Simplify and extend hostname resolution (#155)

Make the hostname resolution more "pluggable", using separate Resolvers
for reading from /etc/hosts and RAINS.

Resolving from the /etc/hosts file (by hostsfileResolver) now reads the
file on every request instead of parsing on startup. This is more
aligned with what e.g. gethostbyname does, making modifications to the
file visible to running applications.

Because it's easy, add an additional resolver that loads
/etc/scion/hosts, in the same format as /etc/hosts. The idea is that by
having a separate file, we can now easily distribute a simple hosts file
from the coordinator.

Remove the fixed hosts table with `AddHost` and the only
half-implemented reverse lookup `GetHostnamesByAddress`.

Add explicit build tag `norains` to build without RAINS support.
Building with `make 'TAGS=!norains'` will build
without rains.
This is handy e.g. when updating the SCION dependency with API breaking
changes here in scion-apps but RAINS has not yet been updated.

Author: matzf

Makefile

@@ -8,6 +8,9 @@ BIN = bin
 #     $GOBIN, else $GOPATH/bin, else $HOME/go/bin
 DESTDIR = $(shell set -a; eval $$( go env ); echo $${GOBIN:-$${GOPATH:-$${HOME}/go}/bin})
 
+# HINT: build with TAGS=norains to build without rains support
+TAGS =
+
 all: lint build
 
 build: scion-bat \
@@ -24,84 +27,84 @@ clean:
 	rm -f bin/*
 
 test: lint
-	go test -v ./...
+	go test -v -tags=$(TAGS) ./...
 
 setup_lint:
 	@# Install golangci-lint (as dumb as this looks, this is the recommended way to install)
 	curl -sfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh| sh -s -- -b $$(go env GOPATH)/bin v1.26.0
 
 lint:
 	@type golangci-lint > /dev/null || ( echo "golangci-lint not found. Install it manually or by running 'make setup_lint'."; exit 1 )
-	golangci-lint run
+	golangci-lint run --build-tags=$(TAGS)
 
 install: all
   # Note: install everything but the examples
 	mkdir -p $(DESTDIR)
 	cp -t $(DESTDIR) $(BIN)/scion-*
 
 integration: all
-	go test -v -tags=integration ./... ./_examples/helloworld/
+	go test -v -tags=integration,$(TAGS) ./... ./_examples/helloworld/
 
 .PHONY: scion-bat
 scion-bat:
-	go build -o $(BIN)/$@ ./bat/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./bat/
 
 .PHONY: scion-bwtestclient
 scion-bwtestclient:
-	go build -o $(BIN)/$@ ./bwtester/bwtestclient/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./bwtester/bwtestclient/
 
 .PHONY: scion-bwtestserver
 scion-bwtestserver:
-	go build -o $(BIN)/$@ ./bwtester/bwtestserver/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./bwtester/bwtestserver/
 
 .PHONY: scion-imagefetcher
 scion-imagefetcher:
-	go build -o $(BIN)/$@ ./camerapp/imagefetcher/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./camerapp/imagefetcher/
 
 .PHONY: scion-imageserver
 scion-imageserver:
-	go build -o $(BIN)/$@ ./camerapp/imageserver/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./camerapp/imageserver/
 
 .PHONY: scion-netcat
 scion-netcat:
-	go build -o $(BIN)/$@ ./netcat/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./netcat/
 
 .PHONY: scion-sensorfetcher
 scion-sensorfetcher:
-	go build -o $(BIN)/$@ ./sensorapp/sensorfetcher/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./sensorapp/sensorfetcher/
 
 .PHONY: scion-sensorserver
 scion-sensorserver:
-	go build -o $(BIN)/$@ ./sensorapp/sensorserver/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./sensorapp/sensorserver/
 
 .PHONY: scion-ssh
 scion-ssh:
-	go build -o $(BIN)/$@ ./ssh/client/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./ssh/client/
 
 .PHONY: scion-sshd
 scion-sshd:
-	go build -o $(BIN)/$@ ./ssh/server/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./ssh/server/
 
 .PHONY: scion-webapp
 scion-webapp:
-	go build -o $(BIN)/$@ ./webapp/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./webapp/
 
 .PHONY: example-helloworld
 example-helloworld:
-	go build -o $(BIN)/$@ ./_examples/helloworld/
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./_examples/helloworld/
 
 .PHONY: example-shttp-client
 example-shttp-client:
-	go build -o $(BIN)/$@ ./_examples/shttp/client
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./_examples/shttp/client
 
 .PHONY: example-shttp-server
 example-shttp-server:
-	go build -o $(BIN)/$@ ./_examples/shttp/server
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./_examples/shttp/server
 
 .PHONY: example-shttp-fileserver
 example-shttp-fileserver:
-	go build -o $(BIN)/$@ ./_examples/shttp/fileserver
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./_examples/shttp/fileserver
 
 .PHONY: example-shttp-proxy
 example-shttp-proxy:
-	go build -o $(BIN)/$@ ./_examples/shttp/proxy
+	go build -tags=$(TAGS) -o $(BIN)/$@ ./_examples/shttp/proxy

README.md

@@ -75,14 +75,14 @@ directory, or summarized in the file `gen/sciond_addresses.json`.
 
 
 #### Hostnames
-Hostnames are resolved by scanning the `/etc/hosts` file and by a RAINS lookup.
+Hostnames are resolved by scanning `/etc/hosts`, `/etc/scion/hosts` and by a RAINS lookup.
 
-Hosts can be added to `/etc/hosts` by adding lines like this:
+Hosts can be added to `/etc/hosts`, or `/etc/scion/hosts` by adding lines like this:
 
 ```
 # The following lines are SCION hosts
-17-ffaa:1:10,[10.0.8.100]	server1
-18-ffaa:0:11,[10.0.8.120]	server2
+17-ffaa:1:10,[10.0.8.100] server1
+18-ffaa:0:11,[10.0.8.120] server2
 ```
 
 The RAINS resolver address can be configured in `/etc/scion/rains.cfg`.

bwtester/README.md

@@ -29,19 +29,19 @@ The packet contents are filled with a Pseudo-Random Generator (PRG) based on AES
 
 The wireline protocol is as follows:
 * 'N' new bwtest request
-  	> Request: 'N', encoded bwtest parameters client->server, encoded bwtest parameters server->client
-	> 
-	> Success response: 'N', 0
-	> 
-	> Failure response: 'N', number of seconds to wait until next request is sent
+  > Request: 'N', encoded bwtest parameters client->server, encoded bwtest parameters server->client
+  > 
+  > Success response: 'N', 0
+  > 
+  > Failure response: 'N', number of seconds to wait until next request is sent
 * 'R' result request
-  	> Request: 'R', encoded client sending PRG key
-	>
-	> Success response: 'R', 0, encoded result data
-	>
-	> Not ready response: 'R', number of seconds to wait until result should be ready by
-	>
-	> Not found response: 'R', 127
+  > Request: 'R', encoded client sending PRG key
+  >
+  > Success response: 'R', 0, encoded result data
+  >
+  > Not ready response: 'R', number of seconds to wait until result should be ready by
+  >
+  > Not found response: 'R', 127
 
 ## bwtestclient