[устранено] mpc & localhost

С "сетью" все в порядке: все тесты положительны :)

Проблему устранил.
Отчасти дело было в /etc/hosts, а именно в дефолтной строке:
::1 localhost
для IPv6 (хотя им и не пользуюсь)

Не знаю, или это баг MPC, или чего другого, но разгадку дал следующий кусок strace mpc:

open("/etc/hosts", O_RDONLY)            = 3
fcntl64(3, F_GETFD)                     = 0
fcntl64(3, F_SETFD, FD_CLOEXEC)         = 0
fstat64(3, {st_mode=S_IFREG|0644, st_size=1060, ...}) = 0
mmap2(NULL, 131072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7ea6000
read(3, "# /etc/hosts: Local Host Databas"..., 131072) = 1060
close(3)                                = 0
munmap(0xb7ea6000, 131072)              = 0
open("/etc/hosts", O_RDONLY)            = 3
fcntl64(3, F_GETFD)                     = 0
fcntl64(3, F_SETFD, FD_CLOEXEC)         = 0
fstat64(3, {st_mode=S_IFREG|0644, st_size=1060, ...}) = 0
mmap2(NULL, 131072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7ea6000
read(3, "# /etc/hosts: Local Host Databas"..., 131072) = 1060
close(3)                                = 0
munmap(0xb7ea6000, 131072)              = 0
open("/etc/gai.conf", O_RDONLY)         = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=2139, ...}) = 0
fstat64(3, {st_mode=S_IFREG|0644, st_size=2139, ...}) = 0
mmap2(NULL, 131072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7ea6000
read(3, "# Configuration for getaddrinfo("..., 131072) = 2139
read(3, "", 131072)                     = 0
close(3)                                = 0
munmap(0xb7ea6000, 131072)              = 0
socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET6, sin6_port=htons(6600), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 0
getsockname(3, {sa_family=AF_INET6, sin6_port=htons(33788), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 0
close(3)                                = 0
socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(6600), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
getsockname(3, {sa_family=AF_INET, sin_port=htons(33788), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
close(3)                                = 0
socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
fcntl64(3, F_GETFL)                     = 0x2 (flags O_RDWR)
fcntl64(3, F_SETFL, O_RDWR|O_NONBLOCK)  = 0
connect(3, {sa_family=AF_INET, sin_port=htons(6600), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation now in progress)
socket(PF_INET6, SOCK_STREAM, IPPROTO_TCP) = 4
fcntl64(4, F_GETFL)                     = 0x2 (flags O_RDWR)
fcntl64(4, F_SETFL, O_RDWR|O_NONBLOCK)  = 0
connect(4, {sa_family=AF_INET6, sin6_port=htons(6600), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = -1 EINPROGRESS (Operation now in progress)
select(5, [4], NULL, NULL, {10, 0})     = 1 (in [4], left {10, 0})
recv(4, 0x80633e8, 50000, 0)            = -1 ECONNREFUSED (Connection refused)
write(2, "MPD_HOST and/or MPD_PORT environ"..., 59MPD_HOST and/or MPD_PORT environment variables are not set
) = 59
brk(0x8099000)                          = 0x8099000
write(2, "error: problems getting a respon"..., 86error: problems getting a response from "localhost" on port 6600 : Connection refused
) = 86
exit_group(1)                           = ?
Process 5832 detached

Не понятно, чего это он пристает к IPv6, когда по IPv4 всё, что нужно выяснено.