Netfilter - ACLs

Netfilter is the packet filtering framework in Cumulus Linux as well as most other Linux distributions. There are a number of tools available for configuring ACLs in Cumulus Linux:

  • iptables, ip6tables, and ebtables are Linux userspace tools used to administer filtering rules for IPv4 packets, IPv6 packets, and Ethernet frames (layer 2 using MAC addresses).
  • cl-acltool is a Cumulus Linux-specific userspace tool used to administer filtering rules and configure default ACLs. cl-acltool operates on various configuration files and uses iptables, ip6tables, and ebtables to install rules into the kernel. In addition, cl-acltool programs rules in hardware for interfaces involving switch port interfaces, which iptables, ip6tables and ebtables cannot do on their own.

Traffic Rules

Chains

Netfilter describes the way that packets are classified and controlled in the Linux kernel. Cumulus Linux uses the Netfilter framework to control the flow of traffic to, from, and across the switch. Netfilter does not require a separate software daemon to run; it is part of the Linux kernel itself. Netfilter asserts policies at layers 2, 3 and 4 of the OSI model by inspecting packet and frame headers based on a list of rules. Rules are defined using syntax provided by the iptables, ip6tables and ebtables userspace applications.

The rules created by these programs inspect or operate on packets at several points in the life of the packet through the system. These points are known as chains:

  • PREROUTING touches packets before they are routed
  • INPUT touches packets after they are determined to be destined for the local system but before they are received by the control plane software
  • FORWARD touches transit traffic as it moves through the switch
  • OUTPUT touches packets that are sourced by the control plane software before they are put on the wire
  • POSTROUTING touches packets immediately before they are put on the wire but after the routing decision is made

Tables

When building rules to affect the flow of traffic, the individual chains can be accessed by tables. Linux provides three tables by default:

  • Filter classifies traffic or filters traffic
  • NAT applies Network Address Translation rules
  • Mangle alters packets as they move through the switch

Each table has a set of default chains that can be used to modify or inspect packets at different points of the path through the switch. Chains contain the individual rules to influence traffic. Each table and the default chains they support are shown below. Tables and chains in green are supported by Cumulus Linux, those in red are not supported (that is, they are not hardware accelerated) at this time.

Rules

Rules are the items that classify traffic to be acted upon. Rules are applied to chains, which are attached to tables, similar to the graphic below.

Rules have several different components; the examples below highlight those different components.

  • Table: The first argument is the table. The second example does not specify a table; the filter table is implied if a table is not specified.
  • Chain: The second argument is the chain. Each table supports several different chains. See Tables above.
  • Matches: The third argument is the match. You can specify multiple matches in a single rule. However, the more matches you use in a rule, the more memory that rule consumes.
  • Jump: The jump specifies the target of the rule; what action to take if the packet matches the rule. If this option is omitted in a rule, then matching the rule has no effect on the fate of the packet, but the counters on the rule are incremented.
  • Targets: The target can be a user-defined chain (other than the one this rule is in), one of the special built-in targets that decides the fate of the packet immediately (like DROP), or an extended target. See Supported Rule Types below for different target examples.

How Rules Are Parsed and Applied

All the rules from each chain are read from iptables, ip6tables, and ebtables and entered in order into either the filter table or the mangle table. The rules are read from the kernel in the following order:

  • IPv6 (ip6tables)
  • IPv4 (iptables)
  • ebtables

When rules are combined and put into one table, the order determines the relative priority of the rules; iptables and ip6tables have the highest precedence and ebtables has the lowest.

The Linux packet forwarding construct is an overlay for how the silicon underneath processes packets. Be aware of the following:

  • The order of operations for how rules are processed is not perfectly maintained when you compare how iptables and the switch silicon process packets. The switch silicon reorders rules when switchd writes to the ASIC, whereas traditional iptables execute the list of rules in order.

  • All rules, except for POLICE and SETCLASS rules, are terminating; after a rule matches, the action is carried out and no more rules are processed. In the example below, the SETCLASS action applied with the --in-interface option, creates the internal ASIC classification, and continues to process the next rule, which does the rate-limiting for the matched protocol:

    -A $INGRESS_CHAIN --in-interface $INGRESS_INTF -p udp --dport $BFD_ECHO_PORT -j SETCLASS --class 7
    -A $INGRESS_CHAIN -p udp --dport $BFD_ECHO_PORT -j POLICE --set-mode pkt --set-rate 2000 --set-burst 2000
    
  • When processing traffic, rules affecting the FORWARD chain that specify an ingress interface are performed before rules that match on an egress interface. As a workaround, rules that only affect the egress interface can have an ingress interface wildcard (only swp+ and bond+ are supported) that matches any interface applied so that you can maintain order of operations with other input interface rules. For example, with the following rules:

    -A FORWARD -i $PORTA -j ACCEPT
    -A FORWARD -o $PORTA -j ACCEPT   <-- This rule is performed LAST (because of egress interface matching)
    -A FORWARD -i $PORTB -j DROP
    

    If you modify the rules like this, they are performed in order:

    -A FORWARD -i $PORTA -j ACCEPT
    -A FORWARD -i swp+ -o $PORTA -j ACCEPT   <-- These rules are performed in order (because of wildcard match on the ingress interface)
    -A FORWARD -i $PORTB -j DROP
    
  • When using rules that do a mangle and a filter lookup for a packet, Cumulus Linux processes them in parallel and combines the action.

  • If a switch port is assigned to a bond, any egress rules must be assigned to the bond.

  • When using the OUTPUT chain, rules must be assigned to the source. For example, if a rule is assigned to the switch port in the direction of traffic but the source is a bridge (VLAN), the traffic is not affected by the rule and must be applied to the bridge.

