What version of Go are you using (go version)?

$ go version
go version go1.15 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/usr/local/google/home/leguo/.cache/go-build"
GOENV="/usr/local/google/home/leguo/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/usr/local/google/home/leguo/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/usr/local/google/home/leguo/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/google-golang"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/google-golang/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build869140076=/tmp/go-build -gno-record-gcc-switches"

What did you do?

syslog.New() uses default syslog socket "/dev/log". To specify socket path, syslog.Dial("unixgram", "/dev/log", ...) can be used. The behaviors should be the same but but they are not. Running the attached example code:

syslog_debug.go.txt

$ go run syslog_debug.go; tail /var/log/debug -n 100 | grep 'Syslog debug: logging with'

What did you expect to see?

Oct 9 17:13:33 foo.bar.com default[1804115]: Syslog debug: logging with default socket.
Oct 9 17:13:33 foo.bar.com customized[1804115]: Syslog debug: logging with customized socket.

What did you see instead?

Oct 9 17:13:33 foo.bar.com default[1804115]: Syslog debug: logging with default socket.
Oct 9 17:13:33 foo.bar.com foo.bar.com customized[1804115]: Syslog debug: logging with customized socket.

Host name appears twice in the customized socket. As a result, the hostname will be parsed as programname by rsyslogd.

The reason is that "unixgram" is not considered as a local socket in
https://golang.org/src/log/syslog/syslog.go, line 164:
field "local" will not be set if network is unixgram.

In https://golang.org/src/log/syslog/syslog.go, line 284:
If local is not set, the hostname will be output.

We need to run a dedicated syslog server in Linux. /dev/log cannot be used since it is used by other services.