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2ef6804ead
This change adds all the kernel parts of a network stack. The network stack is partial but implements many of the important parts. Add if(4) network interface abstraction. Network interfaces are registered in a global list that can be iterated and each assigned an unique integer identifier. Add reference counted packets with a cache that recycles recent packets. Add support for lo(4) loopback and ether(4) ethernet network interfaces. The /dev/lo0 loopback device is created automatically on boot. Add arp(4) address resolution protocol driver for translation of inet(4) network layer addresses into ether(4) link layer addresses. arp(4) entries are cached and evicted from the cache when needed or when the entry has not been used for a while. The cache is limited to 256 entries for now. Add ip(4) internet protocol version 4 support. IP fragmentation and options are not implemented yet. Add tcp(4) transmission control protocol sockets for a reliable transport layer protocol that provides a reliable byte stream connection between two hosts. The implementation is incomplete and does not yet implement out of band data, options, and high performance extensions. Add udp(4) user datagram protocol sockets for a connectionless transport layer that provides best-effort delivery of datagrams. Add ping(4) sockets for a best-effort delivery of echo datagrams. Change type of sa_family_t from unsigned short to uint16_t. Add --disable-network-drivers to the kernel(7) options and expose it with a bootloader menu. tix-iso-bootconfig can set this option by default. Import CRC32 code from libz for the Ethernet checksum. This is a compatible ABI change that adds features to socket(2) (AF_INET, IPPROTO_TCP, IPPROTO_UDP, IPPROTO_PING), the ioctls for if(4), socket options, and the lo0 loopback interface. This commit is based on work by Meisaka Yukara contributed as the commit bbf7f1e8a5238a2bd1fe8eb1d2cc5c9c2421e2c4. Almost no lines of this work remains in this final commit as it has been rewritten or refactored away over the years, see the individual file headers for which files contain remnants of this work. Co-authored-by: Meisaka Yukara <Meisaka.Yukara@gmail.com>
103 lines
3.8 KiB
Groff
103 lines
3.8 KiB
Groff
.Dd June 5, 2017
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.Dt ARP 4
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.Os
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.Sh NAME
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.Nm arp
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.Nd address resolution protocol
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.Sh SYNOPSIS
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.In netinet/if_ether.h
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.In netinet/in.h
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.Sh DESCRIPTION
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The Address Resolution Protocol (ARP) provides resolution of network layer
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addresses to link layer addresses on the local network.
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ARP requests asks for the link layer address of a network layer address and ARP
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replies contains the link layer address of the requested network layer
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address.
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Requests are broadcast on the local network, while replies are unicast back to
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the sending machine.
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.Pp
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The
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.Xr kernel 7
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uses the ARP to resolve Internet Protocol version 4 addresses
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.Xr ( inet 4 )
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to Ethernet addresses
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.Xr ( ether 4 )
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in order to transmit Internet Protocol version 4
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.Xr ( ip 4 )
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datagrams on Ethernet network interfaces
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.Xr ( if 4 ) .
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.Pp
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The
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.Xr kernel 7
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maintains a cache of ARP replies for every network interface, which is actively
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populated whenever there is a need to transmit to a network layer address, and
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passively populated with the source addresses of ARP requests from other hosts.
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Network layer datagrams are queued whenever a network layer address needs to
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be resolved.
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Queued datagrams are transmitted when the destination link layer address has
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been resolved, or are discarded if the resolution times out.
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.Sh IMPLEMENTATION NOTES
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The transmission queue is limited to 16 packets for each network layer address.
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.Pp
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ARP requests are attempted three times, each attempt timing out after a second.
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If each request fails, the cache entry is evicted.
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.Pp
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When an network address is resolved, its cache entry remains valid for 60
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seconds.
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Upon expiry, if the cache entry was unused, it is evicted.
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Otherwise, its network address is renewed by resolving it again with three
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attempts.
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In the meanwhile, the entry cache continues to be used for routing.
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If the renewal fails, the cache entry is evicted.
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.Pp
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The ARP cache can contain up to 256 entries.
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If the cache is full,
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the least recently used cache entry is evicted when a network address is resolved
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that is not currently in the cache, and the source addresses from received ARP
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messages are not passively added to the cache.
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.Pp
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The ARP cache uses a hash table with 256 entries, using a linked list in case
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of hash collisions.
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The hash is the the bytewise xor (exclusive or) of every byte in the network
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address.
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This hash is perfect if the subnet contains no more than 256 addresses, and
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degrades in quality for larger subnets, at worst needing to linearly scan the
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whole ARP cache.
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.Pp
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The ARP cache is purged when the network interface's
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.Xr ether 4
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or
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.Xr inet 4
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configuration changes.
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Packets in the ARP transmission queue are dropped.
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.Sh SEE ALSO
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.Xr ether 4 ,
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.Xr if 4 ,
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.Xr inet 4 ,
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.Xr kernel 7
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.Sh STANDARDS
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.Rs
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.%A D. Plummer
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.%D November 1982
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.%R STD 37
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.%R RFC 826
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.%T \&An Ethernet Address Resolution Protocol
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.Re
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.Sh BUGS
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The ARP timeout is not configurable.
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.Sh SECURITY CONSIDERATIONS
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The source network layer and source link layer addresses of received ARP request
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and replies are trusted.
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If the router does not validate ARP messages on the network are consistent with
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the DHCP leases, an attacker will be able to engage in an ARP spoofing attack
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that would allow denial of service, man in the middle, and session hijacking
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attacks.
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.Pp
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The cache is limited to 256 entries per interface and if the local subnet is
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larger than 256 addresses, then if an attacker is capable of making the local
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system concurrently transmit to 256 addresses on the local network not currently
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in the ARP cache, then doing so would purge the whole transmission queue.
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If the attacker can do this faster than the machines on the local network can
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answer ARP requests, transmission service may be denied entirely, or at the
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least be significantly degraded as the higher layers wait a little while before
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they retransmit.
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