英文:
Sendmail has at least a 5 second delay before accepting emails. On Ubuntu, not Redhat
问题
我配置了一台全新安装的运行Sendmail的Ubuntu 22服务器。当我将电子邮件发送到25端口时，Sendmail响应其“SMTP Sendmail greeting”并开始接受电子邮件之间至少有5秒的延迟。以下是TCPDUMP的输出。我在其他旧版本的Ubuntu上也发现了类似的问题，它们运行的是Sendmail 8.15.2。
在运行Sendmail 8.14.7的旧Redhat 7邮件服务器上并没有发生这种情况。我不确定这是Sendmail还是操作系统的问题。我需要配置一台新的Redhat 9服务器进行比较，但我想尽量避免这样做。感谢任何建议。这种延迟会让使用Thunderbird的人感到不安，因为Thunderbird会显示一个发送进度弹窗（Outlook会隐藏它）。如果我安装的新服务器明显比旧服务器差，Thunderbird用户将不会满意！谢谢。
tcpdump -v host xxx.psfc.mit.edu
17:09:31.959120 IP（tos 0x0，ttl 127，id 45733，offset 0，flags [DF]，proto TCP（6），length 40）
xxx.psfc.mit.edu.59375 > psfcmail-test.psfc.mit.edu.smtp：Flags [.], cksum 0x00b3（正确），ack 1，win 8212，length 0
17:09:36.996785 IP（tos 0x0，ttl 64，id 10232，offset 0，flags [DF]，proto TCP（6），length 240）
psfcmail-test.psfc.mit.edu.smtp > xxx.psfc.mit.edu.59375：Flags [P.]，cksum 0xee78（不正确-> 0xbf0f），seq 1:201，ack 1，win 502，length 200：SMTP，length: 200
220 psfcmail-test.psfc.mit.edu ESMTP Sendmail 8.15.2/8.15.2/Debian-22ubuntu3; Tue, 7
英文:
I configured a fresh install of an Ubuntu 22 server that runs Sendmail. There is at least a constant 5 second, between when I send an email to port 25 and when sendmail responds with it's "SMTP Sendmail greeting" and starts accepting the email. The output of the TCPDUMP is shown below. Older versions of Ubuntu different computers that I have, show a similar issue, running Sendmail 8.15.2.
This does not happen on my old Redhat 7 email server, running Sendmail 8.14.7 I don't know if this is a Sendmail or OS issue. I would have to configure a new Redhat 9 server to compare with. Something that I would like to avoid doing. Thanks for any suggestions. This delay will annoy people running Thunderbird, as Thunderbird display a popup sending bar progress window (Outlook hides it). Thunderbird users will not be happy, if I install a new server that is clearly worse than the old one! Thanks.
tcpdump -v host xxx.psfc.mit.edu
17:09:31.959120 IP (tos 0x0, ttl 127, id 45733, offset 0, flags [DF], proto TCP (6), length 40)
xxx.psfc.mit.edu.59375 > psfcmail-test.psfc.mit.edu.smtp: Flags [.], cksum 0x00b3 (correct), ack 1, win 8212, length 0
17:09:36.996785 IP (tos 0x0, ttl 64, id 10232, offset 0, flags [DF], proto TCP (6), length 240)
psfcmail-test.psfc.mit.edu.smtp > xxx.psfc.mit.edu.59375: Flags [P.], cksum 0xee78 (incorrect -> 0xbf0f), seq 1:201, ack 1, win 502, length 200: SMTP, length: 200
220 psfcmail-test.psfc.mit.edu ESMTP Sendmail 8.15.2/8.15.2/Debian-22ubuntu3; Tue, 7
答案1
得分: 1
5秒的延迟使身份查询成为第一个嫌疑对象（请参阅sendmail FAQ），而DNS解析器问题成为第二个嫌疑对象。
- 在现代sendmail配置中，5秒被用作身份查询的默认超时时间。
- 当第一个DNS解析器没有响应时，通常需要5秒来向第二个DNS解析器发送DNS查询。
Sendmail-FAQ：为什么连接到SMTP端口需要这么长时间？
英文:
5s delay makes ident queries the fist suspect (see sendmail FAQ) and DNS resolver problem the second.
- In modern sendmail configuration 5s is used as default timeout for ident queries.
- It takes usually 5s to send DNS query to second DNS resolver when the first does not respond.
Sendmail-FAQ: Why do connections to the SMTP port take such a long time?
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