Home > Articles

This chapter is from the book

This chapter is from the book

7.4 syslog

The syslog facility isn’t a program used to evaluate system performance, but rather a set of library calls and a daemon, syslogd, that records information that the system and its programs think is worth logging. All email server applications mentioned in this book, but especially sendmail, use syslog to log data about their behavior, and it would be unwise for an email administrator to ignore this fact. The syslog package was originally developed as a part of the sendmail distribution to help maintain the information it would log. It was later adopted by the rest of Berkeley UNIX around the BSD 4.1 timeframe, and from there spread to other UNIX versions. Now it has become so ubiquitous that the fact that its origin is tied to sendmail has been largely forgotten.

7.4.1 syslog and sendmail

During an SMTP message reception, sendmail logs the sender information at the end of the message transaction, whether the message is actually sent or not. If the message is accepted, then the log entry occurs after the end of the DATA phase. If the message is rejected, then the log entry is made immediately after the rejection. During a successful SMTP message reception, sendmail logs the recipient information at the end of the session, although the precise timing can vary depending on sendmail’s delivery mode. At the conclusion of each failed delivery attempt, that attempt is also logged. At least two log entries for each successful delivery and one log entry for each unsuccessful attempt, plus various other entries for start-up, encountering errors, STARTTLS information, and so on, will be made. The two types of log entries mentioned initially make up the bulk of the log messages generated, however, and are the two that occur as a result of a successful delivery. All of these log messages can result in a lot of data, and this information can provide significant insight into what is happening on the server.

Here is an example sendmaillog entry for a successful message delivery:

Mar 12 14:39:49 discovery sendmail[44639]: g2CMdnkq044639: 
 from=<npc@acm.org>, size=1047, class=0, nrcpts=1,
 proto=ESMTP, daemon=MTA, relay=mail.acm.org [] 
Mar 12 14:39:49 discovery sendmail[44641]: g2CMdnkq044639:
 to=<npc@gangofone.com>, delay=00:00:00, xdelay=00:00:00, 
 mailer=local, pri=30052, dsn=2.0.0, stat=Sent

The first question asked might be, “How do we count the total number of messages that flow through the system?” This question isn’t as simple to answer as it might seem. The answer depends on whether one wants to count the number of SMTP connections, the number of unique messages as sent by a sender, or the number of messages (often sent to multiple recipients) that end up in someone’s mailbox somewhere. Each of these metrics is a valid choice, but in my judgment the bulk of the work is done for each successful message recipient, so I generally choose to count the number of syslog entries with both the to= pattern and stat=Sentin them. I call that measure the number of messages that the system has successfully processed, mindful that it is merely one statistic that is much more nebulous than it appears at first glance.

If we consider the format of these log entries to contain a set of fields delimited by whitespace, the first three fields contain information about the date and time when the log entry was made. This information can be parsed to track the busiest time of day for the server. In the fromentry, the eighth field contains the size of the message in bytes, which we can use to find out the average message size handled by the system. On the same entry, the tenth field lists the number of recipients per message, another interesting statistic to track. In the toentry, the information in the delayand xdelayfields are of particular interest. The delayfield measures the total amount of elapsed time between the receipt of the message and this particular delivery attempt. The xdelayfield, which stands for transaction delay, measures the amount of time consumed on this particular delivery attempt, which should reveal something about the current connectivity to a particular site.

A great deal more information available in the logs can be extracted for various purposes, but at this point the next step will be left to the imagination of the reader. Section 2.1.1 of the Sendmail Installation and Operation Guide provides additional information on the sendmaillog entries.

A similar set of information can be extracted from the logs left by any of the POP or IMAP daemons discussed in this book. Combined with other statistical information gathered with the tools described here, one can plot number of processes versus load average, connection rates versus disk activity, and so on to obtain a thorough understanding of any email server’s performance. These checks can be easily automated, and at least the most basic ones should be part of an email administrator’s baselining effort.

7.4.2 syslogand Performance

If a server handles a large volume of email, the resources consumed by syslogin writing out the many log entries can be significant. On very large servers, mounting /var/logor its equivalent on its own disk might be appropriate. Beyond this point, an additional syslogissue directly affects performance that should be mentioned. On Linux systems, by default the syslog daemon will fsync() its log files after each entry is written to them. On a busy email server, this operation can cause a measurable slowdown. In most organizations, email server logs aren’t so critical. This behavior can be switched off by preceding the appropriate entry in the /etc/syslog.conf file with “-”:

mail.* -/var/adm/mail

If logging continues to pose a performance problem for a host, it may be appropriate to log the information to a dedicated remote logging host. If this step is taken, replacing the mail. entries in /etc/syslog.conf with one like the following may be appropriate:

mail.* @loghost.example.com

The loghost.example.com machine may end up aggregating log information for a large number of hosts. Because the host name is included in each log entry, it should be straightforward to split the entries out again on the log host if desired. On the log host, a RAID system with a high-performance filesystem may be mounted on /var/logto handle this load.

One downside to remote logging is that syslog sends its messages to the log host using UDP. Thus, if a log message becomes lost en route, it will not be retransmitted. This behavior makes this method less useful if saving each log message is critical. One way to work around it is to replace the default syslog daemon with syslog-ng [SYS] or one of several packages with similar feature sets that support logging over TCP.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020