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nic_router/README: adapt to new ICMP features
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Martin Stein
Christian Helmuth
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@ -7,11 +7,22 @@
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Brief
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#####
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The 'nic_router' component can be used to individually route IPv4 packets
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between multiple NIC sessions. Thereby, it can translate between different
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subnets. The component supports IP routing, TCP and UDP routing, the
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partitioning of the TCP and UDP port spaces, port forwarding, NAT, and can
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also act as DHCP server and as DHCP client.
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The 'nic_router' component can be used to achieve a controlled mediation
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between multiple NIC sessions on network or transport level. NIC sessions are
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assigned to domains. The rules configured by the user then mediate between
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these domains. This is a brief overview of the features thereby provided:
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* Acting as hub between NIC session with the same domain,
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* routing of UDP and TCP according to destination IP address and port,
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* routing of ICMP and IPv4 according to destination IP address,
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* port forwarding for UDP and TCP,
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* NAPT for UDP, TCP and ICMP "Echo",
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* forwarding of ICMP "Destination Unreachable" according to the UDP, TCP or
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ICMP "Echo" connection it refers to,
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* acting as DHCP server or client per domain,
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* provide per-domain network statistics via a report session,
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* print out header information for each packet received or sent,
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* and be fully re-configurable at runtime.
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Basics
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@ -24,7 +35,7 @@ server. Inside the component, uplink and downlinks are treated the same. Its
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routing algorithm is ultimately controlled through the configuration. NIC
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sessions are assigned to domains. Each domain represents one subnet and a
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corresponding routing configuration. Currently, each domain can contain
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only one NIC session at a time. The assigment of sessions to domains is
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only one NIC session at a time. The assignment of sessions to domains is
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controlled through the the common Genode session-policy tag:
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! <policy label_prefix="http_server" domain="http_servers" />
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@ -87,6 +98,9 @@ meaning:
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---------------------------------------------------------------
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<ip dst="X" /> | Routing IP packets that target
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| IP range X
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---------------------------------------------------------------
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<icmp dst="X" /> | Routing ICMP packets that target
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| IP range X
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A detailed explanation of the different routing rules is given in the
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following sections of this document. For all rules marked with a star, the
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@ -98,15 +112,36 @@ additional back-routing rule for that.
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Now having this variety of ways of routing a packet, it is absolutely legal
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that for one packet the domain may contain multiple rules that are applicable.
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And additionally, there may even be a link state that fits. The router's
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choice, however, is always deterministic. It follows a simple priority scheme:
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choice, however, is always deterministic. It follows this priority scheme:
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1) Link states
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2) Port forwarding rules
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3) Longest prefix match amongst TCP respectively UDP rules
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3.1) Subrule that permits any port
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3.2) Subrules that permit specific ports
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:For TCP and UDP:
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1) Domain-local IP traffic
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2) Link states
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3) Port forwarding rules
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4) Longest prefix match amongst TCP respectively UDP rules
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4.1) Subrule that permits any port
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4.2) Subrules that permit specific ports
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5) Longest prefix match amongst IP rules
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:For ICMP "Echo":
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1) Domain-local IP traffic
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2) Link states
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3) Longest prefix match amongst ICMP rules
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4) Longest prefix match amongst IP rules
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:For ICMP "Destination Unreachable" with embedded UDP, TCP or ICMP "Echo":
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1) Domain-local IP traffic
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2) Link states
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3) Longest prefix match amongst IP rules
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:For IP with unsupported transport-layer protocol:
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1) Domain-local IP traffic
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2) Longest prefix match amongst IP rules
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IP rules
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########
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@ -132,6 +167,35 @@ to unexpected leakage of local IP addresses and ports, you should use the
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combination of IP rules and NAT only with great care.
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ICMP rules
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##########
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These are examples for ICMP rules:
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! <icmp dst="10.0.2.0/24" domain="intranet" />
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! <icmp dst="192.168.1.18/32" domain="my_server" />
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! <icmp dst="0.0.0.0/0" domain="uplink" />
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ICMP rules only apply to ICMP "Echo" packets from sessions of the surrounding
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domain. The 'dst' attribute is compared with the IP destination of the packet.
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The rule with the longest prefix match is taken. The packet is then routed to
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the domain given in the rule.
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For bidirectional traffic, you'll need only one ICMP rule describing the
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client-to-server direction. The server-sided domain doesn't need a rule as the
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router correlates replies to the client-sided rule (and only those) via a link
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state (Section [Link states]) that was created at the clients initial request.
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ICMP rules consider whether the router shall apply NAT (Section [Configuring
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NAT]) for the client side. If this is the case, source IP and ICMP query ID
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are replaced by the router's IP identity and a free ICMP query ID at the
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server-sided domain. Also the corresponding link state takes this in account
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to change back the destination of the replies.
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The router also forwards ICMP errors. This is described in section
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[Link states].
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TCP and UDP rules
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#################
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@ -199,16 +263,16 @@ and port are translated.