  • If all transit traffic needs to have a rule applied, use the FORWARD chain, not the OUTPUT chain.

  • ebtable rules are put into either the IPv4 or IPv6 memory space depending on whether the rule utilizes IPv4 or IPv6 to make a decision. Layer 2-only rules that match the MAC address are put into the IPv4 memory space.

Rule Placement in Memory

INPUT and ingress (FORWARD -i) rules occupy the same memory space. A rule counts as ingress if the -i option is set. If both input and output options (-i and -o) are set, the rule is considered as ingress and occupies that memory space. For example:

-A FORWARD -i swp1 -o swp2 -s 10.0.14.2 -d 10.0.15.8 -p tcp -j ACCEPT

If you set an output flag with the INPUT chain, you see an error. For example, running cl-acltool -i on the following rule:

-A FORWARD,INPUT -i swp1 -o swp2 -s 10.0.14.2 -d 10.0.15.8 -p tcp -j ACCEPT

generates the following error:

error: line 2 : output interface specified with INPUT chain error processing rule '-A FORWARD,INPUT -i swp1 -o swp2 -s 10.0.14.2 -d 10.0.15.8 -p tcp -j ACCEPT'

However, removing the -o option and interface make it a valid rule.

Nonatomic Update Mode and Atomic Update Mode

In Cumulus Linux, atomic update mode is enabled by default. However, this mode limits the number of ACL rules that you can configure.

To increase the number of configurable ACL rules, configure the switch to operate in nonatomic mode.

Instead of reserving 50% of your TCAM space for atomic updates, incremental update uses the available free space to write the new TCAM rules and swap over to the new rules after this is complete. Cumulus Linux then deletes the old rules and frees up the original TCAM space. If there is insufficient free space to complete this task, the original nonatomic update is performed, which interrupts traffic.

You can enable nonatomic updates for switchd, which offer better scaling because all TCAM resources are used to actively impact traffic. With atomic updates, half of the hardware resources are on standby and do not actively impact traffic.

Incremental nonatomic updates are table based, so they do not interrupt network traffic when new rules are installed. The rules are mapped into the following tables and are updated in this order:

  • mirror (ingress only)
  • ipv4-mac (can be both ingress and egress)
  • ipv6 (ingress only)

The incremental nonatomic update operation follows this order:

  1. Updates are performed incrementally, one table at a time without stopping traffic.
  2. Cumulus Linux checks if the rules in a table have changed since the last time they were installed; if a table does not have any changes, it is not reinstalled.
  3. If there are changes in a table, the new rules are populated in new groups or slices in hardware, then that table is switched over to the new groups or slices.
  4. Finally, old resources for that table are freed. This process is repeated for each of the tables listed above.
  5. If sufficient resources do not exist to hold both the new rule set and old rule set, the regular nonatomic mode is attempted. This interrupts network traffic.
  6. If the regular nonatomic update fails, Cumulus Linux reverts back to the previous rules.

To always start switchd with nonatomic updates:

  1. Edit /etc/cumulus/switchd.conf.

  2. Add the following line to the file:

    acl.non_atomic_update_mode = TRUE
    
  3. Restart switchd:

    cumulus@switch:~$ sudo systemctl restart switchd.service

    Restarting the switchd service causes all network ports to reset, interrupting network services, in addition to resetting the switch hardware configuration.

During regular non-incremental nonatomic updates, traffic is stopped first, then enabled after the new configuration is written into the hardware completely.

Use iptables, ip6tables, and ebtables Directly

Using iptables, ip6tables, ebtables directly is not recommended because any rules installed in these cases only are applied to the Linux kernel and are not hardware accelerated using synchronization to the switch silicon. Running cl-acltool -i (the installation command) resets all rules and deletes anything that is not stored in /etc/cumulus/acl/policy.conf.

For example, the following rule appears to work:

cumulus@switch:~$ sudo iptables -A INPUT -p icmp --icmp-type echo-request -j DROP

The rule appears when you run cl-acltool -L:

cumulus@switch:~$ sudo cl-acltool -L ip
-------------------------------
Listing rules of type iptables:
-------------------------------

TABLE filter :
Chain INPUT (policy ACCEPT 72 packets, 5236 bytes)
pkts bytes target prot opt in out source destination
0 0 DROP icmp -- any any anywhere anywhere icmp echo-request

However, the rule is not synchronized to hardware when applied in this way and running cl-acltool -i or reboot removes the rule without replacing it. To ensure all rules that can be in hardware are hardware accelerated, place them in /etc/cumulus/acl/policy.conf and install them by running cl-acltool -i.

Estimate the Number of Rules

To estimate the number of rules you can create from an ACL entry, first determine if that entry is an ingress or an egress. Then, determine if it is an IPv4-mac or IPv6 type rule. This determines the slice to which the rule belongs. Use the following to determine how many entries are used up for each type.

By default, each entry occupies one double wide entry, except if the entry is one of the following:

  • An entry with multiple comma-separated input interfaces is split into one rule for each input interface (listed after --in-interface below). For example, this entry splits into two rules:

    -A FORWARD --in-interface swp1s0,swp1s1 -p icmp -j ACCEPT
    
  • An entry with multiple comma-separated output interfaces is split into one rule for each output interface (listed after --out-interface below). This entry splits into two rules:

    -A FORWARD --in-interface swp+ --out-interface swp1s0,swp1s1 -p icmp -j ACCEPT
    
  • An entry with both input and output comma-separated interfaces is split into one rule for each combination of input and output interface (listed after --in-interface and --out-interface below). This entry splits into four rules:

    -A FORWARD --in-interface swp1s0,swp1s1 --out-interface swp1s2,swp1s3 -p icmp -j ACCEPT
    
  • An entry with multiple layer 4 port ranges is split into one rule for each range (listed after --dports below). For example, this entry splits into two rules:

    -A FORWARD --in-interface swp+ -p tcp -m multiport --dports 1050:1051,1055:1056 -j ACCEPT
    

    Port ranges are only allowed for ingress rules.

