Hello,
tl;dr: Postfix keeps open unix sockets that seem useless to me. Why?
First note I am new to postfix, so I may be wrongly interpreting some
commands; but using it on a VPS with a limit of 500 non-TCP sockets
triggered the limit. So I tried to search why it is using so many sockets.
It currently uses 65 unix-domain sockets:
# ss -exmp | grep master | wc -l
65
Among these, two are shared with other processes:
# ss -exmp | grep master
[...]
u_str ESTAB 0 0 * 329917704
* 0 users:(("qmgr",5104,5),("master",5093,33))
[...]
u_str ESTAB 0 0 * 329917708
* 0 users:(("tlsmgr",5315,5),("master",5093,36))
[...]
The 63 others are all (almost) the same line:
# ss -exmp | grep master
u_str ESTAB 0 0 * 329917676
* 0 users:(("master",5093,15))
u_str ESTAB 0 0 * 329917677
* 0 users:(("master",5093,16))
u_str ESTAB 0 0 * 329917685
* 0 users:(("master",5093,19))
[...]
This line seems weird to me: it is a socket with a connection that is
established, but used only by "master".
Now looking at netstat's output:
# netstat -anx --program | grep "5093/master"
unix 2 [ ACC ] STREAM LISTENING 329917757
5093/master private/retry
[... Normal sockets associated to a file]
unix 2 [ ACC ] STREAM LISTENING 329917753
5093/master private/error
unix 3 [ ] STREAM CONNECTED 329917808 5093/master
unix 3 [ ] STREAM CONNECTED 329917807 5093/master
unix 3 [ ] STREAM CONNECTED 329917804 5093/master
unix 3 [ ] STREAM CONNECTED 329917803 5093/master
unix 3 [ ] STREAM CONNECTED 329917800 5093/master
unix 3 [ ] STREAM CONNECTED 329917799 5093/master
unix 3 [ ] STREAM CONNECTED 329917796 5093/master
unix 3 [ ] STREAM CONNECTED 329917795 5093/master
[... This goes on for a number of lines]
unix 3 [ ] STREAM CONNECTED 329917676 5093/master
unix 2 [ ] DGRAM 329917651 5093/master
# # Getting more precisely the number of sockets not associated to files
# netstat -anx --program | grep "5093/master[[:space:]]*$" | wc -l
65
So I guess the 65 sockets without file listed by netstat are the same as
the 63+2 sockets listed by ss.
Which means 63 sockets are open by master, yet are neither reachable
through a file nor currently used by any process.
Finally, it is not even pre-reserving sockets so as not to have to
allocate new ones after startup:
# postconf -nf
[...]
default_process_limit = 1
[...]
# postconf -Mf
smtp inet n - - - - smtpd
[... Summing the last digits (with a 1 instead of a -) yields 28]
So postfix would need at most 28 sockets to communicate, should all the
processes be started at the same time. Thus not needing 65.
There are still two reasons why sockets could be needed: authentication
is managed through dovecot, and user and domain management is managed
through mysql. However, I still could not understand why 63 sockets
should just lie around, neither bound nor bindable to.
Is there a reason unknown to me, why these sockets exist? (they are
re-created after a postfix reboot)
Thanks in advance,
Leo Gaspard