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Link states
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###########
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Each time a packet gets routed by using a TCP, UDP, or port-forwarding rule,
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the router creates a link state. From then on, all packets that belong
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Each time a packet gets routed by using a TCP, UDP, ICMP or port-forwarding
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rule, the router creates a link state. From then on, all packets that belong
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to the exchange this first packet initiated and come from one of the two
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involved domains are routed by the link state and not by a rule. The costs
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for the link state are paid by the session that sent the first packet.
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involved domains are routed by the link state and not by a rule. The costs for
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the link state are paid by the session that sent the first packet.
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If a link state exists for a packet, it is unambiguously correlated through
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the source IP and port as well as the destination IP and port. This is also
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the case if the transfer includes NAT no matter of what kind or for which
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side.
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If a link state exists for a packet, it is unambiguously correlated either
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through source IP and port plus destination IP and port or, for ICMP, through
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source and destination IP plus ICMP query ID. This is also the case if the
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transfer includes NAT no matter of what kind or for which side.
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It is desirable to discard a link state as soon as it is not needed anymore.
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The more precise this is done, the more efficient can NIC sessions use their
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@ -220,19 +284,20 @@ the NIC router shall discard a link state can be configured in the <config>
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tag of the router for each link type separately:
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! <config udp_idle_timeout_sec="30"
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! tcp_idle_timeout_sec="50">
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! tcp_idle_timeout_sec="50"
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! icmp_idle_timeout_sec="5">
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This would set the maximum UDP idle time to 30 and the maximum TCP idle time
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to 50 seconds. You should choose these values with care. If they are too low,
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replies that normally need no routing rule may get lost. If it is too high,
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link states are held longer than necessary.
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This would set the maximum ICMP idle time to 5, the maximum UDP idle time to
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30 and the maximum TCP idle time to 50 seconds. You should choose these values
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with care. If they are too low, replies that normally need no routing rule may
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get lost. If it is too high, link states are held longer than necessary.
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For UDP link states, this timeout is the only condition that leads to a
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discard. This is better known as hole punching. It allows peers to keep alive
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a UDP pseudo-connection through the router by frequently sending empty packets.
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The need for such a pseudo-connection arises from the router's demand to
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support NAT for UDP transfers and the consequence of keeping the corresponding
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mapping information.
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For UDP and ICMP link states, this timeout is the only condition that leads to
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a discard. This is better known as hole punching. It allows peers to keep
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alive a UDP or ICMP pseudo-connection through the router by frequently sending
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empty packets. The need for such a pseudo-connection arises from the router's
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demand to support NAT for UDP and ICMP transfers and the consequence of
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keeping the corresponding mapping information.
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The lifetime management of TCP link states, in contrast, is more complex. In
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addition to the common timeout, they may be discarded also after the router
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@ -242,6 +307,12 @@ in the <config> tag too:
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! <config tcp_max_segm_lifetime_sec="20">
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As long as there is a link state for a connection, the router also forwards
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ICMP "Destination Unreachable" packets that contain a packet of this
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connection embedded in their payload. The embedded packet is adapted according
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to the NAT configuration of the link state as well as the outer IPv4 packet
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that contains the ICMP.
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Configuring NAT
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###############
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@ -274,11 +345,16 @@ And even combined:
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! <nat domain="intranet" tcp-ports="43" udp-ports="21" />
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If one of the two attributes is not set, this means that no port shall be used
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for this protocol which effectively disables it. Thus, at least one of the two
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attributes must be set for the NAT rule to be sensible. Restricting the port
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usage is necessary to avoid that a client can run Denial-of-Service attacks
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against the destination domain by occupying all of its ports.
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The same goes for ICMP query IDs:
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! <nat domain="intranet" tcp-ports="43" udp-ports="21" icmp-ids="102" />
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If one of the port or ID attributes is not set, this means that no port or ID
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shall be used for this protocol which effectively disables it. Thus, at least
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one of these attributes must be set for the NAT rule to be sensible.
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Restricting the port usage is necessary to avoid that a client can run
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Denial-of-Service attacks against the destination domain by occupying all of
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its ports or IDs.
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Configuring DHCP server functionality
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@ -378,7 +454,7 @@ Log router decisions and optional hints.
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! <config verbose_packets="yes">
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Log most important protocol header fields of each packet that is received or
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sent by the router (ETH, IPv4, ARP, UDP, TCP, DHCP).
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sent by the router (ETH, IPv4, ARP, UDP, TCP, DHCP, ICMP).
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! <config verbose_domain_state="yes">
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@ -390,7 +466,8 @@ connected, current IPv4 config).
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! <config/>
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Log most important protocol header fields of each packet that is received or
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sent at a specific domain (ETH, IPv4, ARP, UDP, TCP, DHCP).
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sent at a specific domain (ETH, IPv4, ARP, UDP, TCP, DHCP, ICMP).
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Examples
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########
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