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+# Syslog protocol support {#syslog_doc}
+
+VPP provides [RFC5424](https://tools.ietf.org/html/rfc5424) syslog protocol
+logging, which is used to transport event messages across network. VPP
+currently suports UDP transport based on
+[RFC5426](https://tools.ietf.org/html/rfc5426).
+
+The syslog message has the following format:
+* header
+* structured data
+* free-form message
+
+The header contains, priority, version, timestamp, hostname, application,
+process id and message id. It is followed by structured data, which  provides
+a mechanism to express event data in easily parsable format. Structured data
+can contain zero, one or multiple structured data elements. Structured data
+element contains name-value pairs. Structured data can by followed by free-form
+message.
+
+Following example explains how to use the internal APIs to genrate syslog
+message:
+```{.c}
+   #include <vnet/syslog/syslog.h>
+
+   ...
+
+   syslog_msg_t syslog_msg;
+
+   /* Check if syslog logging is enabled */
+   if (!syslog_is_enabled ())
+     return;
+
+   /* Severity filer test */
+   if (syslog_severity_filter_block (severity))
+     return;
+
+   /* Initialize syslog message header */
+   syslog_msg_init (&syslog_msg, facility, severity, "NAT", "SADD");
+
+   /* Create structured data element */
+   syslog_msg_sd_init (&syslog_msg, "nsess");
+   /* Add structured data element parameters (name-value pairs) */
+   syslog_msg_add_sd_param (&syslog_msg, "SSUBIX", "%d", ssubix);
+   syslog_msg_add_sd_param (&syslog_msg, "SVLAN", "%d", svlan);
+   syslog_msg_add_sd_param (&syslog_msg, "IATYP", "IPv4");
+   syslog_msg_add_sd_param (&syslog_msg, "ISADDR", "%U",
+                            format_ip4_address, isaddr);
+   syslog_msg_add_sd_param (&syslog_msg, "ISPORT", "%d", isport);
+   syslog_msg_add_sd_param (&syslog_msg, "XATYP", "IPv4");
+   syslog_msg_add_sd_param (&syslog_msg, "XSADDR", "%U",
+                            format_ip4_address, xsaddr);
+   syslog_msg_add_sd_param (&syslog_msg, "XSPORT", "%d", xsport);
+   syslog_msg_add_sd_param (&syslog_msg, "PROTO", "%d", proto);
+
+   /* Send syslog message */
+   syslog_msg_send (&syslog_msg);
+```
+
+Example above produces following syslog message:
+   <134>1 2018-11-12T11:25:30.252715Z 172.16.4.1 NAT 5901 SADD [nsess SSUBIX="0" SVLAN="0" IATYP="IPv4" ISADDR="172.16.1.2" ISPORT="6303" XATYP="IPv4" XSADDR="10.0.0.3" XSPORT="16253" PROTO="6"]
+
+To add free-form message use:
+```{.c}
+   syslog_msg_add_msg (&syslog_msg, "event log entry");
+```