Match on VLAN IDs on Layer 2 Interfaces

You can match on VLAN IDs on layer 2 interfaces for ingress rules. The following example matches on a VLAN and DSCP class, and sets the internal class of the packet. This can be combined with ingress iptable rules to get extended matching on IP fields.

[ebtables]
-A FORWARD -p 802_1Q --vlan-id 100 -j mark --mark-set 0x66

[iptables]
-A FORWARD -i swp31 -m mark --mark 0x66 -m dscp --dscp-class CS1 -j SETCLASS --class 2

Install and Manage ACL Rules with NCLU

NCLU provides an easy way to create custom ACLs in Cumulus Linux. The rules you create live in the /var/lib/cumulus/nclu/nclu_acl.conf file, which gets converted to a rules file, /etc/cumulus/acl/policy.d/50_nclu_acl.rules. This way, the rules you create with NCLU are independent of the two default files in /etc/cumulus/acl/policy.d/ 00control_plane.rules and 99control_plane_catch_all.rules, as the content in these files might get updated after you upgrade Cumulus Linux.

Instead of crafting a rule by hand then installing it using cl-acltool, NCLU handles many of the options automatically. For example, consider the following iptables rule:

-A FORWARD -i swp1 -o swp2 -s 10.0.14.2 -d 10.0.15.8 -p tcp -j ACCEPT

You create this rule, called EXAMPLE1, using NCLU like this:

cumulus@switch:~$ net add acl ipv4 EXAMPLE1 accept tcp source-ip 10.0.14.2/32 source-port any dest-ip 10.0.15.8/32 dest-port any
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

All options, such as the -j and -p, even FORWARD in the above rule, are added automatically when you apply the rule to the control plane; NCLU figures it all out for you.

You can also set a priority value, which specifies the order in which the rules get executed and the order in which they appear in the rules file. Lower numbers are executed first. To add a new rule in the middle, first run net show config acl, which displays the priority numbers. Otherwise, new rules get appended to the end of the list of rules in the nclu_acl.conf and 50_nclu_acl.rules files.

If you need to hand edit a rule, do not edit the 50_nclu_acl.rules file. Instead, edit the nclu_acl.conf file.

After you add the rule, you need to apply it to an inbound or outbound interface using net add int acl. The inbound interface in our example is swp1:

cumulus@switch:~$ net add int swp1 acl ipv4 EXAMPLE1 inbound
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

After you commit your changes, you can verify the rule you created with NCLU by running net show configuration acl:

cumulus@switch:~$ net show configuration acl
acl ipv4 EXAMPLEv4 priority 10 accept tcp source-ip 10.0.14.2/32 source-port any dest-ip 10.0.15.8/32 dest-port any

interface swp1
acl ipv4 EXAMPLE1 inbound

Or you can see all of the rules installed by running cat on the 50_nclu_acl.rules file:

cumulus@switch:~$ cat /etc/cumulus/acl/policy.d/50_nclu_acl.rules
[iptables]
# swp1: acl ipv4 EXAMPLE1 inbound
-A FORWARD --in-interface swp1 --out-interface swp2 -j ACCEPT -p tcp -s 10.0.14.2/32 -d 10.0.15.8/32 --dport 110

For INPUT and FORWARD rules, apply the rule to a control plane interface using net add control-plane:

cumulus@switch:~$ net add control-plane acl ipv4 EXAMPLE1 inbound
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

To remove a rule, use net del acl ipv4|ipv6|mac RULENAME:

cumulus@switch:~$ net del acl ipv4 EXAMPLE1
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

This deletes all rules from the 50_nclu_acl.rules file with that name. It also deletes the interfaces referenced in the nclu_acl.conf file.

Install and Manage ACL Rules with cl-acltool

You can manage Cumulus Linux ACLs with cl-acltool. Rules are first written to the iptables chains, as described above, and then synchronized to hardware via switchd.

To examine the current state of chains and list all installed rules, run:

cumulus@switch:~$ sudo cl-acltool -L all
 -------------------------------
Listing rules of type iptables:
-------------------------------

TABLE filter :
Chain INPUT (policy ACCEPT 90 packets, 14456 bytes)
pkts bytes target prot opt in out source destination
0 0 DROP all -- swp+ any 240.0.0.0/5 anywhere
0 0 DROP all -- swp+ any loopback/8 anywhere
0 0 DROP all -- swp+ any base-address.mcast.net/8 anywhere
0 0 DROP all -- swp+ any 255.255.255.255 anywhere ...

To list installed rules using native iptables, ip6tables and ebtables, use the -L option with the respective commands:

cumulus@switch:~$ sudo iptables -L
cumulus@switch:~$ sudo ip6tables -L
cumulus@switch:~$ sudo ebtables -L

To flush all installed rules, run:

cumulus@switch:~$ sudo cl-acltool -F all

To flush only the IPv4 iptables rules, run:

cumulus@switch:~$ sudo cl-acltool -F ip

If the install fails, ACL rules in the kernel and hardware are rolled back to the previous state. Errors from programming rules in the kernel or ASIC are reported appropriately.

Install Packet Filtering (ACL) Rules

cl-acltool takes access control list (ACL) rules input in files. Each ACL policy file contains iptables, ip6tables and ebtables categories under the tags [iptables], [ip6tables] and [ebtables].

Each rule in an ACL policy must be assigned to one of the rule categories above.

