Can two applications listen to the same port

Question

Can two applications on the same machine bind to the same port and IP address  Taking it a step further  can one app listen to requests coming from a certain IP and the other to another remote IP   I know I can have one application that starts off two threads  or forks  to have similar behavior  but can two applications that have nothing in common do the same

User · Answer

Another way is use a program listening in one port that analyses the kind of traffic (ssh, https, etc) it redirects internally to another port on which the "real" service is listening.

For example, for Linux, sslh: https://github.com/yrutschle/sslh

User · Answer

If by applications you mean multiple processes then yes but generally NO    For example Apache server runs multiple processes on same port  generally 80  It s done by designating one of the process to actually bind to the port and then use that process to do handovers to various processes which are accepting connections

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I have tried the following  with socat   socat TCP-L 8080 fork reuseaddr -   And even though I have not made a connection to the socket  I cannot listen twice on the same port  in spite of the reuseaddr option   I get this message  which I expected before    2016 02 23 09 56 49 socat 2667  E bind 5   AF 2 0 0 0 0 8080   16   Address already in use

User · Answer

Yes and no   Only one application can actively listen on a port   But that application can bequeath its connection to another process   So you could have multiple processes working on the same port

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When you create a TCP connection  you ask to connect to a specific TCP address  which is a combination of an IP address  v4 or v6  depending on the protocol you re using  and a port   When a server listens for connections  it can inform the kernel that it would like to listen to a specific IP address and port  i e   one TCP address  or on the same port on each of the host s IP addresses  usually specified with IP address 0 0 0 0   which is effectively listening on a lot of different  TCP addresses   e g   192 168 1 10 8000  127 0 0 1 8000  etc    No  you can t have two applications listening on the same  TCP address   because when a message comes in  how would the kernel know to which application to give the message   However  you in most operating systems you can set up several IP addresses on a single interface  e g   if you have 192 168 1 10 on an interface  you could also set up 192 168 1 11  if nobody else on the network is using it   and in those cases you could have separate applications listening on port 8000 on each of those two IP addresses

User · Answer

The answer differs depending on what OS is being considered  In general though   For TCP  no  You can only have one application listening on the same port at one time  Now if you had 2 network cards  you could have one application listen on the first IP and the second one on the second IP using the same port number   For UDP  Multicasts   multiple applications can subscribe to the same port   Edit  Since Linux Kernel 3 9 and later  support for multiple applications listening to the same port was added using the SO REUSEPORT option  More information is available at this lwn net article

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Short answer   Going by the answer given here  You can have two applications listening on the same IP address  and port number  so long one of the port is a UDP port  while other is a TCP port   Explanation   The concept of port is relevant on the transport layer of the TCP IP stack  thus as long as you are using different transport layer protocols of the stack  you can have multiple processes listening on the same  lt ip-address gt   lt port gt  combination   One doubt that people have is if two applications are running on the same  lt ip-address gt   lt port gt  combination  how will a client running on a remote machine distinguish between the two  If you look at the IP layer packet header  https   en wikipedia org wiki IPv4 Header   you will see that bits 72 to 79 are used for defining protocol  this is how the distinction can be made   If however you want to have two applications on same TCP  lt ip-address gt   lt port gt  combination  then the answer is no  An interesting exercise will be launch two VMs  give them same IP address  but different MAC addresses  and see what happens - you will notice that some times VM1 will get packets  and other times VM2 will get packets - depending on ARP cache refresh    I feel that by making two applications run on the same  lt op-address gt   lt port gt  you want to achieve some kind of load balancing  For this you can run the applications on different ports  and write IP table rules to bifurcate the traffic between them    Also see  user6169806 s answer

User · Answer

In principle  no   It s not written in stone  but it s the way all APIs are written  the app opens a port  gets a handle to it  and the OS notifies it  via that handle  when a client connection  or a packet in UDP case  arrives   If the OS allowed two apps to open the same port  how would it know which one to notify   But    there are ways around it    As Jed noted  you could write a  master  process  which would be the only one that really listens on the port and notifies others  using any logic it wants to separate client requests    On Linux and BSD  at least  you can set up  remapping  rules that redirect packets from the  visible  port to different ones  where the apps are listening   according to any network related criteria  maybe network of origin  or some simple forms of load balancing

User · Answer

Yes Definitely  As far as i remember From kernel version 3 9  Not sure on the version  onwards support for the SO REUSEPORT was introduced  SO RESUEPORT allows binding to the exact same port and address  As long as the first server sets this option before binding its socket    It works for both TCP and UDP  Refer to the link for more details  SO REUSEPORT  Note  Accepted answer no longer holds true as per my opinion

User · Answer

Yes   From this article  https   lwn net Articles 542629      The new socket option allows multiple sockets on the same host to bind to the same port

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If at least one of the remote IPs is already known  static and dedicated to talk only to one of your apps  you may use iptables rule  table nat  chain PREROUTING  to redirect incomming traffic from this address to  shared  local port to any other port where the appropriate application actually listen

User · Answer

You can make two applications listen for the same port on the same network interface   There can only be one listening socket for the specified network interface and port  but that socket can be shared between several applications   If you have a listening socket in an application process and you fork that process  the socket will be inherited  so technically there will be now two processes listening the same port

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Just to share what  jnewton mentioned  I started an nginx and an embedded tomcat process on my mac  I can see both process runninng at 8080    LT lt XXXX gt -MAC   b0 lt XXX gt   sudo netstat -anp tcp   grep LISTEN tcp46      0      0    8080                                        LISTEN      tcp4       0      0    8080                                        LISTEN

User · Answer

No   Only one application can bind to a port at a time  and behavior if the bind is forced is indeterminate   With multicast sockets -- which sound like nowhere near what you want -- more than one application can bind to a port as long as SO REUSEADDR is set in each socket s options   You could accomplish this by writing a  master  process  which accepts and processes all connections  then hands them off to your two applications who need to listen on the same port   This is the approach that Web servers and such take  since many processes need to listen to 80   Beyond this  we re getting into specifics -- you tagged both TCP and UDP  which is it  Also  what platform

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You can have one application listening on one port for one network interface  Therefore you could have    httpd listening on remotely accessible interface  e g  192 168 1 1 80 another daemon listening on 127 0 0 1 80   Sample use case could be to use httpd as a load balancer or a proxy

User · Answer

Yes  for TCP  you can have two programs listen on the same socket  if the programs are designed to do so  When the socket is created by the first program  make sure the SO REUSEADDR option is set on the socket before you bind    However  this may not be what you want  What this does is an incoming TCP connection will be directed to one of the programs  not both  so it does not duplicate the connection  it just allows two programs to service the incoming request  For example  web servers will have multiple processes all listening on port 80  and the O S sends a new connection to the process that is ready to accept new connections   SO REUSEADDR   Allows other sockets to bind   to this port  unless there is an active listening socket bound to the port already  This enables you to get around those  Address already in use  error messages when you try to restart your server after a crash

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Yes    Multiple listening TCP sockets  all bound to the same port  can co-exist  provided they are all bound to different local IP addresses  Clients can connect to whichever one they need to  This excludes 0 0 0 0  INADDR ANY   Multiple accepted sockets can co-exist  all accepted from the same listening socket  all showing the same local port number as the listening socket  Multiple UDP sockets all bound to the same port can all co-exist provided either the same condition as at  1  or they have all had the SO REUSEADDR option set before binding  TCP ports and UDP ports occupy different namespaces  so the use of a port for TCP does not preclude its use for UDP  and vice versa    Reference  Stevens  amp  Wright  TCP IP Illustrated  Volume II

[sockets] Can two applications listen to the same port?

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