Re: flexfax: Cannot create data socket (127.0.0.1, 4558): Address already in use

[David Woolley <david@djwhome.demon.co.uk>]

> How can I make sure this error does not occur? Is it a configuration
> error in Sendmail or HylaFAX or a bug? Is it absolutely necessary that

As Hylafax uses SO_REUSEADDR, I assume it is a bug in your OS (Linux
has this bug).  The OS is probably using 4558 as a dynamic port number,
when it should be using port numbers which start just a little below 
50,000, up to the maximum.

> the data socket always is on port 4558 rather than an arbitrary free
> socket? Would a passive connection solve the problem? If so, where can I

This is the same protocol as used by FTP.  FTP uses a port in the
privileged server range, whereas Hylafax uses one in the unprivileged
server range.  Although it could probably retry, this would fall
foul of any firewall configuration set up to permit the use of the
standard port.

> find some documentation about this feature?

Try Unix Network Programming.