See man cl-acltool(5) for ACL rule details. For iptables rule syntax, see man iptables(8). For ip6tables rule syntax, see man ip6tables(8). For ebtables rule syntax, see man ebtables(8).

See man cl-acltool(5) and man cl-acltool(8) for further details on using cl-acltool. Some examples are listed here and more are listed later in this chapter.

By default:

  • ACL policy files are located in /etc/cumulus/acl/policy.d/.
  • All *.rules files in this directory are included in /etc/cumulus/acl/policy.conf.
  • All files included in this policy.conf file are installed when the switch boots up.
  • The policy.conf file expects rules files to have a .rules suffix as part of the file name.

Here is an example ACL policy file:

[iptables]
-A INPUT --in-interface swp1 -p tcp --dport 80 -j ACCEPT
-A FORWARD --in-interface swp1 -p tcp --dport 80 -j ACCEPT

[ip6tables]
-A INPUT --in-interface swp1 -p tcp --dport 80 -j ACCEPT
-A FORWARD --in-interface swp1 -p tcp --dport 80 -j ACCEPT

[ebtables]
-A INPUT -p IPv4 -j ACCEPT
-A FORWARD -p IPv4 -j ACCEPT

You can use wildcards or variables to specify chain and interface lists to ease administration of rules.

Currently only swp+ and bond+ are supported as wildcard names. There might be kernel restrictions in supporting more complex wildcards like swp1+ etc.

swp+ rules are applied as an aggregate, not per port. If you want to apply per port policing, specify a specific port instead of the wildcard.

INGRESS = swp+
INPUT_PORT_CHAIN = INPUT,FORWARD

[iptables]
-A $INPUT_PORT_CHAIN --in-interface $INGRESS -p tcp --dport 80 -j ACCEPT

[ip6tables]
-A $INPUT_PORT_CHAIN --in-interface $INGRESS -p tcp --dport 80 -j ACCEPT

[ebtables]
-A INPUT -p IPv4 -j ACCEPT

You can write ACL rules for the system into multiple files under the default /etc/cumulus/acl/policy.d/ directory. The ordering of rules during installation follows the sort order of the files based on their file names.

Use multiple files to stack rules. The example below shows two rules files separating rules for management and datapath traffic:

cumulus@switch:~$ ls /etc/cumulus/acl/policy.d/
00sample_mgmt.rules 01sample_datapath.rules
cumulus@switch:~$ cat /etc/cumulus/acl/policy.d/00sample_mgmt.rules

INGRESS_INTF = swp+
INGRESS_CHAIN = INPUT

[iptables]
# protect the switch management
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -s 10.0.14.2 -d 10.0.15.8 -p tcp -j ACCEPT
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -s 10.0.11.2 -d 10.0.12.8 -p tcp -j ACCEPT
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -d 10.0.16.8 -p udp -j DROP

cumulus@switch:~$ cat /etc/cumulus/acl/policy.d/01sample_datapath.rules
INGRESS_INTF = swp+
INGRESS_CHAIN = INPUT, FORWARD

[iptables]
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -s 192.0.2.5 -p icmp -j ACCEPT
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -s 192.0.2.6 -d 192.0.2.4 -j DROP
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -s 192.0.2.2 -d 192.0.2.8 -j DROP

Install all ACL policies under a directory:

cumulus@switch:~$ sudo cl-acltool -i -P ./rules
Reading files under rules
Reading rule file ./rules/01_http_rules.txt ...
Processing rules in file ./rules/01_http_rules.txt ...
Installing acl policy ...
Done.

Apply all rules and policies included in /etc/cumulus/acl/policy.conf:

cumulus@switch:~$ sudo cl-acltool -i

In addition to ensuring that the rules and policies referenced by /etc/cumulus/acl/policy.conf are installed, this removes any currently active rules and policies that are not contained in the files referenced by /etc/cumulus/acl/policy.conf.

Specify the Policy Files to Install

By default, Cumulus Linux installs any .rules file you configure in /etc/cumulus/acl/policy.d/. To add other policy files to an ACL, you need to include them in /etc/cumulus/acl/policy.conf. For example, for Cumulus Linux to install a rule in a policy file called 01_new.datapathacl, add include /etc/cumulus/acl/policy.d/01_new.rules to policy.conf, as in this example:

cumulus@switch:~$ sudo nano /etc/cumulus/acl/policy.conf

#
# This file is a master file for acl policy file inclusion
#
# Note: This is not a file where you list acl rules.
#
# This file can contain:
# - include lines with acl policy files
#   example:
#     include <filepath>
#
# see manpage cl-acltool(5) and cl-acltool(8) for how to write policy files 
#

include /etc/cumulus/acl/policy.d/01_new.datapathacl

Hardware Limitations on Number of Rules

The maximum number of rules that can be handled in hardware is a function of the following factors:

  • The mix of IPv4 and IPv6 rules; Cumulus Linux does not support the maximum number of rules for both IPv4 and IPv6 simultaneously.
  • The number of default rules provided by Cumulus Linux.
  • Whether the rules are applied on ingress or egress.
  • Whether the rules are in atomic or nonatomic mode; nonatomic mode rules are used when nonatomic updates are enabled (see above).

If the maximum number of rules for a particular table is exceeded, cl-acltool -i generates the following error:

error: hw sync failed (sync_acl hardware installation failed) Rolling back .. failed.

In the table below, the default rules count toward the limits listed. The raw limits below assume only one ingress and one egress table are present.

The NVIDIA Spectrum ASIC has one common TCAM for both ingress and egress, which can be used for other non-ACL-related resources. However, the number of supported rules varies with the TCAM profile specified for the switch.

