULL, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x7fc404f7d8d0}, 8) = 0
rt_sigaction( SIGPIPE, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x7fc404f7d8d0}, NULL, 8) = 0
poll([{fd=9, events=POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND}], 1, 0) = 0 (Timeout )
rt_sigaction(SIGPIPE, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x7fc404f7d8d0}, NULL, 8) = 0
poll([{fd=9, events=POLLIN}], 1, 1000) = 0 (Timeout)

The main problem is - raise the service back, the port is busy - until you kill the php process, the port is free
. Moreover, periodically different php processes can occupy this port on all interfaces, including ipv6

Why does this happen and how to deal with it?

Answer 1

Ports for outgoing connections are by default selected from the range 32768-60999 (set by sysctl net.ipv4.ip_local_port_range).
The port 50100 you're using falls into this range, and when PHP tries to connect to a downed service, it eventually hits that port and connects to itself.
You can solve the problem by setting the port range in curl:
curl_setopt($ch, CURLOPT_LOCALPORT, 12345); // Start port
curl_setopt($ch, CURLOPT_LOCALPORTRANGE, 10); // Number of ports in the range
In this example, ports 12345 to 12355 will be used.