From 3b4bb947517d0d9010120aaa1c3989fd6abf278e Mon Sep 17 00:00:00 2001 From: Ghanan Gowripalan Date: Thu, 19 Aug 2021 13:51:40 -0700 Subject: Add loopback interface as an ethernet-based device ...to match Linux behaviour. We can see evidence of Linux representing loopback as an ethernet-based device below: ``` # EUI-48 based MAC addresses. $ ip link show lo 1: lo: mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 # tcpdump showing ethernet frames when sniffing loopback and logging the # link-type as EN10MB (Ethernet). $ sudo tcpdump -i lo -e -c 2 -n tcpdump: verbose output suppressed, use -v[v]... for full protocol decode listening on lo, link-type EN10MB (Ethernet), snapshot length 262144 bytes 03:09:05.002034 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 66: 127.0.0.1.9557 > 127.0.0.1.36828: Flags [.], ack 3562800815, win 15342, options [nop,nop,TS val 843174495 ecr 843159493], length 0 03:09:05.002094 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 66: 127.0.0.1.36828 > 127.0.0.1.9557: Flags [.], ack 1, win 6160, options [nop,nop,TS val 843174496 ecr 843159493], length 0 2 packets captured 116 packets received by filter 0 packets dropped by kernel ``` Wireshark shows a similar result as the tcpdump example above. Linux's loopback setup: https://github.com/torvalds/linux/blob/5bfc75d92efd494db37f5c4c173d3639d4772966/drivers/net/loopback.c#L162 PiperOrigin-RevId: 391836719 --- pkg/tcpip/link/ethernet/ethernet.go | 23 +++++++++++++++++++---- 1 file changed, 19 insertions(+), 4 deletions(-) (limited to 'pkg/tcpip/link/ethernet') diff --git a/pkg/tcpip/link/ethernet/ethernet.go b/pkg/tcpip/link/ethernet/ethernet.go index b427c6170..b9db273d0 100644 --- a/pkg/tcpip/link/ethernet/ethernet.go +++ b/pkg/tcpip/link/ethernet/ethernet.go @@ -42,6 +42,14 @@ type Endpoint struct { nested.Endpoint } +// LinkAddress implements stack.LinkEndpoint. +func (e *Endpoint) LinkAddress() tcpip.LinkAddress { + if l := e.Endpoint.LinkAddress(); len(l) != 0 { + return l + } + return header.UnspecifiedEthernetAddress +} + // DeliverNetworkPacket implements stack.NetworkDispatcher. func (e *Endpoint) DeliverNetworkPacket(_, _ tcpip.LinkAddress, _ tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) { hdr, ok := pkt.LinkHeader().Consume(header.EthernetMinimumSize) @@ -57,18 +65,22 @@ func (e *Endpoint) DeliverNetworkPacket(_, _ tcpip.LinkAddress, _ tcpip.NetworkP // Capabilities implements stack.LinkEndpoint. func (e *Endpoint) Capabilities() stack.LinkEndpointCapabilities { - return stack.CapabilityResolutionRequired | e.Endpoint.Capabilities() + c := e.Endpoint.Capabilities() + if c&stack.CapabilityLoopback == 0 { + c |= stack.CapabilityResolutionRequired + } + return c } // WritePacket implements stack.LinkEndpoint. func (e *Endpoint) WritePacket(r stack.RouteInfo, proto tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) tcpip.Error { - e.AddHeader(e.Endpoint.LinkAddress(), r.RemoteLinkAddress, proto, pkt) + e.AddHeader(e.LinkAddress(), r.RemoteLinkAddress, proto, pkt) return e.Endpoint.WritePacket(r, proto, pkt) } // WritePackets implements stack.LinkEndpoint. func (e *Endpoint) WritePackets(r stack.RouteInfo, pkts stack.PacketBufferList, proto tcpip.NetworkProtocolNumber) (int, tcpip.Error) { - linkAddr := e.Endpoint.LinkAddress() + linkAddr := e.LinkAddress() for pkt := pkts.Front(); pkt != nil; pkt = pkt.Next() { e.AddHeader(linkAddr, r.RemoteLinkAddress, proto, pkt) @@ -83,7 +95,10 @@ func (e *Endpoint) MaxHeaderLength() uint16 { } // ARPHardwareType implements stack.LinkEndpoint. -func (*Endpoint) ARPHardwareType() header.ARPHardwareType { +func (e *Endpoint) ARPHardwareType() header.ARPHardwareType { + if a := e.Endpoint.ARPHardwareType(); a != header.ARPHardwareNone { + return a + } return header.ARPHardwareEther } -- cgit v1.2.3