ProfileAtomic Mode IPv4 RulesAtomic Mode IPv6 RulesNonatomic Mode IPv4 RulesNonatomic Mode IPv6 Rules
default5002501000500
ipmc-heavy75050015001000
acl-heavy1750100035002000
ipmc-max100050020001000
ip-acl-heavy75000150000

Even though the table above specifies that zero IPv6 rules are supported with the ip-acl-heavy profile, Cumulus Linux does not prevent you from configuring IPv6 rules. However, there is no guarantee that IPv6 rules work under the ip-acl-heavy profile.

Supported Rule Types

The iptables/ip6tables/ebtables construct tries to layer the Linux implementation on top of the underlying hardware but they are not always directly compatible. Here are the supported rules for chains in iptables, ip6tables and ebtables.

To learn more about any of the options shown in the tables below, run iptables -h [name of option]. The same help syntax works for options for ip6tables and ebtables.

root@leaf1# ebtables -h tricolorpolice
...
tricolorpolice option:
--set-color-mode STRING setting the mode in blind or aware
--set-cir INT setting committed information rate in kbits per second
--set-cbs INT setting committed burst size in kbyte
--set-pir INT setting peak information rate in kbits per second
--set-ebs INT setting excess burst size in kbyte
--set-conform-action-dscp INT setting dscp value if the action is accept for conforming packets
--set-exceed-action-dscp INT setting dscp value if the action is accept for exceeding packets
--set-violate-action STRING setting the action (accept/drop) for violating packets
--set-violate-action-dscp INT setting dscp value if the action is accept for violating packets
Supported chains for the filter table:
INPUT FORWARD OUTPUT

iptables and ip6tables Rule Support

Rule ElementSupportedUnsupported
MatchesSrc/Dst, IP protocol
In/out interface
IPv4: icmp, ttl,
IPv6: icmp6, frag, hl,
IP common: tcp (with flags), udp, multiport, DSCP, addrtype
Rules with input/output Ethernet interfaces are ignored
Inverse matches
Standard TargetsACCEPT, DROPRETURN, QUEUE, STOP, Fall Thru, Jump
Extended TargetsLOG (IPv4/IPv6); UID is not supported for LOG
TCP SEQ, TCP options or IP options
ULOG
SETQOS
DSCP
Unique to Cumulus Linux:
SPAN
ERSPAN (IPv4/IPv6)
POLICE
TRICOLORPOLICE
SETCLASS

ebtables Rule Support

Rule ElementSupportedUnsupported
Matchesether type
input interface/wildcard
output interface/wildcard
Src/Dst MAC
IP: src, dest, tos, proto, sport, dport
IPv6: tclass, icmp6: type, icmp6: code range, src/dst addr, sport, dport
802.1p (CoS)
VLAN
Inverse matches
Proto length
Standard TargetsACCEPT, DROPRETURN, CONTINUE, Jump, Fall Thru
Extended TargetsULOG
LOG
Unique to Cumulus Linux:
SPAN
ERSPAN
POLICE
TRICOLORPOLICE
SETCLASS

Other Unsupported Rules

  • Rules that have no matches and accept all packets in a chain are currently ignored.
  • Chain default rules (that are ACCEPT) are also ignored.

Considerations

Splitting rules across the ingress TCAM and the egress TCAM causes the ingress IPv6 part of the rule to match packets going to all destinations, which can interfere with the regular expected linear rule match in a sequence. For example:

A higher rule can prevent a lower rule from being matched unexpectedly:

Rule 1: -A FORWARD --out-interface vlan100 -p icmp6 -j ACCEPT

Rule 2: -A FORWARD --out-interface vlan101 -p icmp6 -s 01::02 -j ACCEPT

Rule 1 matches all icmp6 packets from to all out interfaces in the ingress TCAM.`

This prevents rule 2 from getting matched, which is more specific but with a different out interface. Make sure to put more specific matches above more general matches even if the output interfaces are different.

When you have two rules with the same output interface, the lower rule might match unexpectedly depending on the presence of the previous rules.

Rule 1: -A FORWARD --out-interface vlan100 -p icmp6 -j ACCEPT

Rule 2: -A FORWARD --out-interface vlan101 -s 00::01 -j DROP

Rule 3: -A FORWARD --out -interface vlan101 -p icmp6 -j ACCEPT

Rule 3 still matches for an icmp6 packet with sip 00:01 going out of vlan101. Rule 1 interferes with the normal function of rule 2 and/or rule 3.

When you have two adjacent rules with the same match and different output interfaces, such as:

Rule 1: -A FORWARD --out-interface vlan100 -p icmp6 -j ACCEPT

Rule 2: -A FORWARD --out-interface vlan101 -p icmp6 -j DROP

Rule 2 never matches on ingress. Both rules share the same mark.

Common Examples

Control Plane and Data Plane Traffic

You can configure quality of service for traffic on both the control plane and the data plane. By using QoS policers, you can rate limit traffic so incoming packets get dropped if they exceed specified thresholds.

Counters on POLICE ACL rules in iptables do not currently show the packets that are dropped due to those rules.

Use the POLICE target with iptables. POLICE takes these arguments:

  • --set-class value sets the system internal class of service queue configuration to value.
  • --set-rate value specifies the maximum rate in kilobytes (KB) or packets.
  • --set-burst value specifies the number of packets or kilobytes (KB) allowed to arrive sequentially.
  • --set-mode string sets the mode in KB (kilobytes) or pkt (packets) for rate and burst size.

For example, to rate limit the incoming traffic on swp1 to 400 packets per second with a burst of 100 packets per second and set the class of the queue for the policed traffic as 0, set this rule in your appropriate .rules file:

-A INPUT --in-interface swp1 -j POLICE --set-mode pkt --set-rate 400 --set-burst 100 --set-class 0

Here is another example of control plane ACL rules to lock down the switch. You specify them in /etc/cumulus/acl/policy.d/00control_plane.rules:

View the contents of the file ...

