Centos – Limit concurrent connections to Postfix server

centosconcurrencypostfixsmtpd

I'm having trouble limiting the number of concurrent connections from the same client to my Postfix server. Limiting the maximal number of concurrent processes, from the same or different clients, is done easily.

Here are the two parameters that I'm using on main.cf to do so:

default_process_limit = 50
smtpd_client_connection_count_limit = 25

When testing, I run 100 concurrent connections (threads) from the same client, but it limits at 50 smtpd processes instead of just 25. Here is how I'm counting the smtpd processes at the server:

ps -C smtpd | wc -l

Does anyone know why this happens?

Best Answer

If you want to change the maximal number of smtpd running you should make your changes in /etc/postfix/master.cf. You change - to 50 in the 7th column then restart postfix with postfix restart (or a distro specific substitute).

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
smtp      inet  n       -       -       -       -       smtpd

change this to

smtp      inet  n       -       -       -       50      smtpd

smtpd_client_connection_count_limit limits how many connections an smtp client can make.

It does not say how many smtpd servers will run by default. (smtpd_client_connection_count_limit is by default half of the default process limit.)

Related Solutions

How to “filter” postfix-generated bounce messages

In this case you do it too complicated. This is the easy way.

You need to have this (additional) in your main.cf:

notify_classes = resource, software, bounce, 2bounce
bounce_notice_recipient = random.bounce@example.com
2bounce_notice_recipient = random.bounce@example.com
transport_maps = hash:/etc/postfix/transport_maps   #or another file. See below.

in the /etc/postfix/transport_maps put

random.bounce@example.com    smtp:127.0.0.2:25

then postmap /etc/postfix/transport_maps. Afterwards postfix reload or restart Postfix.

This will notify the random.bounce@example.com user of all bounces and as this user is said to be transported through 127.0.0.2:25 you get all these mails through that server. Replace the addresses/IP/port according to your environment.

Linux – NGINX timeout after +200 concurrent connections

You will need to dump your network connections during the test. While the server may have near zero load, your TCP/IP stack could be billing up. Look for TIME_WAIT connections in a netstat output.

If this is the case, then you will want to check into tuning tcp/ip kernel paramters relating to TCP Wait states, TCP recyling, and similar metrics.

Also, you have not described what is being tested.

I always test:

static content (image or text file)
simple php page (phpinfo for example)
application page

This may not apply in your case but is something I do when performance testing. Testing different types of files can help you pinpoint the bottlneck.

Even with static content, testing different size of files is important as well to get timeouts and other metrics dialed in.

We have some static content Nginx boxes handling 3000+ active connections. So it Nginx can certainly do it.

Update: Your netstat shows a lot of open connections. May want to try tuning your TCP/IP stack. Also, what file are you requesting? Nginx should quickly close the port.

Here is a suggestion for sysctl.conf:

net.ipv4.ip_local_port_range = 1024 65000
net.ipv4.tcp_rmem = 4096 87380 8388608
net.ipv4.tcp_fin_timeout = 30
net.ipv4.tcp_keepalive_time = 30
net.ipv4.tcp_tw_recycle = 1
net.ipv4.tcp_tw_reuse = 1

These values are very low but I have had success with them on high concurrency Nginx boxes.

Best Answer

Related Solutions

How to “filter” postfix-generated bounce messages

Linux – NGINX timeout after +200 concurrent connections

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