What is the theoretical maximum number of open TCP connections that a modern Linux box can have

Question

Assuming infinite performance from hardware  can a Linux box support  65536 open TCP connections   I understand that the number of ephemeral ports   lt 65536  limits the number of connections from one local IP to one port on one remote IP   The tuple  local ip  local port  remote ip  remote port  is what uniquely defines a TCP connection  does this imply that more than 65K connections can be supported if more than one of these parameters are free   e g  connections to a single port number on multiple remote hosts from multiple local IPs   Is there another 16 bit limit in the system   Number of file descriptors perhaps

User · Answer

If you are thinking of running a server and trying to decide how many connections can be served from one machine  you may want to read about the C10k problem  and the potential problems involved in serving lots of clients simultaneously

User · Answer

A single listening port can accept more than one connection simultaneously  There is a  64K  limit that is often cited  but that is per client per server port  and needs clarifying  Each TCP IP packet has basically four fields for addressing  These are  source ip source port destination ip destination port  lt ----- client ------ gt   lt --------- server ------------ gt   Inside the TCP stack  these four fields are used as a compound key to match up packets to connections  e g  file descriptors   If a client has many connections to the same port on the same destination  then three of those fields will be the same - only source port varies to differentiate the different connections   Ports are 16-bit numbers  therefore the maximum number of connections any given client can have to any given host port is 64K  However  multiple clients can each have up to 64K connections to some server s port  and if the server has multiple ports or either is multi-homed then you can multiply that further  So the real limit is file descriptors   Each individual socket connection is given a file descriptor  so the limit is really the number of file descriptors that the system has been configured to allow and resources to handle   The maximum limit is typically up over 300K  but is configurable e g  with sysctl  The realistic limits being boasted about for normal boxes are around 80K for example single threaded Jabber messaging servers

User · Answer

If you used a raw socket  SOCK RAW  and re-implemented TCP in userland  I think the answer is limited in this case only by the number of  local address  source port  destination address  destination port  tuples   2 64 per local address    It would of course take a lot of memory to keep the state of all those connections  and I think you would have to set up some iptables rules to keep the kernel TCP stack from getting upset  amp  or responding on your behalf

[tcp] What is the theoretical maximum number of open TCP connections that a modern Linux box can have

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