Set DSCP on Transit Traffic

The examples here use the mangle table to modify the packet as it transits the switch. DSCP is expressed in decimal notation in the examples below.

[iptables]

#Set SSH as high priority traffic.
-t mangle -A FORWARD -p tcp --dport 22  -j DSCP --set-dscp 46

#Set everything coming in SWP1 as AF13
-t mangle -A FORWARD --in-interface swp1 -j DSCP --set-dscp 14

#Set Packets destined for 10.0.100.27 as best effort
-t mangle -A FORWARD -d 10.0.100.27/32 -j DSCP --set-dscp 0

#Example using a range of ports for TCP traffic
-t mangle -A FORWARD -p tcp -s 10.0.0.17/32 --sport 10000:20000 -d 10.0.100.27/32 --dport 10000:20000 -j DSCP --set-dscp 34

Verify DSCP Values on Transit Traffic

The examples here use the DSCP match criteria in combination with other IP, TCP, and interface matches to identify traffic and count the number of packets.

[iptables]

#Match and count the packets that match SSH traffic with DSCP EF
-A FORWARD -p tcp --dport 22 -m dscp --dscp 46 -j ACCEPT

#Match and count the packets coming in SWP1 as AF13
-A FORWARD --in-interface swp1 -m dscp --dscp 14 -j ACCEPT
#Match and count the packets with a destination 10.0.0.17 marked best effort
-A FORWARD -d 10.0.100.27/32 -m dscp --dscp 0 -j ACCEPT

#Match and count the packets in a port range with DSCP AF41
-A FORWARD -p tcp -s 10.0.0.17/32 --sport 10000:20000 -d 10.0.100.27/32 --dport 10000:20000 -m dscp --dscp 34 -j ACCEPT

Check the Packet and Byte Counters for ACL Rules

To verify the counters using the above example rules, first send test traffic matching the patterns through the network. The following example generates traffic with mz (or mausezahn), which can be installed on host servers or even on Cumulus Linux switches. After traffic is sent to validate the counters, they are matched on switch1 using cl-acltool.

Policing counters do not increment on switches with the Spectrum ASIC.

# Send 100 TCP packets on host1 with a DSCP value of EF with a destination of host2 TCP port 22:

cumulus@host1$ mz eth1 -A 10.0.0.17 -B 10.0.100.27 -c 100 -v -t tcp "dp=22,dscp=46"
  IP:  ver=4, len=40, tos=184, id=0, frag=0, ttl=255, proto=6, sum=0, SA=10.0.0.17, DA=10.0.100.27,
      payload=[see next layer]
  TCP: sp=0, dp=22, S=42, A=42, flags=0, win=10000, len=20, sum=0,
      payload=

# Verify the 100 packets are matched on switch1

cumulus@switch1$ sudo cl-acltool -L ip
-------------------------------
Listing rules of type iptables:
-------------------------------
TABLE filter :
Chain INPUT (policy ACCEPT 9314 packets, 753K bytes)
  pkts bytes target     prot opt in     out     source               destination
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
  pkts bytes target     prot opt in     out     source               destination
  100  6400 ACCEPT     tcp  --  any    any     anywhere             anywhere             tcp dpt:ssh DSCP match 0x2e
    0     0 ACCEPT     all  --  swp1   any     anywhere             anywhere             DSCP match 0x0e
    0     0 ACCEPT     all  --  any    any     10.0.0.17            anywhere             DSCP match 0x00
    0     0 ACCEPT     tcp  --  any    any     10.0.0.17            10.0.100.27          tcp spts:webmin:20000
    dpts:webmin:2002

# Send 100 packets with a small payload on host1 with a DSCP value of AF13 with a destination of host2:

cumulus@host1$ mz eth1 -A 10.0.0.17 -B 10.0.100.27 -c 100 -v -t ip
  IP:  ver=4, len=20, tos=0, id=0, frag=0, ttl=255, proto=0, sum=0, SA=10.0.0.17, DA=10.0.100.27,
      payload=

# Verify the 100 packets are matched on switch1

cumulus@switch1$ sudo cl-acltool -L ip
-------------------------------
Listing rules of type iptables:
-------------------------------
TABLE filter :
Chain INPUT (policy ACCEPT 9314 packets, 753K bytes)
  pkts bytes target     prot opt in     out     source               destination
  chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
  pkts bytes target     prot opt in     out     source               destination
  100  6400 ACCEPT     tcp  --  any    any     anywhere             anywhere             tcp dpt:ssh DSCP match 0x2e
  100  7000 ACCEPT     all  --  swp3   any     anywhere             anywhere             DSCP match 0x0e
  100  6400 ACCEPT     all  --  any    any     10.0.0.17            anywhere             DSCP match 0x00
    0     0 ACCEPT     tcp  --  any    any     10.0.0.17            10.0.100.27          tcp spts:webmin:20000 dpts:webmin:2002

# Send 100 packets on host1 with a destination of host2:

cumulus@host1$ mz eth1 -A 10.0.0.17 -B 10.0.100.27 -c 100 -v -t ip
 IP:  ver=4, len=20, tos=56, id=0, frag=0, ttl=255, proto=0, sum=0, SA=10.0.0.17, DA=10.0.100.27,
     payload=

