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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[DOCS-5196] maxIncomingConnections has neither a default or maximum value of 65536</title>
                <link>https://jira.mongodb.org/browse/DOCS-5196</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;p&gt;The docs for 3.0 incorrectly state that the default value of maxIncomingConnections is 65536.  This is based on &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-5026&quot; title=&quot;Fix 1MM net.maxIncomingConnections&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-5026&quot;&gt;&lt;del&gt;DOCS-5026&lt;/del&gt;&lt;/a&gt;, however that ticket is wrong on two levels, and should be reverted.&lt;/p&gt;

&lt;ol&gt;
	&lt;li&gt;&lt;b&gt;The &lt;a href=&quot;https://github.com/mongodb/mongo/blob/3ec53687da4702d348f1b55eda5e01bd26f36c4b/src/mongo/util/net/listen.h#L46&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;default value in the code is 1000000&lt;/a&gt;&lt;/b&gt;, the same as in 2.6:
&lt;p/&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;src/mongo/util/net/listen.h&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;46:    const int DEFAULT_MAX_CONN = 1000000;&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;
&lt;p&gt;maxIncomingConnections is simply an upper bound that the server applies, in addition to anything the OS might do.  Any other upper bounds that might happen to be imposed by any given system/environment/transport are irrelevant for the maxIncomingConnections server option.  (At most, they could be noted, but certainly the &lt;em&gt;default&lt;/em&gt; server config value should not be misdocumented because of them.)
&lt;br class=&quot;atl-forced-newline&quot; /&gt; &lt;br class=&quot;atl-forced-newline&quot; /&gt;&lt;/p&gt;&lt;/li&gt;
	&lt;li&gt;&lt;b&gt;TCP has no limit of 65536 incoming connections to a single destination port.&lt;/b&gt;  Even if it did, Unix Domain Sockets (UDS), which are a valid transport to use in some cases, have no notoin of &quot;ports&quot; and so no such limit.  Perhaps this has been confused with the maximum number of source (aka ephemeral) ports that can be used for &lt;b&gt;outgoing&lt;/b&gt; connections?
&lt;br class=&quot;atl-forced-newline&quot; /&gt; &lt;br class=&quot;atl-forced-newline&quot; /&gt;
Since the &quot;TCP is limited to 64k ports&quot; notion is a common misconception, I have proven it to be false empirically by creating ~126k TCP connections to a standalone mongod.  The logfile is attached.
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;2015-04-10T04:26:58.296+0000 I NETWORK  [initandlisten] connection accepted from 172.31.2.238:28517 #127009 (126802 connections now open)&lt;/span&gt;&lt;/pre&gt;
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&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;This isn&apos;t easy &amp;#8212; you need more than 1 client host connecting to the server, and there are a whole raft of settings that have to be fiddled with (on both the server and client hosts), but it&apos;s certainly possible.  The client hosts only need to do something like&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;a = []; for (i=0; i&amp;lt;100000;i++) a.push(connect(&quot;172.31.19.134:27017/test&quot;))&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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&lt;p/&gt;
&lt;p&gt;in a &lt;tt&gt;mongo --nodb&lt;/tt&gt; shell.  In fact if you look carefully in the logs you&apos;ll see where the clients stop connecting for a while.  This is because they ran out of ephemeral ports, which is by default 32768-61000 on Linux, and I had to increase it to 1-65535 and start the mongo shells connecting again.
&lt;br class=&quot;atl-forced-newline&quot; /&gt; &lt;br class=&quot;atl-forced-newline&quot; /&gt;
In the past I have managed to get to &lt;a href=&quot;https://twitter.com/_devkev_/status/550108800422641664&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;~586k connections&lt;/a&gt; (using Unix Domain Sockets, because it was easier) before the medium-sized host crashed outright.&lt;/p&gt;&lt;/li&gt;
&lt;/ol&gt;
</description>
                <environment></environment>
        <key id="195229">DOCS-5196</key>
            <summary>maxIncomingConnections has neither a default or maximum value of 65536</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="3">Duplicate</resolution>
                                        <assignee username="kay.kim@mongodb.com">Kay Kim</assignee>
                                    <reporter username="kevin.pulo@mongodb.com">Kevin Pulo</reporter>
                        <labels>
                            <label>groom</label>
                    </labels>
                <created>Fri, 10 Apr 2015 05:41:29 +0000</created>
                <updated>Mon, 30 Oct 2023 21:46:20 +0000</updated>
                            <resolved>Fri, 1 Apr 2022 20:29:10 +0000</resolved>
                                                    <fixVersion>Server_Docs_20231030</fixVersion>
                                    <component>manual</component>
                    <component>Server</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="4451547" author="JIRAUSER1264464" created="Fri, 1 Apr 2022 20:29:10 +0000"  >&lt;p&gt;See &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14280&quot; title=&quot;Documentation incorrectly states that configuration parameter net.maxIncomingConnections has a default value of 65536&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14280&quot;&gt;&lt;del&gt;DOCS-14280&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;</comment>
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                                        <issuelink>
            <issuekey id="189927">DOCS-5026</issuekey>
        </issuelink>
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                            <attachment id="70329" name="mongodb.log" size="17312059" author="kevin.pulo@mongodb.com" created="Fri, 10 Apr 2015 05:41:29 +0000"/>
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