$AllowedSender

Type: input configuration parameter

Default: all allowed

Description:

Note: this feature is supported for backward-compatibility, only. The rsyslog team recommends to use proper firewalling instead of this feature.

Allowed sender lists can be used to specify which remote systems are allowed to send syslog messages to rsyslogd. With them, further hurdles can be placed between an attacker and rsyslogd. If a message from a system not in the allowed sender list is received, that message is discarded. A diagnostic message is logged, so that the fact is recorded (this message can be turned off with the “-w” rsyslogd command line option).

Allowed sender lists can be defined for UDP and TCP senders separately. There can be as many allowed senders as needed. The syntax to specify them is:

$AllowedSender <type>, ip[/bits], ip[/bits]

“$AllowedSender” is the parameter - it must be written exactly as shown and the $ must start at the first column of the line. “<type>” is either “UDP” or “TCP” (or “GSS”, if this is enabled during compilation). It must immediately be followed by the comma, else you will receive an error message. “ip[/bits]” is a machine or network ip address as in “192.0.2.0/24” or “127.0.0.1”. If the “/bits” part is omitted, a single host is assumed (32 bits or mask 255.255.255.255). “/0” is not allowed, because that would match any sending system. If you intend to do that, just remove all $AllowedSender parameters. If more than 32 bits are requested with IPv4, they are adjusted to 32. For IPv6, the limit is 128 for obvious reasons. Hostnames, with and without wildcards, may also be provided. If so, the result of revers DNS resolution is used for filtering. Multiple allowed senders can be specified in a comma-delimited list. Also, multiple $AllowedSender lines can be given. They are all combined into one UDP and one TCP list. Performance-wise, it is good to specify those allowed senders with high traffic volume before those with lower volume. As soon as a match is found, no further evaluation is necessary and so you can save CPU cycles.

Rsyslogd handles allowed sender detection very early in the code, nearly as the first action after receiving a message. This keeps the access to potential vulnerable code in rsyslog at a minimum. However, it is still a good idea to impose allowed sender limitations via firewalling.

WARNING: by UDP design, rsyslogd can not identify a spoofed sender address in UDP syslog packets. As such, a malicious person could spoof the address of an allowed sender, send such packets to rsyslogd and rsyslogd would accept them as being from the faked sender. To prevent this, use syslog via TCP exclusively. If you need to use UDP-based syslog, make sure that you do proper egress and ingress filtering at the firewall and router level.

Rsyslog also detects some kind of malicious reverse DNS entries. In any case, using DNS names adds an extra layer of vulnerability. We recommend to stick with hard-coded IP addresses wherever possible.

Sample:

$AllowedSender UDP, 127.0.0.1, 192.0.2.0/24, [::1]/128, *.example.net, somehost.example.com

Copyright 2008-2023 Rainer Gerhards (Großrinderfeld), and Others.