# Verify the 100 packets are matched on switch1

cumulus@switch1$ sudo cl-acltool -L ip
-------------------------------
Listing rules of type iptables:
-------------------------------
TABLE filter :
Chain INPUT (policy ACCEPT 9314 packets, 753K bytes)
  pkts bytes target     prot opt in     out     source               destination
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
  pkts bytes target     prot opt in     out     source               destination
  100  6400 ACCEPT     tcp  --  any    any     anywhere             anywhere             tcp dpt:ssh DSCP match 0x2e
  100  7000 ACCEPT     all  --  swp3   any     anywhere             anywhere             DSCP match 0x0e
    0     0 ACCEPT     all  --  any    any     10.0.0.17            anywhere             DSCP match 0x00
    0     0 ACCEPT     tcp  --  any    any     10.0.0.17            10.0.100.27          tcp spts:webmin:20000 dpts:webmin:2002Still working

Filter Specific TCP Flags

The example solution below creates rules on the INPUT and FORWARD chains to drop ingress IPv4 and IPv6 TCP packets when the SYN bit is set and the RST, ACK, and FIN bits are reset. The default for the INPUT and FORWARD chains allows all other packets. The ACL is applied to ports swp20 and swp21. After configuring this ACL, new TCP sessions that originate from ingress ports swp20 and swp21 are not allowed. TCP sessions that originate from any other port are allowed.

INGRESS_INTF = swp20,swp21

[iptables]
-A INPUT,FORWARD --in-interface $INGRESS_INTF -p tcp --syn -j DROP
[ip6tables]
-A INPUT,FORWARD --in-interface $INGRESS_INTF -p tcp --syn -j DROP

The --syn flag in the above rule matches packets with the SYN bit set and the ACK, RST, and FIN bits are cleared. It is equivalent to using -tcp-flags SYN,RST,ACK,FIN SYN. For example, you can write the above rule as:

-A INPUT,FORWARD --in-interface $INGRESS_INTF -p tcp --tcp-flags SYN,RST,ACK,FIN SYN -j DROP

Control Who Can SSH into the Switch

Run the following NCLU commands to control who can SSH into the switch. In the following example, 10.0.0.11/32 is the interface IP address (or loopback IP address) of the switch and 10.255.4.0/24 can SSH into the switch.

cumulus@switch:~$ net add acl ipv4 test priority 10 accept source-ip 10.255.4.0/24 dest-ip 10.0.0.11/32
cumulus@switch:~$ net add acl ipv4 test priority 20 drop source-ip any dest-ip 10.0.0.11/32
cumulus@switch:~$ net add control-plane acl ipv4 test inbound
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Cumulus Linux does not support the keyword iprouter (typically used for traffic sent to the CPU, where the destination MAC address is that of the router but the destination IP address is not the router).

Example Configuration

The following example demonstrates how several different rules are applied.

Following are the configurations for the two switches used in these examples. The configuration for each switch appears in /etc/network/interfaces on that switch.

Switch 1 Configuration

cumulus@switch1:~$ sudo cat /etc/network/interfaces
...
/etc/network/interfaces
=======================

auto swp1
iface swp1

auto swp2
iface swp2

auto swp3
iface swp3

auto swp4
iface swp4

auto bond2
iface bond2
  bond-slaves swp3 swp4

auto br-untagged
iface br-untagged
  address 10.0.0.1/24
  bridge_ports swp1 bond2
  bridge_stp on

auto br-tag100
iface br-tag100
  address 10.0.100.1/24
  bridge_ports swp2.100 bond2.100
  bridge_stp on
...

Switch 2 Configuration

cumulus@switch2:~$ sudo cat /etc/network/interfaces
...
/etc/network/interfaces
=======================

auto swp3
iface swp3

auto swp4
iface swp4

auto br-untagged
iface br-untagged
  address 10.0.0.2/24
  bridge_ports bond2
  bridge_stp on

auto br-tag100
iface br-tag100
  address 10.0.100.2/24
  bridge_ports bond2.100
  bridge_stp on

auto bond2
iface bond2
  bond-slaves swp3 swp4
...

Egress Rule

The following rule blocks any TCP traffic with destination port 200 going from host1 or host2 through the switch (corresponding to rule 1 in the diagram above).

[iptables] -A FORWARD -o bond2 -p tcp --dport 200 -j DROP

Ingress Rule

The following rule blocks any UDP traffic with source port 200 going from host1 through the switch (corresponding to rule 2 in the diagram above).

[iptables] -A FORWARD -i swp2 -p udp --sport 200 -j DROP

Input Rule

The following rule blocks any UDP traffic with source port 200 and destination port 50 going from host1 to the switch (corresponding to rule 3 in the diagram above).

[iptables] -A INPUT -i swp1 -p udp --sport 200 --dport 50 -j DROP

Output Rule

The following rule blocks any TCP traffic with source port 123 and destination port 123 going from Switch 1 to host2 (corresponding to rule 4 in the diagram above).

[iptables] -A OUTPUT -o br-tag100 -p tcp --sport 123 --dport 123 -j DROP

Combined Rules

The following rule blocks any TCP traffic with source port 123 and destination port 123 going from any switch port egress or generated from Switch 1 to host1 or host2 (corresponding to rules 1 and 4 in the diagram above).

[iptables] -A OUTPUT,FORWARD -o swp+ -p tcp --sport 123 --dport 123 -j DROP

This also becomes two ACLs and is the same as:

[iptables]
-A FORWARD -o swp+ -p tcp --sport 123 --dport 123 -j DROP 
-A OUTPUT -o swp+ -p tcp --sport 123 --dport 123 -j DROP

Layer 2-only Rules/ebtables

The following rule blocks any traffic with source MAC address 00:00:00:00:00:12 and destination MAC address 08:9e:01:ce:e2:04 going from any switch port egress/ingress.

[ebtables] -A FORWARD -s 00:00:00:00:00:12 -d 08:9e:01:ce:e2:04 -j DROP

Considerations

Not All Rules Supported

Not all iptables, ip6tables, or ebtables rules are supported. Refer to the Supported Rules section above for specific rule support.

