Handling syslog clients sending blank fields

Problem

I recently starting centrally collecting logs on my home network using Promtail, Loki and Grafana This lets you easily search and analyse logs for all sorts of things.

One of the services I wanted to collect logs for was my TP-Link Wireless Access Points managed by TP-Link Omada. However, it turns out that the APs weren’t setting the app-name field correctly when sending logs to the remote server.

This resulted in errors in promtail that frequently reset the connection to rsyslogd that was acting as a syslog relay.

This was the error I could see in the logs:

promtail:

level=warn ts=2022-01-04T04:08:21.013648461Z caller=syslogtarget.go:216 msg="error parsing syslog stream" err="expecting an app-name (from 1 to max 48 US-ASCII characters) or a nil value [col 50]

rsyslog:

rsyslogd: omfwd: TCPSendBuf error -2027, destruct TCP Connection to promtail:514 [v8.36.0 try http://www.rsyslog.com/e/2027 ]
rsyslogd-2027: omfwd: TCPSendBuf error -2027, destruct TCP Connection to promtail:514 [v8.36.0 try http://www.rsyslog.com/e/2027 ]
rsyslogd-2007: action 'action 2' suspended (module 'builtin:omfwd'), retry 0. There should be messages before this one giving the reason for suspension. [v8.36.0 try http://www.rsyslog.com/e/2007 ]
rsyslogd-2359: action 'action 2' resumed (module 'builtin:omfwd') [v8.36.0 try http://www.rsyslog.com/e/2359 ]

Environment

Syslog Clients:

Omada Controller 4.4.8
TP-Link EAP 620 HD (x2)

rsyslogd configured as a forwarder to promtail. docker-compose extract below:

version: '3'

services:
#    -- snip --
  loki:
    image: grafana/loki:2.4.1
    restart: unless-stopped
    volumes:
      - ./loki-config.yaml:/mnt/config/loki-config.yaml
    command:
      - --config.file=/mnt/config/loki-config.yaml
  promtail:
    image: grafana/promtail:2.4.1
    restart: unless-stopped
    volumes:
      - /var/log:/var/log
      - ./promtail-config.yaml:/mnt/config/promtail-config.yaml
    command:
      - --config.file=/mnt/config/promtail-config.yaml
  rsyslog:
    image: rsyslog/syslog_appliance_alpine
    restart: unless-stopped
    ports:
      - 514:514/udp
      - 514:514/tcp
    environment:
      - RSYSLOG_CONF=/config/rsyslog.conf
    volumes:
      - ./rsyslog.conf:/config/rsyslog.conf

promtail configuration:

server:
  http_listen_port: 9080
  grpc_listen_port: 0

positions:
  filename: /tmp/positions.yaml

clients:
  - url: http://loki:3100/loki/api/v1/push

scrape_configs:
- job_name: system
  static_configs:
  - targets:
      - localhost
    labels:
      job: varlogs
      __path__: /var/log/*log
  pipeline_stages:
    - regex:
        expression: "^[\\w]+\\s+[\\d]+\\s+[\\d|:]+ (?P<host>[^\\s]+) (?P<tag>[^:\\[]+)"
    - labels:
        host:
        tag:
- job_name: syslog
  syslog:
    listen_address: "0.0.0.0:514"
    idle_timeout: 10m
    label_structured_data: yes
    labels:
      job: "syslog"
  relabel_configs:
    - source_labels: ["__syslog_connection_ip_address"]
      target_label: "ip_address"
    - source_labels: ["__syslog_message_severity"]
      target_label: "severity"
    - source_labels: ["__syslog_message_facility"]
      target_label: "facility"
    - source_labels: ["__syslog_message_app_name"]
      target_label: "appname"
    - source_labels: ["__syslog_message_hostname"]
      target_label: "host"

rsyslog.conf configured according to: https://grafana.com/docs/loki/latest/clients/promtail/scraping/#rsyslog-output-configuration

Solution

The way I solved this problem was by configuring rsyslog to use a modified template that sets the app-name to - (nil) when the app-name field is blank. The resulting configuration is shown below. Note that this solution should work for any field (e.g. hostname) send from mis-behaving syslog clients.

...

:app-name, !isequal, "" {
    action(type="omfwd" protocol="tcp" target= "promtail" port="514" Template="RSYSLOG_SyslogProtocol23Format" TCP_Framing="octet-counted" KeepAlive="on")
}

# RSYSLOG_SyslogProtocol23Format but with app-name hard-coded to '-'
template(name="missingAppName" type="string" string="<%PRI%>1 %TIMESTAMP:::date-rfc3339% %HOSTNAME% - %PROCID% %MSGID% %STRUCTURED-DATA% %msg%\n")

:app-name, isequal, "" {
    action(type="omfwd" protocol="tcp" target= "promtail" port="514" Template="missingAppName" TCP_Framing="octet-counted" KeepAlive="on")
}

...

References

I found the following links useful while working through this problem: