Re: locating blocked port

[Haines Brown <haines@histomat.net>]

On Tue, Jan 31, 2023 at 08:31:38AM -0700, Casey Deccio wrote:
> 
> 
> > On Jan 31, 2023, at 8:05 AM, Haines Brown <haines@histomat.net> wrote:
> > 
> > I have an  application that refuses to start because  its port is 
> > blocked. But I have difficulty knowing what port it is

> I would try strace, which shows you all system calls being made.  In 
> this case, it is probably bind() that is returning an error.

I do 	 

  $ strace -e trace=%net java -jar /usr/local/share/JabRef/JabRef-3.2.jar 2>&1 | grep bind
        at java.base/sun.nio.ch.Net.bind0(Native Method)
        at java.base/sun.nio.ch.Net.bind(Net.java:555)
        at java.base/sun.nio.ch.Net.bind(Net.java:544)
        at java.base/sun.nio.ch.NioSocketImpl.bind(NioSocketImpl.java:643)
        at java.base/java.net.ServerSocket.bind(ServerSocket.java:388)

But this does not tell me what is bound to what port. 

> For example:
> 
> $ cat test.py 
> #!/usr/bin/env python3

In /usr/bin there is a jabref file, but it does not specify what port 
it uses.

> import socket
> s = socket.socket()
> try:
>     s.bind(('0.0.0.0', 56))
> except:
>     pass
> $ python3 test.py # doesn't print any output

> $ strace -e trace=%net python3 test.py 2>&1 | grep bind
> bind(3, {sa_family=AF_INET, sin_port=htons(56), sin_addr=inet_addr("0.0.0.0")}, 16) = -1 EACCES (Permission denied)
> 
> The value of sin_port is what you are looking for.

My run of the strace command as above does not return the string 
sin_port.,
> 
> > How do I know from this what port the java application tried to use?

> >  So I try:
> > 
> >    $ sudo ss -pt state listening 'sport = :56'
> >    Recv-Q   Send-Q    Local Address:Port   Peer Address:Port Process
> > 
> >  This seems a null return. Dores this mean jabref is not using port 
> >  56?
> 
> Well, it tells you that nothing (including jabref) is listening on 
> TCP port 56, but it won't tell you about why something *failed* to listen.  See strace above. 

I don't see in refurn for strace that anyhing is not listening to 
port 56.

 Casey, thanks for your effort to help, but I'm too ill infomred to 
benefit from your advice.

-- 

     Haines Brown 
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