ACL Log Policer Limits Traffic

To protect the CPU from overloading, traffic copied to the CPU is limited to 1 pkt/s by an ACL Log Policer.

Bridge Traffic Limitations

Bridge traffic that matches LOG ACTION rules are not logged in syslog; the kernel and hardware identify packets using different information.

Log Actions Cannot Be Forwarded

Logged packets cannot be forwarded. The hardware cannot both forward a packet and send the packet to the control plane (or kernel) for logging. To emphasize this, a log action must also have a drop action.

SPAN Sessions that Reference an Outgoing Interface

SPAN sessions that reference an outgoing interface create mirrored packets based on the ingress interface before the routing/switching decision. See SPAN Sessions that Reference an Outgoing Interface and Use the CPU Port as the SPAN Destination in the Network Troubleshooting section.

iptables Interactions with cl-acltool

Because Cumulus Linux is a Linux operating system, the iptables commands can be used directly. However, consider using cl-acltool instead because:

  • Without using cl-acltool, rules are not installed into hardware.
  • Running cl-acltool -i (the installation command) resets all rules and deletes anything that is not stored in /etc/cumulus/acl/policy.conf.

For example, running the following command works:

cumulus@switch:~$ sudo iptables -A INPUT -p icmp --icmp-type echo-request -j DROP

And the rules appear when you run cl-acltool -L:

cumulus@switch:~$ sudo cl-acltool -L ip
-------------------------------
Listing rules of type iptables:
-------------------------------
TABLE filter :
Chain INPUT (policy ACCEPT 72 packets, 5236 bytes)
pkts bytes target  prot opt in   out   source    destination

0     0 DROP    icmp --  any  any   anywhere  anywhere      icmp echo-request

However, running cl-acltool -i or reboot removes them. To ensure all rules that can be in hardware are hardware accelerated, place them in the /etc/cumulus/acl/policy.conf file, then run cl-acltool -i.

Hardware Limitations

Due to hardware limitations in the Spectrum ASIC, BFD policers are shared between all BFD-related control plane rules. The following default rules share the same policer in the 00control_plan.rules file:

[iptables]
-A $INGRESS_CHAIN -p udp --dport $BFD_ECHO_PORT -j POLICE --set-mode pkt --set-rate 2000 --set-burst 2000
-A $INGRESS_CHAIN -p udp --dport $BFD_PORT -j POLICE --set-mode pkt --set-rate 2000 --set-burst 2000
-A $INGRESS_CHAIN -p udp --dport $BFD_MH_PORT -j POLICE --set-mode pkt --set-rate 2000 --set-burst 2000

[ip6tables]
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -p udp --dport $BFD_ECHO_PORT -j POLICE --set-mode pkt --set-rate 2000 --set-burst 2000 --set-class 7
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -p udp --dport $BFD_PORT -j POLICE --set-mode pkt --set-rate 2000 --set-burst 2000 --set-class 7
-A $INGRESS_CHAIN --in-interface $INGRESS_INTF -p udp --dport $BFD_MH_PORT -j POLICE --set-mode pkt --set-rate 2000 --set-burst 2000 --set-class 7

To work around this limitation, set the rate and burst of all 6 of these rules to the same values with the --set-rate and --set-burst options.

Where to Assign Rules

  • If a switch port is assigned to a bond, you must assign any egress rules to the bond.
  • When using the OUTPUT chain, you must assign rules to the source. For example, if a rule is assigned to the switch port in the direction of traffic but the source is a bridge (VLAN), the traffic is not affected by the rule and must be applied to the bridge.
  • If all transit traffic needs to have a rule applied, use the FORWARD chain, not the OUTPUT chain.

ACL Rule Installation Failure

After an ACL rule installation failure, a generic error message like the following is displayed:

cumulus@switch:$ sudo cl-acltool -i -p 00control_plane.rules
Using user provided rule file 00control_plane.rules
Reading rule file 00control_plane.rules ...
Processing rules in file 00control_plane.rules ...
error: hw sync failed (sync_acl hardware installation failed)
Installing acl policy... Rolling back ..
failed.

INPUT Chain Rules

INPUT chain rules are implemented using a trap mechanism. Packets headed to the CPU are assigned trap IDs. The default INPUT chain rules are mapped to these trap IDs. However, if a packet matches multiple traps, they are resolved by an internal priority mechanism that might be different from the rule priorities. Packets might not get policed by the default expected rule, but by another rule instead. For example, ICMP packets headed to the CPU are policed by the LOCAL rule instead of the ICMP rule. Also, multiple rules might share the same trap. In this case the policer that is applied is the largest of the policer values.

To work around this issue, create rules on the INPUT and FORWARD chains (INPUT,FORWARD).

Hardware Policing of Packets in the Input Chain

On certain platforms, there are limitations on hardware policing of packets in the INPUT chain. To work around these limitations, Cumulus Linux supports kernel based policing of these packets in software using limit/hashlimit matches. Rules with these matches are not hardware offloaded, but are ignored during hardware install.

ACLs Do not Match when the Output Port on the ACL is a Subinterface

Packets do not get matched when a subinterface is configured as the output port. The ACL matches on packets only if the primary port is configured as an output port. If a subinterface is set as an output or egress port, the packets match correctly.

For example:

-A FORWARD --out-interface swp49s1.100 -j ACCEPT

Egress ACL Matching on Bonds

ACL rules that match on an outbound bond interface are not supported. For example, the following rule is not supported:

[iptables]
-A FORWARD --out-interface <bond_intf> -j DROP

To work around this issue, duplicate the ACL rule on each physical port of the bond. For example:

[iptables]
-A FORWARD --out-interface <bond-member-port-1> -j DROP
-A FORWARD --out-interface <bond-member-port-2> -j DROP