On 27.03.2020 14:27, André Warnier (tomcat/perl) wrote:
On 26.03.2020 20:42, Eric Robinson wrote:-----Original Message----- From: Olaf Kock <tom...@olafkock.de> Sent: Thursday, March 26, 2020 2:06 PM To: users@tomcat.apache.org Subject: Re: Does Tomcat/Java get around the problem of 64K maximum client source ports?Hi Eric, On 26.03.20 18:58, Eric Robinson wrote:Greetings, Many people say the maximum number of client ports is 64K. However,TCP connections only require unique sockets, which are defined as...local_IP:local_port -> remote_ip:remote_port Theoretically, it is possible for a client process to keep using the same localsource port, as long as the connections are to a unique destinations. For example on a local machine, the following connections should be possible...192.168.5.100:1400 -> 192.168.5.200:3306 192.168.5.100:1400 -> 192.168.5.201:3306 192.168.5.100:1400 -> 192.168.5.202:3306 192.168.5.100:1400 -> 192.168.5.203:3306 I've seen this demonstrated successfully here: https://serverfault.com/questions/326819/does-the-tcp-source-port-have -to-be-unique-per-host As someone on that page pointed out, while it is possible, it does notcommonly occur in practice "because most TCP APIs don't provide a way to create more than one connection with the same source port, unless they have different source IP addresses." This leads to the 64K maximum client port range, but it is really a limitation of the APIs, not TCP.So how does tomcat handle things? Is it limited to a maximum 64K clientsource ports, or is it 64K per destination, as it should be? To be honest, I can't remember to have seen a web- or application server that accepts 64K concurrent connections at all, let alone to a single client. I can't come up with any reason to do so, I'd assume that there's a DOS attack if I get 100 concurrent incoming connections from a single IP, and a programming error if the server sets up more than 1K outgoing connections Maybe I'm missing the obvious, or have only administered meaningless installations, but I fail to see the real world relevance of this question.I don't blame you for being puzzled, but this not about tomcat accepting connections. It's about tomcat acting as the client, where MySQL is the server. I'm referring to client connections from tomcat to MySQL. We have about 1800 instances of tomcat running. This question comes up once in a while when tomcat can't connect to MySQL. Trust me, it can be an issue.--EricThe question is : is there even any Java API (method) (or even OS API) which allows a client to open a (client, non-LISTEN) socket AND specify the client IP address and/or port ?I mean, if there is none, then the question may be interesting in the absolute, but ultimately pointless.I believe that the IP address of client packets, will be determined by which /route/ the OS determines that the target server address can be reached (which will determine through which network interface the packets "go out", which will determine the sender IP address inserted in the packets). I don't think that the application-level software (here a java webapp) can determine this in advance. And the client port will be decided by the OS-level TCP stack, in function of which ones are not yet in use (which a java webapp can also not determine in advance).Example of creating a client socket : Socket echoSocket = new Socket(hostName, portNumber);The hostname (or IP address of ditto) and port numbers passed as arguments, are the IP:port of the server you are connecting /to/, not the ones of the local socket.
Addendum : https://stackoverflow.com/questions/11129212/tcp-can-two-different-sockets-share-a-port/11129641From which I gather that, although it may be possible (in some languages or using some API) to open several client connections (to different target IP/port) using the same local port number, you may still have other issues when doing this : - the host's TCP/IP stack has to keep track of all the simultaneous connections, and if the list gets very large, this may become very resource-intensive
- there may be a limit at the OS level, to how many sockets are allowed at the same time- to each connection, corresponds some kind of a "file descriptor". There may be limits, at the process level (in this case the JVM running tomcat), or even at the host level, to how many of those may be in use at any one time (So if you were running on a host one JVM for a tomcat instance, it would compete with other processes running on the same host and also using file descriptors)
It seems also thus possible that the reason why you are running against a problem not being able to make additional connections to a MySql server, is not necessarily the usage of all 64K port numbers, but one of the other limitations above.
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