Routing basics
See also: IP Layer Network Administration, IP routing tutorial
Routing is the process of selecting a path to send network traffic. There are several routing protocols for dynamic routing, specifically B.A.T.M.A.N. and OLSR for mesh networking, however static routing is typically enough for most use cases. Routing is handled by a kernel component and can be configured by the user space tool ip from the package iproute2. Note that by default OpenWrt announces IPv6 default route only for GUA and applies source filter for IPv6 that allows routing only for prefixes delegated from the upstream router.
How it works
Basic routing
In a trivial case, the route is selected by the traffic destination, highest netmask and lowest metric.
Routing rules are expected to be in their default state and can be basically ignored, the same applies to routing tables other than the main table.
Policy-based routing
See also: Policy-based routing
In general case, the kernel iterates over the routing rules from lower to higher numeric priority values.
By default, only the main routing table contains the default route and the corresponding rule is automatically created with a priority of 32766.
If we utilize custom routing tables with ip4table and/or ip6table options, netifd creates the rules for each local address as source and each local subnet as destination with respective priorities of 10000 and 20000.
To override the default route in the main table, we can add a rule with a priority of about 30000, or assign each upstream interface to a separate routing table and lookup it after the main table.
Configuration
See also: Static routes, Routing rules
/etc/config/network is the UCI configuration file where all routing related adjustments are made in OpenWrt.
Default routing tables
See also: ip-route
| ID | Name | Description |
|---|---|---|
0 | unspec | Special table matching all table names/IDs. |
253 | default | Reserved table, empty by default. |
254 | main | Routing table with all non-policy routes. |
255 | local | Special table with local and broadcast addresses. |
Edit /etc/iproute2/rt_tables to customize routing tables.
Default routing rules
See also: ip-rule
| Priority | Match | Lookup table | Description |
|---|---|---|---|
0 | anything | local | High priority routing for local and broadcast addresses. |
10000 | local addresses as source | <custom> | A list of rules for each local address created by netifd when using ip4table or ip6table. |
20000 | local subnets as destination | <custom> | A list of rules for each local subnet created by netifd when using ip4table or ip6table. |
32766 | anything | main | Non-policy routing, can be overridden with other rules by the administrator, also offers routes for tunnel endpoints. |
32767 | IPv4 traffic | default | Post-processing for IPv4 traffic missed by previous rules. |
90000+ | traffic from local system | <custom> | A list of rules for each interface created by netifd when using ip4table or ip6table, works as failover for traffic from the local system. |
4200000000+ | IPv6 traffic to interfaces | - | A list of terminating rules automatically created for each IPv6 interface. |
Testing
Verify your routing traceroute and traceroute6.
traceroute openwrt.org traceroute6 openwrt.org
Check your IP and DNS provider.
Test routing for a specific destination.
ip route get 198.51.100.1 ip route get 198.51.100.1 from 192.168.1.1 ip route get 2001:db8::1 ip route get 2001:db8::1 from fd00:1::1
Troubleshooting
Collect and analyze the following information.
# Restart services service log restart; service network restart; sleep 10 # Log and status logread; ifstatus wan; ifstatus wan6 # Runtime configuration ip address show; ip route show table all ip rule show; ip -6 rule show; nft list ruleset # Persistent configuration uci show network; uci show dhcp; uci show firewall grep -v -e "^#" -e "^$" /etc/iproute2/rt_tables